Category Archives: TECHNIQUES

IMAGING TECHNIQUES OF PHARYNX AND LARYNX

IMAGING TECHNIQUES OF PHARYNX AND LARYNX ARE VERY EASY, WE HERE WILL TRY TO SUMMERIZE THEM.

THANKS TO DR/ DAVID SUTTON MD, FRCP, FRCR, DMRD, FCan.AR (Hon) OUR BELOVED TEACHER

PLAIN XRAY:

  • Lateral Film is the best film projection, as Pharynx and Larynx clear of the cervical spine . as in Fig (1). The Film is placed against the shoulder and the incident beam is centered on :
  • Jaw Angle if we want Nasopharynx.
  • Thyroid Cartilage If we want Larynx.

 

Fig. 1 

(A) Some of the structures demonstrated on a plain lateral view of the neck. Xerography, although giving an excellent demonstration of the air-soft-tissue interface, is no longer available because of the high radiation dose used. 

(B) High-kV lateral neck gives an adequate demonstration of the soft tissue anatomy.

BARIUM SWALLOW

  • More benefit to opacify lesions below the crico-pharyngeus that cannot be assessed with a laryngeal mirror.
  • Tumours of the pharynx will be outlined by barium coating, especially those in the piriform fossae ( difficult to be seen with a mirror).
  • The normal larynx will appear as a `filling defect’ in the frontal projection with contrast in the piriform fossa on either side.
  • This is well shown on the oblique projection, obtained with the patient swallowing while the head is turned to one side.
  • When the larynx fails in its primary function as a protective sphincter for the lungs, ‘spill-over’ will occur to give a `barium laryngogram’.
  • This problem is seen more and more in an aging people, who dysphagia is due to mild stroke.

 

Fig . 2

Lateral View of normal Barium Swallow. The epiglottis (arrow) folds over the larynx as barium passes down into the esophagus .

Cine Radiology

  • It gives a good demonstration of degustation process.
  •  Passage of the bolus across the back of the tongue with an elevation of the larynx and tilting of the epiglottis down over the closed larynx is shown (Fig. 2).
  • Contrast then passes through the open cricopharyngeus into the esophagus.
  • Minor functional disorders of swallowing can only be shown by this technique (Fig. 3).
  • Videofluoroscopy is an alternative means of assessing swallowing function and a technique for assessment of oral or pharyngeal dysphagia.

Fig.3 

Cine barium swallow. Four frames in 1 s, frontal and lateral views. The patient had mild dysphagia due to unilateral vagal paralysis. Various
normal and abnormal features are demonstrated. 
Lateral view. 
1. The vallecula fills with barium and is then partially effaced by the normal backward compression of the tongue. 
2. The epiglottis is partially immobile and only tilts down to the transverse position, not fully covering the laryngeal inlet. 
3. Barium enters the vestibule of the larynx. This may occasionally be observed in asymptomatic subjects but is usually an indication of failure of the laryngeal sphincters ('spill-over'). 
4. At the same time the relation of the postphyaryngeal wall to the cervical spine does not change, indicating paralysis of the middle pharyngeal constrictor. 
5. Cricopharyngeus contracts and relaxes normally. 
AP view. 
6. The epiglottis shows little movement. 
7. The right side of the pharynx contracts normally but the left remains flaccid and filled with barium. 
8. Cricopharyngeus contracts and releases normally.

CT

  • CT is now the optimum method of imaging the outlines of the nasopharynx but also shows the soft-tissue structures of the infratemporal fossa and parapharyngeal space, which lie alongside the Naso- and oropharynx.
  • Formerly this region could be studied radiologically only in an indirect way by examination of surrounding bony structures or by invasive contrast examinations such as sialography and angiography.
  • The infratemporal fossa is situated below the skull base and is bounded by the pharyngeal musculature medially and the mandible laterally.
  • The most prominent and easily recognized structures within the infratemporal fossa are the pterygoid muscles.The anatomy of this region is depicted in Fig. 4.
  • Below, the infratemporal fossa is continuous with the parapharyngeal space.
    The role of CT for lesions in this region may be defined as:
    • To complement direct examination of lesions in the postnasal space
    .• To assess the size and situation and relations of a well-defined
    mass for prospective surgical removal, or the extent of local deep infiltration for radiotherapy planning.
    • To assess the relation of the mass to the great vessels and the parotid gland: by combining CT with contrast enhancement more accurate differentiation becomes possible.Contrast enhancement should be assessed in the vascular phase by intravenous infusion or bolus injection. Further sections in the coronal plane may give more information. Respiratory movement is less of a problem with the new fast scanners.
  • Scanning is begun at the level of the thyroid bone and sequential scans
    of 5 min slices are made in a caudal direction.
  • The shape of the airway alters as sequential scans are viewed. Above the rounded hypopharynx, it is bisected by the crescentic epiglottis.
  • Further down, the median and lateral glossoepiglottic folds delineate the valleculae.
  • Below this, the airway assumes a triangular shape and the piriform sinuses are seen as two lateral appendages separated by the aryepiglottic folds.
  • At the level of the cords the shape changes to the characteristic glottic chink or boat shape with the sharp anterior commissure extending right up to the thyroid cartilage in the midline (Fig. 5).
  • In the subglottic area, there is an even, symmetric oval shape which gives way at the level of the first tracheal ring to an oval flattened posteriorly, which may he likened to the shape of a horseshoe. 
  • CT provides a non-invasive, quick and effective radiological investigation of the larynx, and is not uncomfortable for the patient.
  • It can be done without risk in the face of respiratory obstruction, or after suspected laryngeal injury.
  • It gives an accurate assessment of laryngeal anatomy and involvement by tumor, particularly at the glottic level.
  • The value of such an assessment is greatly increased if partial laryngectomy is contemplated, but this is an unusual operation in the UK where carcinoma of the larynx is treated with radiotherapy and/or total laryngectomy.

 

Fig.4

Axial CT of soft tissues below the skull base. (A) Normal section through antra and postnasal space. The arrowheads indicate the openings of
the eustachian tubes. m = ramus of mandible; s = styloid process; p = pterygoid muscles. (B) Section at a slightly lower level passes through the soft palate
(sp). Tensor and levator palatini blend with the pharyngeal constrictors (c) to give a dense muscle mass. The enlarged but otherwise normal parotid gland
has a lower attenuation, i.e. appears darker, than the masseter muscle in front of it but not as dark as the fatty tissue in the parapharyngeal space. Thus the
medial limit of the deep parotid lobe can be defined (arrowhead). These features are best shown by MRI.

Fig.5
Axial CT Scan of the normal Larynx at the level of true vocal cords. 
Note the diamond shape of the airway with the cords in abduction.
Contrast is given , so CCA (A) and IJV (I) are shown.

MRI

  • MRI is superior over CT for neck masses.
  • MRI show neck vessels without the need to contrast.
  • T1WI gives the best spatial resolution.
  • T weighted Protocols give the best view of muscle invasion by carcinoma esp in Tongue base.
  • CT with contrast is superior over MRI in the evaluation of Neck Lymph Nodes, however, none of them can differentiate neoplastic from inflammatory hyperplasia, yet metabolic techniques like FDG-PET, SPECT is promising in that.
  • Sometimes the presence of Fat can obscure lesions, so fat saturation techniques are very useful, STIR protocol the most dependent one show increase signal intensity from most of the tumors especially Parotid Tumors. But It can’t be used with gadolinium contrast.
  •   Chemical Shift fat saturation techniques can show recurrent tumor after gadolinium injection.
  • Now Fat Saturation Techniques, used with T1 fast spin echo OR Post-gadolinium T protocols are more useful in defining Neck Lesions .   

 

Fig.6 (A) upper image: Sagittal MRI, T 1 -weighted, shows good differentiation between the tongue muscles, the genioglossi in the floor of the mouth and the surrounding fat, especially in the pre-epiglottic space (arrow).  (B) lowe image : Sagittal film in a child revealing subglottic stenosis (arrow)



Fig.7

Chordoma. Sagittal MR section, T,-weighted, shows a nonhomogeneous mass in the nasopharynx and replacing the basisphenoid.The tumor has burst out of its capsule and is displacing the brainstem posteriorly.

MRI ANATOMICAL POSITIONING

Introduction

Magnetic Resonance Imaging could be identified as a medical diagnostic procedure that makes different organs images by using the principle of nuclear magnetic resonance.

It generates thin-sections of any a part of the body – from any angle and direction, So Imaging can form such an image once the body is exposed to electromagnetic field.

MRI creates a powerful magnetic field and also the tiny biological “magnets” within the body consisting of protons set within the nucleus of the atom are attractable, As the nucleon possesses basic magnetic properties.

First, imaging creates a gentle state of magnetism inside the body by inserting it in a very steady magnetic field.

Second, the imaging stimulates the body with radio waves to vary the steady-state orientation of protons.

Third, the imaging machine stops the radio waves and registers the body’s magnetic force transmission.

Fourth, the transmitted signal is accustomed construct internal pictures of the body by computerized axial imaging.

An imaging image isn’t a photograph. it’s truly a computerized map or image of radio signals emitted by the organs. imaging is superior to CAT scan as a result of CAT scan is mistreatment radiation, imaging uses harmless radio waves.

The sole uncommon preparation is that every one removable golden objects should be left outside the scanning space, together with removable hearing aids, dentures, and different prosthetic devices.

Credit cards may be broken by the imaging as a result of magnetic codes may be full of the imaging magnet.

Magnetic Resonance Imaging could be a powerful diagnostic tool within the medical imaging market place because of the procedure of alternative for the visualization of sentimental tissue.

The imaging business is manufacturing over a pair of,000 units p.a. U. s. is portrayed with four-hundredth of the planet promoting and production of imaging. there’s associate degree rising agreement that the imaging incorporates a broad application in smaller hospital and clinics. (1)

 

History of MRI

1882 – Nikola Tesla discovered the Rotating magnetic field in the national capital, Hungary. This was a basic discovery in physics.

Figure (1); Nikole Tesla The genius who lit the world,
The Discoverer of Magnetic Field and the God father of The current MRI (2)

 

1937 – Columbia University prof Isidor I. Rabi operating within the Pupin physic Laboratory in NY town, discovered the quantum phenomenon dubbed nuclear magnetic resonance (NMR).

1950’s – Herman Carr creates a one-dimensional MR image.

1956 – International Electro-technical Commission-Committee of Action declared the MR creation of The at Munich, Deutschland by.

All magnetic resonance imaging machines are a label in “Tesla Units”. The strength of a magnetic field is measured in Tesla or Gauss Units.

The strength of a field is measured in Tesla or Gauss Units. The more sturdy the field, the more sturdy the quantity of radio signals which might be evoked from the body’s atoms and so the higher the standard of resonance imaging footage.

1972 – Raymond Damadian applies for a patent, that describes the idea of nuclear magnetic resonance being employed for the higher purpose. He illustrates major components of magnetic resonance imaging machine in his application.

1973– Paul Lauterbur, a chemist Associate in Nursingd a nuclear resonance pioneer at the State University N Y, Stony Brook, created the first nuclear resonance image. it completely was of a tubing.1974 – Raymond Damadian receives his patent.

1977  Raymond Damadian with his two post-doctoral students, Michael Goldsmith and Larry Minkoff build the first MRI scanner at New York’s Downstate Medical Center. And They do first MRI scan.

Figure (2): shows First MR image ever of the Human organ. 1977 (3)

last 40 years:

1983 – Ljunggren and Tweig introduce k-space.

1986 – Le Bihan writes about diffusion weighted imaging (DWI).

1987 – Real time MR imaging of the heart is developed.

1991 – Filler and colleagues describe imaging of axonal transport of super magnetic metal oxide particles, a technique, which later becomes important in imaging of neural tracts.

1993 – Functional MR imaging of the brain is introduced.

1994 – The first intraoperative MR unit developed by GE and Harvard is installed at the Brigham and Women’s Hospital in Boston.

2000‘s – Cardiac MRI, Body MRI, fetal imaging, practical MR imaging is more developed and become routine in several imaging centers. analysis centers create important strides forward in imaging cartilage on high field scanners. the amount of free standing magnetic resonance imaging centers, most of that utilize low or moderate field MR scanners considerably will increase.(4)

HEAD Imaging

Figure (3): shows A sample patient positioning in an eight channel brain coil (5).

Head is considered the first Organ that MRI was practically used to scan. The brain is the most complicated organ in the body.

MRI has made an advanced progress in the diagnosis of many diseases. Here we are, we will go A step-by-step approach to head and brain MR imaging as given below:

Patient Preparation:

 

Routine Procedure :

The patient consent form should be given to the patient with a detailed explanation of the content. The form should be carefully read, all questions must be answered with clear answers such as “YES” or “NO,” and additional clarifications should be written. It must be signed by the patient or legal guardians and confirmed by MR personnel.

If there are any surgical implants, radiologist on duty has to make a decision based on implant type and MR compatibility.

If there is any suspicion or lack of information on the implant, do not take any risk with the patient safety and do not scan the patient.

If the form is complete with all the information, the patient should change to MR gown and remove any clothing with any metal. It is always a good practice to remove the jewelry as well.

As the last line of patient safety, it is also a good practice to scan patient with a handheld metal detector before taking the patient to MRI room.

Patient Positioning:

The patient head should be centered at the brain coil, chin pointing upward as shown in the figure (3).

The patient should use earplugs for hearing protection with additional headsets and/or immobilization pads should be placed around the head to reduce the noise and gross patient motion.

The head should also be fixed with additional straps for further patient motion reduction while keeping patient safety and comfort as a priority.

We also recommend placing the leg support pads for patient comfort. An alarm bell should be given to the patient and tested.

After landmarking the center of the brain coil or just below the eyes using laser marker lights (while the eyes are closed) or touch sensors, you can send the patient in and start the exam(6).

Routine Brain

Sample Imaging Protocols: Routine brain imaging is used for the patients referred to MRI without any specific diagnosis and it can be applied for general nonspecific headache or even checkup.

Figure (4) show Routine Brain Sequences (7)

Tips and Tricks

Sagittal T1 can be replaced by a sagittal T2 sequence that might be more informative for visualizing craniocervical lesions.
Recently, diffusion weighted imaging (DWI) has been added to routine brain imaging as well. It can also be routinely acquired for older patients.
It is also important to remind that T1 flair as an alternative to T1 imaging can be used for routine brain imaging.

T1 flair sequence provides better gray-white matter contrast and has shorter acquisition times due to higher echo train length. It also further suppresses the CSF resulting in better image quality. However, due to inversion pulse associated with the T1 flair sequence, there are considerable worries regarding postcontrast use of T1 flair sequence.

 

Temporomandibular Joint (TMJ)

Temporomandibular joints (TMJ) joint imaging , we can doe it  with a number of different coils depending on the availability at your site.

Even though today high-density coils with eight channel of above can give you good quality, the TMJ imaging dedicated coils do produce a very nice image quality.

The TMJ dedicated coils usually has a holder device to fix them in the desired position and can be used for bilateral joint imaging with small loop coils as shown in the figure below.                                                                            These loop coils are practically surface coils and can have different diameters. The coil shown in this book is two 3 in. diameter coil.

These coils will get relatively uniform signal within the 3 in. in diameter and 3 in. in depth. Therefore, they provide a high signal for the TMJ joints.

If you have one of the new multichannel brain coils but not the loop coils, you can still follow the guidance in the book to acquire very nice TMJ images.

Quite often TMJ imaging can be done in dynamic imaging or so-called kinematic imaging.

Kinematic imaging requires imaging TMJ joint while the mouth is closed and opened at different levels so that the joint and disc position can be imaged dynamically.

To keep the mouth open at different levels without motion is possible by using kinematic devices from different companies.

However, if you do not have any special hardware for this type of exam, you can still do the kinematic exam by explaining the patient the procedure and give instructions during the scan accordingly.                                              The kinematic exam starts with the mouth shut. Then, we repeat the scan while the patient opens his/her mouth 1–2 cm in each time until we reach the maximum opening.

A step-by-step approach to TMJ MR imaging is given below.

Patient Preparation:

The patient consent form should be given to the patient with a detailed explanation of the content. The form should be carefully read, all questions must be answered with clear answers such as “YES” or “NO,” and additional clarifications should be written.

It must be signed by the patient or legal guardians and confirmed by MR personnel.

Plus Routine Procedure :

As the last line of patient safety, it is also a good practice to scan patient with a handheld metal detector before taking the patient to MRI room.

Patient Positioning:

The TMJ coil is shown in figure (5). The holder enables you to position the coil in the desired location and can be fixed in this location so that it does not move during the scan.

When you position the coil, ask the patient to open and close the mouth while you feel exactly where the TMJ joint is with your hand. The loop coil center should be placed directly at the TMJ joint.

The coils should be as close to the face as possible without disturbing the patient. If you do not have the TMJ dedicated coils, you can use general purpose brain coil for imaging.

Please make sure that you give the patient alarm/buzzer to patient’s hand and test it before sending in. The landmarking should be at the center of loop coils using laser lights or touch sensor.

It is always recommended to let the patient know how long the scan is going to take and also keep communicating frequently to make them as comfortable as possible in the MR bore (8).

Figure (5): A sample patient positioning in a TMJ dedicated loop coils (9)

 

Elbow Imaging

Elbow imaging can be done with several different coils.

If you have a general flexible coil available at your site, you can put the patient feet first on supine position and let the arms at the side. Then you can wrap the coil around the elbow. This is the most comfortable position for the patient.

However, if you do not have any working flexible coils, you can use one of the smaller diameter coils such as the knee, foot, or loop coils to scan the patient head first on prone position. This is also called superman position.

Somewhat contrary to the position name, it is a super uncomfortable position for the patient though. If you have to scan a patient in a cast and cannot be placed straight, you can also use other coils such as shoulder coil for an efficient scan.

Patient Preparation:

Routine Procedure

Patient Positioning:

If you do have a flexible coil, you can position the patient supine, feet first or head first and you can wrap the coil around the elbow of interest. Quite similar to shoulder imaging patient preparation, place additional pads under the patient’s arm to make humerus almost parallel to the table.

The palm of the hand should be pointing upward as well for the best patient position as shown in Figure (6). However, if the patient is unable to stay in this position due to injury or pain, you can position the arm in a more comfortable way.

To reduce gross patient motion artifacts, immobilization straps should be placed over the patient arm at the elbow level or a bit more inferior.

Please make sure that you give the patient alarm/buzzer to patient’s hand and test it. After landmarking the center of the coil using laser lights or touch sensors, you can send the patient in and start the exam.

It is always recommended to let the patient know how long the scan is going to take and also keep communicating frequently to make them as comfortable as possible in the MR bore (10).

Figure (6) shows: A sample patient positioning for an elbow in a general purpose flexible coil. Please
note that the patient palm point upward in this position (external rotation)(11).

Knee Imaging

Knee joints are usually imaged unilaterally using dedicated coils.

The knee and foot coils are usually what we call transmit/receive coils rather than receive only coils.

Transmit/receive coils have the design features to be able to transmit the RF pulse directly from a transmitter element in the coil and receives the signal with receiver elements. This way, we can eliminate possible wrap around
or aliasing artifacts from the other knee.

Most of the MR systems use dedicated single channel (quadrature) transmit-receive knee coils. However, most of the recent MR systems come with multichannel (8 or 16) dedicated transmit/receive knee and/or foot coils.

The utilization of dedicated multichannel knee coils can make significant improvements in MR image SNR and We can use it for either shorter scan time or increased spatial resolution.

If you do not have a dedicated knee coil,So you can use other available coils. However, the image parameters should be modified to compensate for the SNR loss.

Patient Preparation:

Routine Procedures.

Finally, have the patient go to the restroom before the exam.

Patient Positioning:

Place the knee coil straight at the center of the MR table.

When you place the patient’s knee in the coil, insert a small pad under the knee joint to slightly bend the knee (about 15°).

The patella should be aligned with the center of the coil for good positioning.

When the coil top is attached and locked, place additional pads between the knee and coil to further immobilize the knee. These pads can significantly reduce the motion artifacts.

The other knee should be placed as further ways from the coil as possible to prevent any wrapping or aliasing, especially with the receive-only coil.

Please make sure that you give the patient alarm bell to the patient and ask them to test it before sending in.

Mark center of the coil by using laser lights then send the patient in and start the exam.

It is always recommended to let the patient know how long the scan is going to take and also keep communicating frequently to make them as comfortable as possible in the MR bore (12).

Figure (7): shows A sample for feet first and supine patient positioning in a multichannel transmit/ receive knee coil is shown. Please note that this coil design includes a small pad under the knee joint (13).

Figure (8): shows A sample for feet first and supine patient positioning in a multichannel transmit/receive knee coil is shown. Please note that this coil design includes a small angulation around 15°(14).

Liver and biliary system

.
Indications
Why the physician would ask for MRI on Liver and biliary system.

first, if he is looking for Focal lesions or staging of neoplasms, or even benign hepatic disease, especially haemangioma and focal nodular hyperplasia. hemochromatosis.

Gallbladder disease and biliary duct obstruction are also of the important indications.

Finally evaluation of liver infiltrants such as iron or fat.

Equipment

we will need Body coil/volume torso array or multi-coil and  RC bellows. Also, Ear plugs and Pe gating leads are required.

Patient positioning

The patient lies supine on the examination couch with the RC bellows (if required) securely attached.

The patient is positioned so that the longitudinal alignment light lies in the midline, and the horizontal alignment light passes through the level of the third lumbar vertebra or the lower costal margin(15) .

Suggested protocol:

Coronal breath-hold incoherent (spoiled) GRE/SE T1 (Figure 9)

Acts as a localizer if three-plane localization is unavailable, or as a diagnostic sequence.

Thick slices/gap are prescribed relative to the vertical alignment light, from the posterior abdominal muscles to the anterior abdominal wall.

The area from the pubic symphysis to the diaphragm is included in the image.

P 60 mm to A 40 mm

Axial SE/FSE/incoherent (spoiled) GRE T1 and out of phase (Figures 10 and 11)

As for Coronal T1, except prescribe slices from the inferior margin of the liver to the diaphragm.

Figure 11.2 Coronal SE T1 weighted image through the abdomen demonstrating slice prescription boundaries and orientation for axial imaging of the liver.

Delayed scans after contrast enhancement using chemical/spectral presaturation techniques are sometimes necessary to evaluate arterial and venous phases.

Figure (9): Coronal SE T1 weighted image through the abdomen demonstrating slice prescription boundaries and orientation for axial imaging of the liver(16).

Figure (10): Axial FSE T1 weighted image through the liver (17).

Figure (11): Axial incoherent (spoiled) T1 weighted breath-hold image of the liver (18).

 

Additional sequences

SS-FSE (MRCP) (Figure 11)

This sequence provides images in which only fluid-filled spaces such as the gall bladder and biliary ducts return signal.

It is necessary to use very long TEs and TRs to effectively nullify the signal from all tissues except those that have long T2 decay times.

TEs in excess of 200 ms and TRs of more than 10 s are required (see also Pancreas and Salivary glands). If SS-FSE is unavailable then an FSE sequence may be substituted.

Figure (12) Coronal SS-FSE image of the gallbladder (MRCP). Very long values of TR and TE were used to acquire images in which only fluid is seen (19).

 

SS-FSE/GRE-EPI/SE-EPI/diffusion imaging

The use of real-time imaging has applications in the liver and biliary system. This includes biopsies and thermal ablations of liver lesions under real-time MR control.

In addition, diffusion and perfusion techniques of the liver have been developed that may negate the use of contrast agents in the future.

DWI images are overlaid onto T1 weighted acquisitions.

The DWI image set provides pathology information, whereas the T1 weighted acquisition provides anatomical data.

The images produced are not dissimilar to a PET/CT scan.

In addition diffusion tensor imaging used in conjunction with parallel imaging techniques enables differentiation of benign from malignant hepatic lesions and may also assist in the quantification of hepatic fibrosis.

Image optimization

TECHNIQUE ISSUE:

The inherent SNR and CNR of the abdominal contents are usually excellent due to their high proton density, and the use of a torso array coil increases this even further.

In addition, parallel imaging techniques using multi-array coils reduce scan time significantly.

Due to the respiratory artifact, RC or respiratory triggering may be necessary. Alternatively, breath-hold techniques may be used to suspend respiratory motion.

In axial T1 sequences, it is necessary to shorten the TR to less than 400 ms in SE sequences as this is considered the optimum value for demonstrating liver contrast.

As the slice number available per acquisition is reduced with a short TR, two or three acquisitions may be required to cover the whole liver.

Two FSE sequences using TEs of 80 ms and 160 ms are required to characterize haemangiomas, which retain a high signal intensity on late echo images.

Artefact problems

The main source of artifact in the liver is motion caused by respiration, flow and peristalsis.

RC or respiratory triggering is often required, especially on the superior axial slices, due to the proximity of the diaphragm.

However, breath-hold techniques may also be utilized. Pe gating is sometimes used but it often increases the scan time, especially if the patient’s heart rate is slow or cardiac output poor, so that the system cannot trigger efficiently off each R wave.

Commonly, Pe gating does not significantly increase image quality and only serves to lengthen the scan time. Under these circumstances it is advisable to dispense with it.

Spatial presaturation pulses placed S and I to the FOV are necessary to decrease flow motion artifact in the aorta and IVC.

GMN also minimizes flow artifact but, as it increases the signal in vessels and the minimum TE, it is not usually beneficial in T1 weighted sequences.

Bowel motion is often a problem on the lower axial slices of the liver, whereas gastric motion artifact is sometimes evident on the more superior slices. Antispasmodic agents, given IV, IM or subcutaneously prior to the examination, effectively reduce this.

Patient considerations

Careful explanation of the procedure is important.

Ensure that the patient is as comfortable as possible. Some antispasmodic agents given IM may cause nausea but fruit juice given after the study can alleviate this.

Due to excessively loud gradient noise associated with some sequences, ear plugs must always be provided to prevent hearing impairment.

Contrast usage;

Contrast is often beneficial to demonstrate liver metastases.

Weighting depends on the type of contrast media used.

T1 shortening agents such as gadolinium require T1 weighted post-contrast scans.

These can be acquired in conjunction with chemical/spectral presaturation pulses and acquired in multiple phases to evaluate the dynamic contrast enhancement characteristics of hepatic lesions.

T2 weighting is necessary after injection of superparamagnetic T2 shortening (liver specific) agents (see Contrast agents in Part 1).

Scans should be delayed for approximately 1 hour after injection to allow time for uptake of contrast by the liver.

The use of contrast and dynamic imaging to visualize liver vasculature and the biliary system is gaining in popularity. Oral and rectal contrast agents,for evaluation of gastrointestinal disease, are also used

Kidneys and adrenal glands

Indications:

Why the physician asks for MRI on Kidneys and adrenal glands. this happens when we we are looking for: Adrenal masses and hemorrhage Or  Renal masses and haemorrhage.Or renal cell carcinoma.Or Renal transplant rejection.Or Ureteric obstruction.

Equipment

 we will need Body coil/multi-phased array or multi-coil array, RC bellows and  Ear plugs.

Patient positioning

The patient lies supine on the examination couch with the RC bellows securely attached (if required).

The patient is positioned so that the longitudinal alignment light lies in the midline, and the horizontal alignment light passes through the level of the third lumbar vertebra, or the lower costal margin. The kidneys are generally located about four fingers inferior to the xiphoid (20).
 

Suggested protocol

Coronal breath-hold fast incoherent (spoiled) GRE/SE/FSE T1 (Figure 13)

Acts as a localizer if three-plane localization is unavailable. Alternatively it can be used as a diagnostic sequence. Medium slices/gap are prescribed on either side of the vertical alignment light, from the posterior abdominal muscles to the anterior abdominal wall. The area from the pubis symphysis to the diaphragm is included in the image.

Axial incoherent (spoiled) GRE T1 in and out of phase .contrast chemical/spectral presaturation

As for Coronal SE/FSE T1, except medium slices/gap are prescribed from the inferior margin of the kidneys to the superior aspect of the adrenals (Figure 14). The coronal plane may also be useful depending on lesion location. Slices may also be offset to specifically image the adrenals.

 

Figure (13) Coronal incoherent (spoiled) GRE T1 weighted image through the abdomen demonstrating the kidneys(21)

 

Figure (14) Coronal incoherent (spoiled) GRE T1 weighted through the abdomen demonstrating slice prescription boundaries and orientation for axial imaging of the kidneys(22)

Additional sequences

MR urography:

Either FSE or SS-FSE sequences may be used with very long TEs and TRs to produce heavily T2 weighted images in which only fluid that has a very Figure 11.10 Coronal incoherent (spoiled) GRE T1 weighted through the abdomen demonstrating slice prescription boundaries and orientation for axial imaging of the kidneys.for use in the urinary system to visualize the renal collecting system, the ureters, and the bladder.

Diffusion imaging

DWI using SS-EPI acquisition in conjunction with parallel imaging techniques may be useful in the differentiation of malignant adrenal lesions from hyperplasia or adenomas and renal cysts from renal cell carcinomas.

Image Optimization

Technical issues

The inherent SNR and CNR of the abdominal contents are usually excellent due to their high proton density, and the use of a torso array coil increases this even further. In addition, parallel imaging techniques using multi-array coils reduce scan times significantly. Spatial resolution is important, especially when imaging relatively small structures such as the kidneys and adrenal glands, which therefore require thin slices/gap.

However, this is often difficult to achieve when using the body coil, a large FOV and in the presence of respiratory and flow artifact. The use of a torso array coil greatly improves resolution in the abdomen. In addition, parallel imaging techniques can be used to improve resolution whilst

keeping scan times short. SE sequences usually produce the best contrast in the abdomen, but result in fairly lengthy scan times. For this reason, breath-hold GRE or SS-FSE sequences are often preferred. FSE used in conjunction with a rectangular/asymmetric FOV allows PD and T2 images to be obtained in a shorter scan time.

Artefact problems

The main source of artifact in this area is from respiratory movement and flow in the aorta and the IVC. RC or respiratory triggering is often required and significantly reduces respiratory ghosting.

Alternatively, breath-hold techniques may be utilized. Spatial presaturation pulses placed S and I to the FOV are necessary to reduce flow motion artifact arising from the aorta and IVC.

As the kidneys and adrenals are retroperitoneal structures, a spatial presaturation band brought into the FOV and placed over the anterior abdominal wall reduces respiratory artifact significantly without obscuring important anatomy.

GMN also minimizes flow and, in some cases, respiratory motion but it increases the signal in vessels and the minimum TE.

Chemical shift artifact is often troublesome in the kidneys, especially at higher field strengths. This is due to retroperitoneal fat being adjacent to fluid-filled kidneys.

Narrowing the receive bandwidth increases this artifact but, if used in conjunction with fat suppression techniques, results in a significant improvement in SNR and a reduction in chemical shift.

However, this strategy increases the minimum TE and is therefore reserved for T2 weighted sequences.

Bowel motion is also troublesome but is effectively reduced by the administration of antispasmodic agents given IV, IM or subcutaneously prior to the examination.

Patient considerations

Careful explanation of the procedure is important. Ensure that the patient is as comfortable as possible. Some antispasmodic agents given IM may cause nausea but fruit juice given after the study can alleviate this. Due to excessively loud gradient noise associated with some sequences, ear plugs must always be provided to prevent hearing impairment.

Contrast usage

Contrast is sometimes used in conjunction with dynamic imaging to visualize the uptake of contrast in the kidneys (see Dynamic imaging under Pulse sequences in Part 1).

Vascular imaging of the renal arteries is a common technique discussed later (see Vascular imaging later in this chapter). Contrast may also be necessary to increase the conspicuity of the adrenal glands.

Recently functional imaging of the kidneys after the administration of macromolecular contrast agents have been advocated in the evaluation of a variety of renal diseases.

These agents are almost totally excreted by the kidneys, thereby improving the conspicuity of lesions that have different perfusion characteristics.
 

Pancreas

Common indications

Why the physician asks for MRI on Pancreas. if he is looking for Pancreatic Tumors.or Pancreatic duct obstruction.

Equipment

In this examination, we will need Body coil or multi-phased array or multi-array coil.An RC bellows is also needed.and finally,  Ear plugs is requested in this examination.

Patient positioning

The patient lies supine on the examination couch with the RC bellows securely attached.

The patient is positioned so that the longitudinal alignment light lies in the midline, and the horizontal alignment light passes through the level of the third lumbar vertebra, or the lower costal margin (23).

Suggested protocol

Coronal breath-hold fast incoherent (spoiled) GRE/SE T1 Acts as a localizer if three-plane localization is unavailable, or as a diagnostic sequence.

Thick slices/gap are prescribed on either side of the vertical alignment light, from the posterior abdominal muscles to the anterior abdominal wall. The area from the pubic symphysis to the diaphragm is included in the image.

Figure (15): Coronal FSE T1 weighted image through the abdomen demonstrating slice prescription boundaries and orientation for axial imaging of the pancreas (24).

Figure (16): Axial high-resolution FSE T2 of the pancreas (25).

Figure (17): Axial SS-FSE T2 of the pancreas during free breathing(26).

 

Figure (18): Axial fast incoherent (spoiled) T1 weighted image of the pancreas(27).

Diffusion imaging

Diffusion imaging used in conjunction with parallel imaging techniques may be useful to detect pancreatic adenocarcinoma and for differentiation from benign and cystic lesions.

Image optimization: Technical issues

The inherent SNR and CNR of the abdominal contents are usually excellent due to their high proton density, and the use of a torso array coil increases this even further. In addition, parallel imaging techniques using multi-array coils reduce scan times significantly.

Spatial resolution is also important, especially when imaging relatively small structures such as the pancreas that require thin slices/gap. However, good resolution is often difficult to achieve when using the body coil and a large FOV, and in the presence of respiratory and flow artifact.

A torso phased array coil greatly improves the SNR that can then be traded for resolution. In addition, parallel imaging techniques can be used to improve resolution whilst keeping scan times short.

SE sequences usually produce the best contrast in this region, but result in fairly lengthy scan times and therefore FSE is usually used.

Artefact problems

The main source of artifact in this region is from respiratory and flow motion in the aorta and IVC. RC or other respiratory gating techniques are often required and significantly reduce respiratory ghosting.

Alternatively, breath-hold techniques may be utilized. Spatial presaturation pulses placed S and I to the FOV are necessary to reduce flow motion artifact in the aorta and IVC.

GMN also minimizes flow motion but, as it increases the signal in vessels and the minimum TE, it is not usually
beneficial in T1 weighted sequences.

Additional shimming may be required before chemical/spectral presaturation sequences.

Gastric and bowel motion is also troublesome in this area due to the proximity of the stomach and the duodenum to the pancreas.

This artifact is effectively reduced by the administration of antispasmodic agents given IV, IM or subcutaneously prior to the examination.

Patient considerations

The careful explanation is essential if breath-holding sequences are to be performed. Some antispasmodics given IM may cause nausea, which can be remedied by giving the patient fruit juice after the scan. Due to excessively loud gradient noise associated with some sequences, ear plugs must always be provided to prevent hearing impairment.

Contrast usage

Contrast is often necessary for conjunction with dynamic imaging to visualize small pancreatic lesions.

Positive and negative oral contrast agents to delineate bowel, and therefore the pancreas can be useful.

Recently studies have been performed using secretin as an enhancement agent.

This stimulates the release of fluid into the pancreatic duct, thereby improving visualization on T2 weighted images. There may also be a role for secretin in the evaluation of the pancreatic function.

TESTES

Patient Preparation

Firstly The patient has to go to the toilet before the study. Then you should explain the procedure to the patient and then Offer him/her ear protectors or ear plug.

Ask the patient to undress except for underwear and  Ask him/her to remove anything containing metal (hearing aids, hairpins, body jewelry, etc.). finally, you must have an intravenous line placed

Positioning

The Patient is Supine. The  Body array coil (wraparound coil, surface coil, e.g., circular surface
coil) and we don’t forget to cushion the legs.

Sequences

The Scout: coronal, sagittal, and axial.

Sequence 1 coronal T2-weighted.

Slice thickness of 4 mm is determined with slice gap of 0–20 % of slice thickness (0–0.8 mm or factor 1.0–1.2). The  FOV should be small ( e.g., 200 mm) with  Matrix about  512 ( but then with NSA 3–4). we also should do Phase oversampling. Finally, saturation slab is axial superior to the slices for saturation of the vessels

Sequence 2 coronal T1-weighted

Slice thickness of 4 mm is determined with slice gap of 0–20 % of slice thickness (! 0–0.8 mm or factor 1.0–1.2). The FOV should be small (e.g., 200 mm). Phase oversampling should be done. finally, saturation slab is axial superior to the slices for saturation of the vessels.

Figure (19): Testes, coronal, sequences 1 and 2(28)

Figure (20): Testes, axial, sequence 4(29)

Sequence 3 coronal

As sequence 2 but after administration of contrast agent (Gd-DTPA)

Sequence 4 axial

T1-weighted after administration of contrast agent.

slice thickness of 4 mm is determined with slice gap about 20 % of slice thickness ( 0.8 mm or factor 1.2).

The FOV should be small (e.g., 200 mm). Saturation slab is axial superior to the slices for saturation of the vessels (30).
Tips & Tricks

Positioning aid: leave tight-fitting underpants in place (immobilizes the testes).

If necessary, cushion the testes. If a surface coil is used, perhaps place a thin foam pad between testes and coil to prevent too strong a signal in the vicinity of the coil.

BREAST

Patient Preparation

Firstly The patient has to go to the toilet before the study. Then you should explain the procedure to the patient and then Offer him/her ear protectors or ear plug.

Ask the patient to undress completely above the waist and  Ask him/her to remove anything containing metal (hearing aids, hairpins, body jewelry, etc.). finally, you must have an intravenous line placed.

Positioning

The Patient is Prone. We will use the Breast coil. Arms will be alongside the body or in front of the head, forehead resting on the hands(31).

Sequences

Scout: axial, coronal, and parasagittal

Sequence 1 axial T2-weighted

Slice thickness of 4 mm with slice gap: 0–20 % ( 0–0.8 mm or factor 1.0–1.2). Phase encoding gradient should be LR (because of cardiac motion). No Saturation slab.

Figure (21): Breast, axial, sequence 1(32).

Sequence 2 axial T2-weighted 3-D GRE

With Flip angle 25° 1.0 T , TR = 8.5–12 and TE = 5.3–6.1. With Flip angle 20–25° 0.5 T , TR = 7.7–10
and TE = 2.5–3. with Flip angle 25, TR = 24 and TE = 13. with Flip angle 50° and  Slab thickness: 128 mm, Number of partitions: 32. (Effective) slice thickness:  4 mm. The FOV: 30–35 mm. Phase encoding gradient: LR. No Saturation slab.

Sequences 3–8 T1-weighted axial

as sequence 2 but after administration of contrast agent (Gd-DTPA 0.1 mmol/kg body weight), no delay between the sequences (sequence duration 50–90 seconds), and possibly coronal images.

Sequence 9 coronal T1-weighted 3-D GRE

With  Flip angle 20–25°, Slab thickness: 128 mm. A number of partitions: 32. (Effective) slice thickness: 4 mm. The FOV: 30–35 mm. Phase encoding gradient: cephalocaudal. No Saturation slab.

Figure (22): Breast, coronal, sequence 9(33).

Postprocessing

Subtract sequence 2 from, e.g., sequence 4. Dynamic assessment of pathologic contrast enhancements.

Tips & Tricks

For a small breast, cushion the inside of the coil with some padding (reduces motion artifacts)
A tight-fitting T-shirt can also be quite effective in immobilizing the breast

SOFT TISSUES OF THE NECK

Patient Preparation

Firstly The patient has to go to the toilet before the study. Then you should explain the procedure to the patient and then Offer him/her ear protectors or ear plug.

Ask the patient to undress completely above the waist except for underwear and  Ask him/her to remove anything containing metal (hearing aids, hairpins, body jewelry, etc.).

finally necessary, have an intravenous line placed (e.g., if the investigation is for possible tumor)

— Note: Before starting the study ask the patient to swallow mostly during the pauses and to try not to swallow at all during acquisition (i.e., when the scanner is loud) (34).

Positioning

The patient is Supine. We would use the Neck coil, and we shouldn’t forget Cushion of the legs

Sequences

Scout: sagittal and axial (three planes are best)

Figure (23): Soft tissues of the neck, coronal, sequence 1(35).
  

Sequence 1 coronal:

T2-weighted fat-saturated (plot on sagittal slice: more central or more dorsal depending on the purpose of the investigation; outline FOV on axial slice)

Slice thickness: 6 mm with Slice gap: 20 % of it ( 1.2 mm or factor 1.2). FOV:  250 mm

— Saturation slab: axial below the slices for saturation of the blood vessels (if necessary with flow compensation)

Sequence 2 axial (from jugular fossa to base of skull) T2-weighted

Slice thickness: 6 mm with slice gap: 20 % of it ( 1.2 mm or factor 1.2). FOV: approx. 180–200 mm. Saturation slab: axial (parallel) below the slices for saturation of the vessels (if necessary with flow compensation)

Sequence 3 sagittal T2-weighted

Slice thickness: 6 mm with slice gap: 20 % of it (1.2 mm or factor 1.2).saturation slab: axial below the slices for saturation of the blood vessels (if necessary with flow compensation)

Figure (24): Soft tissues of the neck, sagittal, sequence 3(36).
 

Sequence 4 axial T1-weighted

Example:  TR = 450–600     TE = 12–25 otherwise as sequence 2 If contrast agent is administered:

Sequence 5 axial T1-weighted

(as sequence 4 but after administration of contrast agent, e.g., Gd-DTPA)

Sequence 6 coronal

T1-weighted (after administration of contrast agent)
Example:  TR = 450–600   TE = 12–25 otherwise as sequence 1

Tips & Tricks

Positioning aid: center on the upper border of the larynx. For really obese patients use either large flexible wraparound coil or spinal array coil (select upper section)

Conclusion

MRI Invention was considered a revolution in the science of radiological diagnosis, especially in soft tissue pathology. Anatomical positions are variable and all depend on requested examination and condition of the patient. patient preparations in every examination are very important and just very vital in patient survival and keep the Technology and machines well. sequences of every examination are important to cover up all different pathologies from a different view.

.

References:

  1. The article How MRI works? Available at site http://www.teslasociety.com/mri.htm
  2. The Genius Who Lit the World (Nikola Tesla) available at http://www.teslasociety.com/index.html
  3. Figure 20. The interpolated image of the Minkoff scan and the first ever MRI scan of a live human being (4:45 AM July 3, 1977). Major Diagnostic Breakthrough in Multiple Sclerosis Achieved With Advanced UPRIGHT® MRI available at http://www.fonar.com/news/100511.htm.
  4. A quick history of the MRI, available at http://two-views.com/mri-imaging/history.html#sthash.kqJYI7wV.dpbs

5.  Figure 7.1 patient positioning available at M. Elmaoğlu and A. Çelik, MRI Handbook: MR Physics, Patient                     Positioning, and Protocols(1st e); 106.

 

  1. The patient positioning of Head Imaging, MRI CNS available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 106.
  2. Table 7.1 Routine brain protocols and prescription plans, CENTRAL NERVOUS SYSTEM: MRI PROTOCOLS, IMAGING PARAMETERS available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 107.

8.. Patient preparation and positioning, Tempo-mandibular joint available at Elmaoglu – MRI Handbook – MR                   Physics, Patient Positioning, and Protocols (1st e); 182.

  1. Figure (8.2 & 8.3) patient positioning in a TMJ dedicated loop coils, TMJ MRI available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 183.
  2. Patient Positioning, Elbow Imaging available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 191.
  3. Figure (8.12 & 8.13) patient positioning for the elbow, available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 192.
  4. Patient Positioning, Knee Imaging available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 209.
  5. Figure (8.32) feet first and supine patient positioning in a multi-channel transmit/receive knee coil, available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 210.
  6. Figure (8.32) feet first and supine patient positioning in a multi-channel transmit/receive knee coil, available at Elmaoglu – MRI Handbook – MR Physics, Patient Positioning, and Protocols (1st e); 210.

15.Patient positioning, MRI of Liver and biliary system . available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;219.

  1. Figure (11.2) Coronal SE T1 weighted image through the abdomen demonstrating slice
    prescription boundaries and orientation for axial imaging of the liver, MRI of Liver and biliary system . available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;220.

17. Figure (11.3) Axial FSE T1 weighted image through the liver, MRI of Liver and biliary system . available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;221.

 

18. Figure (11.4) Axial incoherent (spoiled) T1 weighted breath-hold image of the liver, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;221.

 

19. Figure (11.7) Coronal SS-FSE image of the gallbladder (MRCP). Very long values of TR and TE were used to acquire images in which only fluid is seen, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;223.

 

20.Patient Positioning, MRI of Kidneys and adrenal glands available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;227.

 

21. Figure 11.9 Coronal incoherent (spoiled) GRE T1 weighted image through the abdomen demonstrating the kidney, MRI Kidney available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;227.

 

22. Figure 11.10 Coronal incoherent (spoiled) GRE T1 weighted through the abdomen, MRI Kidney available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;228.

 

23. Patient positioning, MRI of the pancreas available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;233.

 

24. Figure 11.16 Coronal FSE T1 weighted image through the abdomen.MRI of the Pancreas, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;234.

 

25. Figure 11.17 Axial high-resolution FSE T2 of the pancreas, MRI of the Pancreas, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;235.

 

26. Figure 11.18 Axial SS-FSE T2 of the pancreas during free breathing, MRI of the Pancreas, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;235.

 

27. Figure 11.19 Axial fast incoherent (spoiled) T1 weighted image of the pancreas, MRI of the Pancreas, available at Handbook of MRI Technique (3rd e) by Catherine Westbrook;236.

 

28. Testes, coronal, sequences 1 and 2, MRI of Tests available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 73.

 

29. Testes, axial, sequence 4, MRI of Tests available at: MRI Parameters and Positioning  by Torsten B. Moeller, M.D (1st e); 73.

 

30. Patient Positioning, MRI of Testis, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 72.

 

31. Patient Positioning, MRI of the breast, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 30.

 

32. Breast, axial, sequence 1, MRI of the breast, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 31.

 

33. Breast, coronal, sequence 9, MRI of the breast, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 32.

34. Patient preparations, MRI of the soft tissue of the neck, available at MRI Parameters and Positioning by               Torsten  B. Moeller, M.D (1st e); 17.

  1. Soft tissues of the neck, coronal, sequence 1, MRI of the soft tissue of the neck, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 18.
  2. Soft tissues of the neck, sagittal, sequence 3, MRI of the soft tissue of the neck, available at MRI Parameters and Positioning by Torsten B. Moeller, M.D (1st e); 19.

 

MR FAT SATURATION TECHNIQUES




 Fat suppression is a key consideration in MRI imaging. The following techniques result in fat saturation

  • Short Tau Inversion recovery
  • Spectral pre saturation with inversion recovery
  • Chemical shift selective inversion recovery
  • Dixon method

1) Spectral presaturation with inversion recovery Technique

Spectral pre saturation with inversion recovery technique or SPIR technique is a type of crossbred fat suppression technique(1).

That could acquire an image without fat in it (fat suppression), we can do it by selective protons saturation.

The SPIR technique is useful technique it makes significant attenuation of the fat proton sensitivity while preserving the water proton sensitivity (2).

It uses T1 sequence relaxation and fat frequency excitation then applies of 180-degree radio-frequency inversion pulse to induce a flip angle along the longitudinal axis(3).

This inversion should be followed with a 90-degree excitation pulse which nulls signals of the processing fat molecules as they will not exhibit transverse magnetization hence improving differentiation of pathology.

That could work with RF-pulse fat-selective and also with spoiler gradient, they it ‘s together with residual longitudinal fat magnetization nulling through the mechanism of inversion delay.

the frequency of excitation pulse should be higher than RF pulse (4). This can be helpful only in the fat contenting examed region, as signals coming out of other tissues and water remain unaffected.

2) Chemical shift selective inversion recovery

Chemical shift is the most commonly used technique for fat suppression.

In this technique, we use a selective RF pulse at 90 degrees of narrow bandwidth target processing fat molecules,

so we can process them in the transverse plane (5).

After that immediately gradient spoiler is applied causing them to dephase. Over time imaging sequence begins (d) and no fat signal is seen and the water peak is imaged only (6).

The main disadvantage of this technique is that it should be used at a frequency higher than 1 Tesla and are not working at frequencies less than 0.3 Tesla (7) .

and it should acquire a homogeneous field to work, so we cannot get clear images or findings from an inhomogeneous field like field have a metallic device or any area of abnormal anatomy variants (7).

The main advantage of this technique is that it ‘s very helpful in merging with any pulsed sequences of MRI.

FIGURE (a) to (d) show the different Frequencies of fat suppression selective technique.(8)

Factors that affect chemical shift:

There are two parameters that affect the size of the chemical shift, So in the following sentences, we will discuss the impact of changing these parameters.

Chemical shift artifact is that artifact noticed between fat and water protons due to the difference in the magnetic shields.

it is a powerful artifact which we can use to rule out the existence of lesions consisting of fat  (12).

The artifact of Chemical shift makes dark edge between water and fat interference  (13).

There are several parameters that affect the size of chemical shift artifact including; Magnetic field of the used magnet, gradient strength, and the used bandwidth (14).

the first and most important and crucial parameter is magnetic field strength. In McRobbie et al (2007) textbook, doubling the strength of the magnetic field will increases the chemical shift induced (14).

Moreover, decreasing the gradient strength will increase the chemical shift. Finally, the third parameter that would affect the chemical shift is the bandwidth of the used radio-frequency pulse.

Narrower the bandwidth gives a higher chemical shift while increasing the bandwidth will decrease the chemical shift artifact.

 

 

3) Dixon method:

 

The sequence was named after the man who described it, WT Dixon in 1984(9). It ‘s based on imaging of in and out-of-phase.

we acquire 2 images sets,  the first in-phase TE and the other at out-of-phase TE.

These 2 sets, when added to each other, give ‘water-only’ image while if they subtracted from each other gives ‘fat-only’ image (Fig. below).

So this method is very helpful  in high magnetic susceptibility areas, but it may need magnetic field good homogeneity.(10)

four images of Dixon method, the (A) represent in-phase,(B) represent out-of-phase, (C) represent fat-only, and (D) represent water-only images. (11)

Dixon method has special favors and characters rather than other fat suppression techniques, as its fat signal suppression is less affected by artifacts and more uniform.

also, we can use it with other types of sequences and we can merge ir and use it with different MR sequences, for example, T1 . from a single acquisition, it can give images with or without fat suppression.