Role of HRCT In Interstitial Lung Diseases Diagnosis is a very important topic to discuss .
The interstitial lung diseases (ILDs) are a heterogeneous group of lung disorders that result from damage to
the lung by various forms of inflammation and fibrosis.
By definition, Interstitial Lung Diseases (ILDs) involve the lung interstitium that forms a fibrous skeleton for the lungs, However many of the conditions that traditionally included under the heading of ILDs actually associate with extensive alterations of the alveolar and airway architecture.
For this reason, the terms diffuse infiltrative lung disease or diffuse parenchymal lung disease are preferable. Still, the term ILDs remains in common clinical usage.
Interstitial Lung Diseases represent more than 200 different entities, and various and often-confusing classification systems that we can use .
One useful approach to classification is to separate the ILDs into diseases of known and unknown etiology.
ILD of unknown etiology (65% of all ILDs) , we can subdivide it into the group of idiopathic interstitial pneumonias (IIPs), and a group comprising several rare
but interesting diseases with distinctive clinicopathologic features, such as lymphangioleiomyomatosis, Langerhans cell histiocytosis, pulmonary alveolar proteinosis, and pulmonary alveolar microlithiasis.
Sarcoidosis has an exceptional position within the group of ILDs of unknown cause, as it is relatively common and can present as a systemic disease(1).
CT scanning is the most important noninvasive diagnostic key to the identification and characterization of ILD and aids the radiologist and the clinician in the management of patients who carry this disorder.
Among all noninvasive methods, it provides the highest sensitivity and specificity in the detection of ILD.Also, it has a higher accuracy in comparison to the clinical assessment, lung function tests, and chest radiography in diagnosing a specific disorder, and adds diagnostic accuracy and confidence when added to
the clinical assessment and the chest radiogram. Finally,
CT helps to identify the best location for lung biopsy , and provides an important basis for the follow-up of ILD patients.
Epidemiology
United States:
As a group, diffuse interstitial diseases of the lung are uncommon. Based on the Bernalillo County, NM, USA registry data published in 1994, the overall estimated incidence is approximately 30 cases per 100,000 persons per year.
Rates of interstitial lung disease are somewhat higher in men than in women, Age and occupational exposures affect the epidemiology . of patients referred to a pulmonary disease specialist, an estimated 10-15% have a ILD.
International:
Although little-published data exist comparing worldwide prevalence, significant differences are apparent.
The Bernalillo County study estimated a prevalence of 80.9 cases per 100,000 population in men and 67.2 cases per 100,000 population in women. In comparison, a Japanese study estimated a prevalence of 4.1 cases per 100,000 population; a study in the Czech Republic reported 7-12 cases per 100,000 population; and
data from a Finnish registry indicated 16-18 cases per 100,000 population.
Mortality/Morbidity:
The natural history of diffuse interstitial lung diseases varies among different diagnostic entities and among individuals with the same diagnosis. Note the following:
Some diseases are insidious in onset and gradual but unrelenting in progression (eg, similar to IPF), while other diseases are acute in onset but responsive to therapy (eg, COP).
Diseases that most closely approximate IPF have a similar mortality rate of approximately 50% at 5 years.
In the United States, Chronic obstructive pulmonary disease (COPD) estimate 10-15% of mortality attributed to all types of DPLDs.
Race:
Diffuse interstitial diseases of the lung sometimes show racial predilections. Examples include sarcoidosis, which is more common in those of African ancestry in the United States. In contrast, PLCH, also known as histiocytosis X, primarily affects Caucasians.
Sex:
Several diffuse interstitial diseases of the lung show sexual predilections. IPF affects men more than women (at a ratio of 1.5:1), while LAM and pulmonary tuberous sclerosis exclusively affect women.
The Bernalillo County study estimated an incidence of 31.5 cases per 100,000/year in men and 26.1 cases per 100,000/year in women.
Women are much more likely to develop rheumatologic/connective-tissue disease than men and thus are more likely to experience pulmonary manifestations of those diseases. However, when affected, men with certain rheumatologic diseases (eg, rheumatoid arthritis) are more likely to develop pulmonary manifestations than women.
The pneumoconioses (eg, silicosis) are much more common in men than in women, probably because of higher rates of occupational exposure.
Age:
Many of the diffuse parenchymal lung disease develop over many years and therefore are more prevalent in older adults. For example, most patients with IPF present in the sixth or greater decade of life.
Others forms of interstitial lung disease, such as sarcoidosis, LAM, connective-tissue disease–associated lung disease, and inherited forms of lung disease primarily present in younger adults.(2)
History and Etymology
Computed Tomography (CT) imaging is also known as “CAT scanning” (Computed Axial Tomography).
Tomography is from the Greek word “tomos” meaning “slice” or “section” and “graphia” meaning “describing”.
British engineer Godfrey Hounsfield Invent CT at EMI Laboratories, England with the help of South Africa-born physicist Allan Cormack of Tufts University, Massachusetts.
Hounsfield and Cormack were later awarded the Nobel Peace Prize for their contributions to medicine and science.
”High resolution computed tomography” term was first used by Todo et al in 1982. He also described the usefulness of HRCT imaging in pulmonary diseases .
In 1985, Naidich et al., Nakata et al. and Zerhouni et al. described the fundamental technique of HRCT and published first report on
HRCT (3).
However, The concept of the secondary lobule and its importance in interpreting various conditions dates back to long before the advent of HRCT, a technique which merely facilitated its identification even under normal conditions.
The radiological identification of the secondary lobule came about thanks to ER Heitzman as early as 1969, who developed our understanding of the crucial role of this anatomical structure in his work “The Lung” published in 1973 – about fifteen years before the development of HRCT! (4)
Radiological Anatomy
The lungs:
The right lung has three lobes and the left has two lobes, with the lingula of the left upper lobe corresponding to the right middle lobe.
One terminal bronchiole with lung tissue forms an acinus which, together with vessels, lymphatics and nerves, forms the primary lobule, Three to
five primary lobules form a secondary lobule.
Interlobar Fissures:
The depth of fissures varies from superficial slit to complete separation of lobes. The oblique (major) fissure:
This is similar in both right and left lungs.
It extends from T4/T5 posteriorly to the diaphragm Anteroinferiorly .
The left major fissure is more vertically orientated than the right .
The fissures do not follow a straight plane from top to bottom but are undulating in their course.
The medial aspect of each fissure passes through the hilum The lateral aspect of each fissure is anterior to the medial aspect at the level of the hila and below Above the hila, the relationship changes and the lateral aspect of the fissure is more posterior than the medial aspect.
The transverse (minor) fissure:
This separates the upper and middle lobes of the right lung, It runs horizontally from the hilum to the anterior and lateral surfaces of the right lung at the level of the
fourth costal cartilage.
Its posterior limit is the right oblique fissure, which it meets at the level of the sixth rib in the midaxillary line .It is anatomically complete in only one-third of subjects and is absent in 10%.(5)
The anatomical organization of the lungs consists of the bronchovascular bundles and the secondary lobules(6).
The bronchovascular bundles are made up of the main bronchi, the pulmonary vessels and the interstitial framework around them (central interstitium).(7)
The secondary lobules are the peripheral units of parenchyma where the airways meet the capillaries within the interstitial framework supporting them (peripheral interstitium)
Secondary lobule
Knowledge of anatomy of secondary Lobule is essential for understanding HRCT.
The secondary lobule is the basic anatomic unit of pulmonary structure and function.as Interpretation of interstitial lung diseases is
based on the type of involvement of the secondary lobule.
It is the smallest lung unit that is surrounded by connective tissue septa.It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.
The secondary lobule is supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery.
Pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.
Under normal conditions only a few of these very thin septa will be seen.
There are two lymphatic systems: a central network, that runs along the bronchovascular bundle towards the centre of the lobule and a peripheral network, that is located within the interlobular septa and along the pleural linings.
Centrilobular area is the central part of the secondary lobule. It is usually the site of diseases, that enter the lung through the airways (i.e. hypersensitivity pneumonitis, respiratory bronchiolitis, centrilobular emphysema).
Perilymphatic area is the peripheral part of the secondary lobule.It is usually the site of diseases, that are located in the lymphatics of in the interlobular septa (i.e. sarcoid, lymphangitic carcinomatosis, pulmonary edema).
These diseases are also usually located in the central network of lymphatics that surround the bronchovascular bundle (8).
CT, How It Works:
The CT Scanner generations:
First and second generation:
Known as the rotate-translate type, it is a single X-ray source with either one (first) or a bank of up to 30 (second) detectors. Both move around the patient in 1° increments. Data was collected through a 180° rotation.
Third generation:
The rotate-rotate type are found in most modern scanners. A large number of small detectors arranged in an arc covering a complete patient cross section allows continous data collection through a full 360° rotation. This permits scan times of less than 0.4s.
Fourth generation:
Detectors are arranged in a stationary ring around the patient and the tube rotates. The outer part of the fan beam is always outside the patient and can be used to measure unattenuated radiation and self-calibrate. It requires vastly increased number of detectors which is prohibitively expensive.
Fifth generation:
This is also known as an electron beam scanner. It has electrons focused on and swept round a high voltage target ring to produce X-rays. It has no mechanical parts, thus, rapid scan times are possible (9).
Configuration of a typical CT scanner
The rotational axis is the Z-axis and X-ray beam is collimated as a wide fan shape big enough to cover the patient. In cross section, it has a narrow width parallel to the Z-axis. For a single slice scanner, this width defines the slice thickness.
Behind the patient is an arc of 500–1000 detectors. The radius of the arc is equal to the focal distance, thus each detector is the same distance from the source.
The patient lies on a couch that can be moved longitudinally through the gantry aperture. The gantry is normally perpendicular to the couch but can be tilted up to 30°. It’s mainly used for head scanning so the scan plane can be made parallel to the skull base. Simple third generation scanner.
The CT X-ray tube
Tubes for CT scanners have to be capable of producing prolonged exposure times at high mA.
They usually have two focal spot sizes, the smallest being ~ 0.6mm. It typically operates at 120 kV but the range is between 80 – 140kV. It also has heat capacities in excess of 4MJ.
Filtration
CT algorithms rely on the X-ray beam being monoenergetic, which, of course, it isn’t. To approximate a monoenergetic beam more closely, the X-ray beam is deliberately hardened by adding filtration. This is usually 0.5mm of copper (equivalent to 8mm of aluminium). It produces a mean energy of ~ 70 keV from
120 kV.
Since the patient cross-section is commonly elliptical, X-rays at the periphery tend to pass through less tissue. To compensate for this difference in attenuation across the field of view, some CT scanners have a bow tie filter after the X-ray tube.
This is a filter that is thin in the center and thick at the edges to artificially harden the beam at the edge.
Collimation
A collimator is mounted on the X-ray tube with the beam collimated to a fixed width (usually 50cm).
In the case of single slice scanners, collimation also defines the slice thickness (0.5 – 20mm).
Single slice scanners also have a post-patient collimator that is mounted in front of the detectors and used to reduce scatter reaching the detectors when the slice thickness is less than the detector width.
It is not needed for multi-slice scanners as the full width of the detectors in each row is used to form theimage and shielding from scatter would also shield from direct radiation.
Detector requirements
The detector must be small to allow good spatial resolution (single slice scanners have 600– 900 in an arc).
It should have high detection efficiency for X-rays in the CT energy range. Also, it should work within a fast response time (with negligible afterglow).
Wide dynamic range is very important as wide range of X-ray intensities (e.g. from zero attenuation when the beam passes to the side of the patient to very high attenuation in a lateral projection of a heavy patient).It should be have a stable and noise-free response.
Types of detectors
Older single slice scanners used ionisation chambers that were filled with high atomic number gases (xenon or krypton) at high pressure (20 atm).
Incident X-rays ionise the gas and produce a charge at the collection electrode. As it’s only 60% efficient, it’s not suitable for use in multislice scanners.
All new scanners use solid state detectors which consist of a scintillant (e.g. bismuth germinate) and embedded photodiode to detect output. It has a very high detection efficiency (98%) (10).
CT Chest Technique
High-resolution CT is a sampling examination of the lung, in which thin sections are taken at staggered intervals, revealing both the pattern and the distribution of abnormality, so that a differential diagnosis—or sometimes a single diagnosis—can be rendered. (11)
For patients with Iinterstitial Lung Disease , the identification of the smallest possible structures of the lung parenchyma and the depiction of their abnormalities is of paramount importance for any imaging approach.
Therefore, CT protocols have to utilize thin collimation and high-spatial-frequency reconstruction algorithms to achieve an optimal spatial resolution and consequently, facilitate an optimal assessment of interstitial and airspace disease.
For decades, patients with ILD have traditionally been investigated with HRCT (Mayo et al. 1987). This technique consists of a “step-and-shoot” approach, in which 0.5- to 1-mm collimation scans are obtained at 10- to 20-mm intervals, a small FOV, and a high radiation dose per section.
It provides excellent image quality, free of partial volume and projection artifacts, and combines high sensitivity in the detection of ILD with high accuracy in establishing the correct diagnosis.
This “classic” HRCT technique still plays a decisive role in the noninvasive investigation of patients with pulmonary disease of a diffuse distribution pattern (Hansell 2001).
With the advent of MDCT, volumetric high-resolution imaging has enriched the diagnostic armamentarium of the radiologist. New-generation MDCT scanners allow fast single-breath-hold scanning, volumetric data acquisition with thinly collimated scans, and high-spatial-frequency reconstruction when scanning the entire lung.
They, thus, combine the advantages of “traditional” HRCT and modern spiral scanning techniques.
Volumetric protocols enable the radiologist to detect those abnormalities that might have been missed during the classic HRCT step-and-shoot approach. Moreover, volumetric isotropic data sets permit the reconstruction of high-quality multiplanar images, which help to appreciate better the distribution of disease, for example, to identify a cephalocaudal gradient of disease severity in certain disorders.
Finally, continuous data acquisition allows the generation of MIP images, which are helpful in the detection of micronodular disease and centrilobular abnormalities.
There are also some trade-offs with volumetric HRCT scanning. The radiation dose is 5 to 10 times higher, and the image quality is discretely lower in comparison to classic sequential HRCT.
This image quality reduction is most apparent in the depiction of small septa and of ground-glass opacities, and its clinical significance has yet to be determined. In order to achieve the best possible balance between diagnostic accuracy, exploitation of the advantages of volumetric CT and radiation dose,
the following options exist:
1. Sequential HRCT protocol:
This protocol utilizes high milliampere-second and kilovolt peak values to obtain the best possible image quality.
Thin collimation (1 mm) scans are obtained at 10- or 20- mm intervals. Therefore, the overall radiation dose is a 5th to a 10th in comparison to the standard or high-resolution volumetric protocols.
This protocol may be regarded as “imaging biopsy” in diffuse lung disease, as it detects disease, allows the specification of disease distribution, and helps in establishing a differential diagnosis with high accuracy and confidence levels. It is the protocol of choice in patients with proven ILD, and in those cases that require imaging follow-up during or after therapy.
Because the classic HRCT leaves 9- to 19-mm broad gaps between the scanned sections unexamined, it should not be utilized as sole protocol in patients with suspicion of focal lung disease, in diffuse interstitial disorders where there is an increased risk associated with focal or even malignant abnormalities (such as dermatomyositis/ polymyositis), or in entities with a distinct propensity to involve extrapulmonary sites in the mediastinum, the chest wall, the diaphragm, and the abdomen.
In such instances, a combination .with a standard volumetric protocol (Table 26.1) is highly recommended.
Another caveat is the assessment of patients with suspected air trapping at supine scans.In these cases, it is advisable to perform single slice step-and-shoot scans in prone positions instead of a continuous volumetric examination in order to reduce radiation burden.
2. Volumetric HRCT protocol:
This protocol combines thin collimation with volumetric scanning and high milliampere-second and kilovolt peak values. During scanning, the dose modulation is off.
The result is a high quality contiguous data set, which allows for high-resolution multiplanar and three-dimensional reconstructions with superb image quality.
The latter is similar to that of sequential HRCT scanning, although it does not match it in every detail.
The major disadvantage of this protocol is the radiation dose, which is approximately 10 times higher than that of conventional HRCT.
It is recommended to use this protocol in ILD patients only when high-quality three-dimensional reconstructions are necessary, for example, for generating a data set for CT bronchoscopy.
3. Volumetric standard CT protocol:
Here, the milliampere-second and kilovolt peak values are reduced in comparison to the sequential or volumetric high-resolution protocol, and dose modulation is switched on.
The result is a substantial reduction in dose in comparison to the volumetric high-resolution protocol.
Nevertheless, thin collimation and high-spatial-resolution reconstruction guarantee very good image quality, and volumetric data acquisition a continuous morphologic assessment of the lung investigated, respectively.
This protocol is best used in combination with the classic sequential HRCT protocol in ILD patients.
It provides a volumetric data set and the best high-resolution images, with a reasonable radiation dose that reaches roughly 50% of the dose resulting from the volumetric high resolution protocol.
It is advisable to utilize this combination protocol in all patients with ILD who are imaged for their first time, in cases where the chest radiogram indicates diffuse and focal disease, and in those who are at risk to develop focal disorders on top of a diffuse lung disease process.
4. Volumetric low-dose CT protocols:
In patients with ILD, the low-dose high-resolution CT technique with a reduction of the milliampere-second values to approximately 40 mAs is in our view a valuable alternative to the standard volumetric protocol when combined with the sequential HRCT technique.
It allows for the assessment of the pulmonary parenchyma in slim individuals, visualization of focal abnormalities in the lung parenchyma, and analysis of major airways disorders.
The combination with the classic HRCT approach fosters almost the same advantages as those described for the combination of the standard volumetric protocol with classic HRCT. When using this protocol, one has to keep in mind that the somewhat reduced image quality may limit the diagnostic accuracy when scanning
the parenchyma, the mediastinum, chest wall, and upper abdomen in obese patients (12).
Analysis of Different Pattern of Lung Pathologies On HRCT
I) RETICULAR PATTERN:
The main finding consists of thin interlacing linear opacities creating a more-or-less tight mesh.
This finding is produced by a thickening of the structures of the lobular interstitium, and often of the central interstitium as well.
It’s caused by thickened interlobular or intralobular septa or honeycomb (fibrotic) destruction.
It always represents significant interstitial lung disease (ILD). It may be due to a variety of causes (fluid accumulation, amyloid deposits, cellular infiltration, fibrosis) and the pattern may vary accordingly.
The distribution of the lesions and other associated signs are often useful for diagnosis (13) .
A) Smooth:
Centrally, the alteration is characterized by a uniform thickening of the bronchial walls and an increase in the diameter of the adjacent vessels . Peripherally, the thickening of the interstitium appears as an exaggeration of the interstitial borders and by a fine reticulation crossing them (14).
In the centrilobular region the arteriole is more prominent and the bronchiolar walls, normally not evident in CT, are visible.
Interstitial thickening may be caused by edema, organic substances or cellular infiltration.
The lobular architecture is preserved, only is more recognizable than in the normal lung, at times with an exaggerated appearance
From pattern to disease:
1.Lymphangitic Carcinomatosis:its distribution is often unilateral and patchy . it’s associated with well defined nodules , hilar & mediastinal adenopathy and unilateral pleural effusion.
2.Pulmonary Edema, interstitial:its distribution is Bilateral and diffuse. It’s more prominent at Middle and lower zones and Peribronchovascular areas . it’s gravity dependent . it’s associated with Acinar-sized, ill-defined nodules, patchy ground-glass and consolidations, cardiomegaly and bilateral pleural effusion.
3.Amyloidosis, interstitial:it’s distribution is Bilateral, patchy more prominent in Peripheral and Basal areas.It’s associated with Calcified micronodules, consolidations, mediastinal adenopathies, tracheal thickening. (15)
B) Nodular:
Thickening of the central and/or peripheral interstitium with associated micronodules (►)If the interstitium is thickened simply because of a focal accumulation of cells or substance and the architecture of the lobule is preserved.
Beaded appearance
If the interstitium is thickened because of fibrosis and the nodular elements are due to focal fibrosis, the architecture of the lobule may be distorted.
Dotlike opacities
From pattern to disease:
1. Lymphangitic Carcinomatosis: asdescribed above. 2. Amyloidosis, interstitial: asdescribed above. 3. Asbestosis, Early: Its distribution is Bilateral, diffuse or patchy. It’s more prominent in Peripheral, dorsal and Basal areas. It’s associated with Subpleural dotlike opacities, irregular intralobular reticulation, subpleural lines, parenchymal bands, and pleural plaques.
C) Irregular:
The interstitium presents various degrees of thickening along the lobular margins and peribronchovascular bundles . The interstitial structures show an irregular course with a zigzag conformation which distorts their architecture and renders it increasingly unrecognizable. (16)
This pattern is characteristic of the fibrosing diseases, as fibrosis accounts for the distortion of the lobular anatomy. An irregularly thickened intralobular network is often seen together with the loss of the separation between lobules.
The pattern is also accompanied by traction bronchiectasis and bronchiectasis, with vessels and bronchi following an irregular, corkscrew-like path.
From pattern to disease:
1. Sarcoidosis, fibrosing: Its distribution is Bilateral and Patchy. It’s more prominent in the central region, especially Dorsal and Upper zones. It’s associated with Parahilar conglomerations with traction bronchiectasis, perilymphatic nodules,and hilar-mediastinal adenopathies.
2. Hypersensitivity Pneumonitis (HP), chronic: Its distribution is Bilateral and Patchy. It’s more prominent at Subpleural, but also in peribronchovascular
regions. It’s associated with interface sign, traction bronchiectasis, ground-glass and ill-defined centrilobular nodules, mosaic oligemia with air-trapping.
3. Drug Toxicity: Its distribution is Bilateral and Patchy. It’s associated with ground-glass, consolidations with air-bronchogram,
and possible honeycombing.
4. Collagen vascular diseases, early Its distribution is Bilateral and Diffuse. It’s more prominent in Peripheral, Sub-Pleural and Dorsal and Basal
regions. It’s associated with ground-glass and consolidations with traction bronchiectasis, which are specific signs of each disease.
5. Non-Specific Interstitial Pneumonia (NSIP): its distribution is Bilateral, Uniform, and Patchy . It’s more prominent in Peripheral, Dorsal, and Basal regions.
It’s associated with ground-glass and consolidations with bronchiolectasis, bronchial walls thickening, and rare honeycombing.
6. Usual Interstitial Pneumonia (UIP), early: Its distribution is Bilateral, and Patchy in normal parenchyma. It’s typically Sub-Pleural, especially Dorsal, and
in Basal regions, but, also, periperhal up to Upper regions. It’s associated with Subpleural dotlike opacities, irregular intralobular reticular pattern, subpleural lines, parenchymal bands, and pleural plaque.
7. Asbestosis, early:as described above.
II) NODULAR PATTERN
The main alteration consists of small rounded opacities (micronodules if the diameter is less than 3 mm, macronodules if between 3 mm and 1 cm) which tend to be localized in definite positions within the secondary lobule and in relation to the pleural surface.
The nodular pattern may be due to a variety of granulomatous diseases arising directly in the lung or arriving via the bloodstream down to small vessels where they develop concentrically, or via the bronchi,for example, when a reaction to an inhaled substance develops in a small bronchus and the adjacent area.
A) Centrilobular
The nodules tend to be centered at a certain distance from the pleural surface and at times, also from the interlobular septa . As a consequence, they are separated from the lobular margins, the costal margins and the fissures by a transparent rim.
Centrilobular distribution is more typical of diseases in which the elementary lesions originate from or near the peripheral bronchioles When the adjacent
peribronchiolar airspaces are involved, the nodules tend to present low density and ill defined borders (nodular ground-glass).
These are related to endobronchial and small airway disease.the most peripheral nodules are > 5mm from the pleural surface .a ‘tree in-bud’ appearance suggests endobronchial disease.(17)
From pattern to disease:
1. Respiratory Bronchiolitis-Interstitial Lung Disease (RB-ILD):
Its distribution is Bilateral and Patchy with Uniform distributions more prominent at upper and middle zones. It’s associated with Patchy ground-glass, centrilobular emphysema, bronchial wall thickening, and intralobular reticular pattern (rare).
2. Langerhans’ Cell Histiocytosis (LCH), early:
Its distribution is Bilateral and diffuse with Uniform distribution. It’s more prominent at upper and middle zones. It’s associated with well-defined, high density nodules, possibly cavitated, sparing the costophrenic angles and air-trapping.
3. Lymphocytic Interstitial Pneumonia LIP:
Its distribution is Diffuse and Uniform, with more predominance at the middle and lower zones. It’s associated with dense, well-defined, perilymphatic nodules, ground-glass, nodular reticulation, and thinwalled cysts.
4. Hypersensitivity Pneumonitis (HP), subacute:
Its distribution is Diffuse and Uniform, with more predominance at the middle and lower zones. It’s associated with Patchy ground-glass, at times mixed with areas of lobular air-trapping (head-cheese pattern).
B) Random
These nodules are of a good density with sharp borders mainly because they are confined to the interstitium. Their distribution is reasonably uniform in the secondary lobule and the parenchyma.
At times they can be seen in contact with the extremities of the vascular structures, from which they appear to originate (feeding vessel sign). They have a hematogenous origin.
However, they can also be found near the pleural surface ( not the rule). In short, their spatial distribution appears uniform.
Here we need to define a “ground glass Opacity” term: It used to describe Increased hazy opacity within the lung, not associated with obscured underlying vessels. This finding reflects the relative reduction of the quantity of air in the alveoli, either due to partial filling of the air spaces or thickening of the septa (intralobular interstitium) (18).
From pattern to disease:
1. Silicosis:
Its distribution is Bilateral with some right-sided prevalence. It has a tendency to predominate posteriorly
and has prevalence in the middle and upper zones. It’s associated with Pseudoplaques, “egg-shell”
mediastinal adenopathy, larger opacities and conglomerated parahilar masses.
2. T.B, Miliary:
Its distribution is Bilateral, Symmetrical and Uniform. It’s associated with Diffuse or localized ground-glass Opacity and mediastinal adenopathies with central hypodensities.
3. Metastases:
Its distribution is Bilateral and often Symmetrical. While it’s possible to be subpleural, it’s more prominent in basal regions. It’s associated with nodules of different size, possibily cavitated or calcified, shows a feeding vessel sign, and has mediastinal adenopathies.
C) Perilymphatic
First we need to describe “Peri-lymphatic” as a Term used to describe the distribution of nodules which tend to be concentrated in the perilobular and subpleural interstitium (although they may have an intralobular location) and therefore located along the costal margins and major fissures.(19)
These nodules tend to be prevalent in the perilobular and subpleural interstitium and are, therefore, profuse along the costal margins and the fissures.
They are more common in diseases which spread along the lymphatics, and may therefore be found within the lobule, but also along the vessels and bronchi (beaded appearance).
The nodules have well-defined margins as well as high and uniform opacity.The spatial arrangement of the lesions tends to be patchy, interspersed with areas of normal parenchyma.
From pattern to disease
1. Sarcoidosis, granulomatous:
Its distribution is Bilateral and Patchy. It’s more prominent at Perihilar regions, mainly Dorsal and subpleura.
It has middle and upper zone predominance. It’s associated with Bronchovascular nodules, pseudoplaques,
hilar and mediastinal adenopathies, micronodular ground-glass, and lobular air-trapping. 2. Lymphocytic Interstitial Pneumonia LIP: as described before.
III) ALVEOLAR PATTERN
The main finding consists of opacities resulting from alveolar filling their density varies in proportion to the extent of this filling, from partial (ground-glass) to complete (consolidation) Involvement of the small airways may either take the form of luminal narrowing (indirect signs of which are hypodensities of the distal
parenchyma due to regional oligemia), or filling by various materials (in which case they become ectatic and their opacity can stand out against the surrounding lung parenchyma)(20)
A) Mixed-density, acute
This pattern is recognizable by the presence of the two characteristic findings of the alveolar diseases, ground-glass and consolidation , combined in varying proportions. The ground-glass may be accompanied by a reticular pattern (crazy paving).
The simultaneous findings of bronchial involvement (bronchial wall thickening, bronchiectasis) and ill-defined centrilobular nodules due to alveolar filling is not
uncommon.
The acute form usually presents with bilateral and often extensive consolidations which may change in appearance, location, and size within hours or days.
From pattern to disease
1. Pneumocystis Carinii Pneumonia (PCP):
Its distributions is Bilateral, Symmetrical, and Patchy or Diffuse. It’s often parahilar and more prominent in the middle and upper zones. It’s associated with Walled cysts, crazy paving, hazy micronodules, mediastinal adenopathies, and pleural effusion.
2. Diffuse Alveolar Hemorrhage (DAH) in Wegener’s granulomatosis:
Its distribution is Bilateral and Diffuse or Patchy. It’s more prominent Parahilar or diffuse, and not peripheral.
It shows hazy, centrilobular nodules, crazy paving, large cavitating nodules and mediastinal findings.
3. Acute Interstitial Pneumonia (AIP):
Its distribution is Bilateral, Symmetrical, and Diffuse or Patchy. It’s usually peripheral and gravity dependent.
It’s associated with reticular pattern, parenchymal distortion, traction bronchiectasis, and sporadic honeycombing.
4. Hypersensitivity Pneumonitis (HP), acute:
Its distribution is Bilateral and Patchy, occasionally uniform, but most often basal. It’s associated with Hazy centrilobular nodules, mediastinal adenopathies and mosaic oligemia with air-trapping.
5. Adult Respiratory Distress Syndrome (ARDS):
Its distribution is Bilateral, Symmetrical, and Patchy. The prevalence is in the dependent lung. It is more extensive at the lung bases. It’s associated with Asymmetrical and less gravity-dependent if pulmonary ARDS.
6. Pulmonary Edema (PE), alveolar:
Its distribution is Bilateral, Symmetrical, and Diffuse or Patchy. Its subpleural and gravity dependent. It has Basal prevalence and is associated with redistribution of pulmonary perfusion, smooth reticular pattern,pleural effusion, and cardiomegaly.
B) Mixed-density, chronic:
This pattern is recognizable by the presence of the two characteristic findings of the alveolar diseases, groundglass and consolidation combined in varying proportions.
The ground-glass may be accompanied by a reticular pattern (crazy paving). The simultaneous finding of bronchial involvement bronchial wall thickening, bronchiectasis) and ill-defined centrilobular nodules due to alveolar filling is not uncommon.
The chronic form usually presents with consolidations often localized and patchy which progress slowly, even over weeks or months.
Here we need to describe the term “Crazy Paving”: it used to described Scattered or diffuse groundglass attenuation with superimposed interlobular septal thickening and intra lobular lines. (21)
From pattern to disease:
1. Chronic Eosinophilic Pneumonia (CEP):
Its distribution is Bilateral and Patchy with more prominence at peripheral and subpleural regions and middle and upper zones. It’s associated with Ill-defined nodules, mediastinal adenopathies and rarely pleural effusion.
2. Mucosa-Associated Lymphatic Tissue lymphoma(MALToma):
Its distribution is Bilateral or Unilateral, Diffuse or Patchy, with more prominence in the peribronchial area. It’s associated with Bronchi stretched and thinned within the consolidations; centrilobular nodules, small or large masses, and halo sign.
3. Pulmonary Alveolar Proteinosis (PAP):
Its distribution is Bilateral, Diffuse or Patchy. It’s associated with predominant ground-glass, extensive crazy paving, and sharp interfaces with the healthy parenchyma.
4. BronchioloAlveolar Carcinoma (BAC):
Its distribution is Uni- or Bilateral, Asymmetrical, or Patchy. It’s often peripheral and subpleural as well as basal. It’s associated with possible pseudocavitations, nodules and hazy ground glass, crazy paving, adenopathies, and pleural effusion.
5. Desquamative Interstitial Pneumonia (DIP):
Its distribution is Bilateral, Symmetrical, and Patchy. It’s subpleural but also diffuse prevalently basal.
It’s assocuated with ground-glass predominant, limited parenchymal distortion with traction bronchiolectasis, and microcysts.
6. Cryptogenic Organizing Pneumonia (OP):
Its distribution is Bilateral and Patchy. It’s peripheral but also peribronchial basal. It’s associated with air bronchogram and bronchiolectasis within the opacities, centrilobular nodules with ill-defined margins, and macronodules or masses.
N.B. The Term (Air Brochogram) used to described Visualization of patent bronchial structures within areas of parenchymal consolidation.(22)
7. Drug toxicity:
Its distribution is Bilateral, Symmetrical, and Patchy. It’s peripheral basal and associated with Amiodarone, which has hyperdense consolidations (compared to the muscles), reticular pattern and micronodules, with pleural thickening.
C) Mosaic oligemia with air-trapping
This term used to describe Areas of low parenchymal density in which the vessels are reduced in size and number. The extent of oligemia may be lobular or segmental, while the distribution is typically patchy Mosaic perfusion (23)
The main characteristics of this pattern are areas of patchy hyperlucency , often with lobular distribution, associated with vessels reduced in number and diameter. This pattern is a typical expression of small airway obstruction. The oligemia is due to hypoxic vasoconstriction secondary to alveolar hypoventilation.
The surrounding normal parenchyma appears “relatively” hyperdense partially because of hyperperfusion (pseudo-ground-glass). While oligemia from vascular obstruction (e.g. in patients with chronic pulmonary thromboembolism) does not change during expiration, hypoxic oligemia accentuates (air-trapping) Mosaic perfusion.
From pattern to disease
1.Constrictive Bronchiolitis (CB):
Its distribution is Bilateral, Asymmetrical, and Patchy. It’s associated with direct signs of airway disease (bronchiectasis) and shows pseudo-ground-glass in the normally ventilated areas.
D) Tree-in-bud
This term used to describe Thin branching opacities which terminate with small nodular opacities, usually visible in the lung periphery.
his finding is particularly common in diseases with endobronchial spread of infection.
Its appearance is due to the presence of dilated adjacent bronchioles and air spaces filled with material such as pus, mucous or fluid. (24)
it’s identified by the presence of thin branching opacities in the peripheral lung, which terminate with small nodular opacities of different density.
The branching opacities (the tree) reflect the presence of dilated bronchioles filled with material other than air, whereas the nodular opacities (the buds) are due to clusters of partially or completely filled alveoli, usually with poorly-defined margins (centrilobular nodules).
The tree-in-bud sign is typical of diseases with bronchogenic spread.
From pattern to disease:
1. Infections, endobronchial:
Its distribution is Uni- or Bilateral, and Patchy. It’s variable often in relation to the bronchi. It’s associated with Atypical mycobacteriosis, bronchial wall thickening, bronchiectasis, cavitation, and possibly cavitated consolidations.
IV) CYSTIC PATTERN
The main finding consists of small areas of absolute hyperlucency (cysts) – black holes which, more-or-less, extensively occupy the lung parenchyma. They may or may not be delimited by walls.
Cyst formation may result from bronchial and bronchiolar enlargement due to wall distention, traction, increased endoluminal pressure, or a focal hyperinflation of the air spaces with rupture of the walls.(25)
A) Clusters of grapes
The cysts are arranged in grape-like clusters, often around a stem (the bronchovascular pedicle).
Usually, these lesions have thick walls; their diameter may not be uniform air-fluid levels or inclusions inside the cysts are common.
The fluid may be of varying nature: mucus, pus or blood. An intracystic mass is often due to a mycetoma, more rarely neoplastic; however, only a mycetoma moves when the patient’s position is changed!
At times the cysts may be completely full of material and assume a pseudo-nodular appearance.
From pattern to disease:
1. Bronchiectasis, Cystic, and Cystic Fibrosis (CF):
Its distribution is Uni- or Bilateral, and Patchy. It’s central or peripheral with more prominence at middle and upper zones. It’s associated with air-fluid levels, tubular or varicose bronchiectasis and tree-in-bud, and oligemia with air-trapping.
B) String of pearls
The cysts are arranged in a single layer in the subpleural region and resemble a string of pearls.
Usually, these lesions have thin walls (comparable to the thickness of a fissure), which are interlobula septa at times thickened by minimal fibrosis.
If the diameter of the cysts is greater than 1 cm, the term bulla is used , it tend to have thicker walls owing to a greater quantity of fibrosis
From pattern to disease:
1. Emphysema,Paraseptal:
Its distribution is Uni- or Bilateral, and Patchy. It’s peripheral and subpleural with more prominence at middle and upper zones. It’s associated with Centrilobular emphysema as well as spontaneous pneumothorax.
C) Honeycombing:
This term used to describe Small thick-walled cystic spaces arranged in several concentric layers.
Honeycombing is the radiological hallmark of end-stage lung disease, and therefore traction bronchiectasis and bronchiolectasis as well as interface signs are often present. (26)
This pattern refers to thick-walled, rounded cysts arranged in several layers .
In the affected regions, the pulmonary architecture is distorted and traction bronchiectasis and bronchiolectasis are often present Honeycombing is the expression of the end phase of a number of fibrotic diseases (end-stage lung).
The lung volume characteristically reduces; an early sign of loss of volume is the dislocation of thin structures such as the fissures, whereas in the more advanced phases the bronchovascular bundles and the mediastinum are also displaced.
From pattern to disease:
1. Asbestosis, advanced:
Its distribution is Bilateral, and Patchy. It’s peripheral, subpleural, and basal. we can find it with Traction bronchiectasis and bronchiolectasis, irregular reticular pattern, subpleural lines, and pleural plaques.
2. Collagen vascular diseases, advanced:
Its distribution is Bilateral and Patchy. It’s peripheral, subpleural, and basal. we can find it with Traction bronchiectasis and bronchiolectasis, irregular reticular pattern, and disease-specific signs.
3.Usual Interstitial Pneumonia UIP, advanced:
Its distribution is Bilateral and Patchy. It’s peripheral, subpleural, and basal. we can find it with Traction bronchiectasis and bronchiolectasis,irregular reticular pattern, and mediastinal adenopathies.
D) Random cysts
We can arrange The cysts without obvious aggregations.
Their walls are of variable thickness,and in some diseases they are absent.
The presence of a minute central hyperdensity can indicate the presence of a centrilobular arteriole.
The distribution of the cysts is relatively homogeneous in the affected parenchyma, so their profusion is uniform.
the overall appearance of the diseases presenting with this pattern can be very similar.
The differential diagnosis, therefore, requires a careful assessment of the craniocaudal distribution of the lesions and the involvement of the costophrenic angles.
From pattern to disease:
1. Lymphangioleiomyomatosis (LAM):
Characterized by Thin-walled cysts of variable sizes surrounded by normal lung parenchyma can be seen throughout the lung with interlobular septal thickening.
It may show a dilated thoracic duct.haemorrhages , that we can see as areas of increased attenuation.(27)
. 2. Langerhans’ Cell Histiocytosis (LCH), advanced:
Its distribution is Bilateral, Symmetrical, and Uniformly distributed. It’s more prominent at middle and upper zones, with costophrenic angles spared. It’s associated with thick walls, bizarre coalescent cysts,associated cavitated nodules, and possible pneumothorax.
3. Emphysema,centrilobular:
Its distribution is Bilateral, Symmetrical or Asymmetrical. It’s Uniformly distributed with more prominence at middle and upper zones. It’s assocaited with lack of walls, a visible centrilobular artery, paraseptal emphysema, and saber-sheath trachea.
Conclusion:
Interstitial lung diseases (ILDs) comprises a diverse group of diseases that lead to inflammation and fibrosis of the alveoli, distal airways, and septal interstitium of the lungs.
The ILDs consist of disorders of known cause (e.g., collagen vascular diseases, drug-related diseases) as well as disorders of unknown etiology. The latter include idiopathic interstitial pneumonias (IIPs),sarcoidosis and a group of miscellaneous, rare, but nonetheless interesting, diseases.
In patients with ILD, MDCT enriches the diagnostic armamentarium by allowing volumetric high resolution scanning, i.e.,continuous data acquisition with thin collimation and a high spatial frequency reconstruction algorithm.
CT is a key method in the identification and management of patients with ILD.
It not only improves the detection and characterization of parenchymal abnormalities, but also increases the accuracy of diagnosis.
The spectrum of morphologic characteristics that are indicative of interstitial lung disease is relatively limited and includes four main patttern Reticular (smooth,Nodular,Irregular) ,Nodular (Centrilobular ,Random,perilymphatic) , Alveolar (mixed density acute, mixed density chronic , Mosaic olidemia with air
trapping, Tree in bud) , Cystic (clusters of grapes,string of pearls, honeycombing, random cysts ).
In the correct clinical context, some patterns or combination of patterns, together with the anatomic distribution of the abnormality, i.e., from the lung apex to the base, or peripheral subpleural versus central bronchovascular, can lead the interpreter to a specific diagnosis. However, due to overlap of CT morphology between various entities, complementary lung biopsy is recommended in virtually all ILDS cases.
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