HRCT CHEST PROTOCOLS FOR IMAGING LUNG.pptx

HRCT CHEST PROTOCOLS

1. HRCT CHEST PROTOCOLS

2. Both CT Chest and HRCT chest  are used for diagnosis of chest/lungs related issues. The main difference between CT chest and HRCT chest is the level of detail and the specific imaging techniques used. CT chest  stands for Computed Tomography, is a type of imaging study that uses x rays to create detailed and cross-sectional images of the chest. A CT chest can show the lungs, chest wall, mediastinum (area between the lungs), heart, and blood vessels. A CT chest can provide high-resolution images of lung tissue, but it may not be as detailed as an HRCT chest.

3. HRCT chest (High-Resolution Computed Tomography) is a specific type of CT scan that uses thinner slices and specialized imaging techniques to provide high-resolution images of lung tissue. HRCT chest can provide more detailed images of the lung parenchyma, including the small airways and lung structure, than a standard CT chest. HRCT chest is often used to evaluate lung diseases such as interstitial lung disease, lung nodules, or pulmonary embolism.

4. Indications HRCT is particularly useful in the assessment of diffuse lung conditions involving the interstitium such as: Interstitial lung disease Cystic lung disease Small Airways disease Pulmonary micronodules Bronchiectasis

5. Purpose HRCT is performed in order to visualize small structures of the lung and detect subtle changes of disease that otherwise may be difficult to assess on conventional chest imaging . Prone HRCT imaging is useful in patients with basal disease, eliminating changes due to gravity or dependant atelectasis.

6. Slice Thickness The use of thin sections (≤1.5 mm) is essential if spatial resolution and lung detail are to be optimized . Generally, 1- to 1.25-mm-thick slices are adequate for diagnosis; a clear-cut advantage for thinner slices has not been shown, and may result in either increased image noise or a significantly higher radiation dose . With slices thicker than 1.25 to 1.5 mm, volume averaging within the plane of scan significantly reduces the ability of CT to resolve small structures . The use of 2.5- to 5-mm slice thickness should not be considered adequate for HRCT

7. Reconstruction Algorithm Reconstruction of images using a sharp, high spatial frequency, or high resolution algorithm reduces image smoothing and increases spatial resolution, making structures appear sharper . Using a high-resolution algorithm is a critical element in performing HRCT . In one study of HRCT techniques , the use of a high spatial frequency algorithm to reconstruct scan data resulted in a quantitative improvement in spatial resolution when compared to a standard algorithm; in this study, subjective image quality was also rated more highly with the high spatial frequency algorithm.

8. Kilovolts (Peak), Milliamperes, and Scan Time A sharp or high-resolution reconstruction algorithm, in addition to increasing image detail, increases the visibility of noise in the CT image . This noise usually appears grainy, mottled, or as streaks that can be distracting and may obscure anatomic detail . Because much of this noise is quantum related, it is inversely proportional to the number of photons absorbed (precisely, it is inversely proportional to the square root of the product of mA and scan time) . Consequently, it increases with decreasing mAs or kilovolt peak (kV(p)) and decreases with increased mAs or kV(p) .

9. Field of View and Targeted Reconstruction Scanning should be performed using the smallest field of view (FOV) that will encompass the patient (e.g., 35 cm), as this reduces pixel size. Retrospectively targeting image reconstruction to a single lung instead of the entire thorax significantly reduces the FOV and image pixel size, and thus increases spatial resolution

10. Spatial Resolution of High-Resolution Computed Tomography The inherent or maximum spatial resolution of a CT scanner is determined by the geometry of the data-collecting system and the frequency at which scan data are sampled during the scan sequence . A fundamental relationship exists between pixel size and the size of structures that can be resolved by CT. For optimal matching of image display to the attainable spatial resolution of the scanner, there should be two pixels for the smallest structure resolved .

11. HIGH-RESOLUTION COMPUTED TOMOGRAPHY PROTOCOL MODIFICATIONS: INTERSPACED SCANS VOLUMETRIC SCANNING, PRONE IMAGING, EXPIRATORY COMPUTED TOMOGRAPHY

12. Interspaced Scans HRCT may be performed with individual axial scans being obtained at spaced intervals, usually 1 to 2 cm, without table motion . In this manner, HRCT is intended to “sample” lung anatomy, with the assumptions being that a diffuse lung disease will be visible in at least one of the levels sampled and the findings seen at the levels scanned will be representative of what is present throughout the lung. When interspaced scanning is chosen for HRCT, we consider scans obtained at 1-cm intervals, from the lung apices to bases, to be the most appropriate routine scanning protocol, allowing an adequate sampling of the lung and lung disease regardless of its distribution..

13. Volumetric High-Resolution Computed Tomography The advent of MDCT scanners effectively revolutionized the HRCT technique, and volumetric HRCT using thin detectors (0.5–0.625 mm) is now the routine in many institutions. Although volumetric HRCT generally is acquired using a helical technique (constant table motion during image acquisition), axial scanning of the entire chest can be obtained using scanners capable of a rapid “step-and-shoot” technique.

14. Volumetric HRCT technique has several advantages. It allows (a) complete imaging of the lungs and thorax, (b) viewing of contiguous slices for the purpose of better defining lung abnormalities, (c) reconstruction of scan data in any plane or using maximum-intensity projections (MIPs) or minimum-intensity projections ( MinIPs ), (d) precise level-by-level comparison of studies obtained at different times for evaluation of disease progression or improvement, and (e) the diagnosis of additional thoracic abnormalities .

15. Prone Scanning However, scans obtained with the patient positioned prone are sometimes necessary for diagnosing subtle lung abnormalities. Atelectasis is commonly seen in the dependent lung (i.e., posterior lung on supine scans) in both normal and abnormal subjects, resulting in a so-called dependent density or subpleural line . These normal findings can closely mimic the appearance of early lung fibrosis, and they can be impossible to distinguish from true pathology on supine scans alone. However, if scans are obtained in both supine and prone positions, dependent density can be easily differentiated from true pathology. Normal dependent density disappears in the prone position a true abnormality remains visible regardless of whether it is dependent or nondependent

16. Expiratory High-Resolution Computed Tomography As an adjunct to routine inspiratory images, expiratory HRCT scans have proved useful in the evaluation of patients with a variety of obstructive lung diseases . On expiratory scans, focal or diffuse air trapping may be diagnosed in patients with large or small airway obstruction or emphysema. It has been shown that the presence of air trapping .

17. Air trapping visible using expiratory or postexpiratory HRCT techniques has been recognized in patients with emphysema chronic airways disease asthma cystic fibrosis bronchiolitis obliterans the cystic lung diseases associated with Langerhans histiocytosis and tuberous sclerosis bronchiectasis airways disease related to AIDS and small airways disease associated with thalassemia .

18. Additional Technical Modifications : Reduction of Cardiac Motion Artifacts Electrocardiographically Triggered High-Resolution Computed Tomography Segmented Reconstruction Gantry Angulation

19. Reduction of Cardiac Motion Artifacts HRCT scans obtained in a routine fashion may be degraded by cardiac motion. Several motion-related artifacts may be seen, particularly in the left paracardiac region. HRCT using electrocardiographic (ECG) triggering of scan acquisition, reduced gantry rotation time, and segmented reconstruction of scan data have all been used in an attempt to reduce these artifacts. ECG triggering resulted in a significant reduction in motion artifacts in the middle lobe, lingula, and left lower lobe, but no differences in diagnostic outcome were found between triggered and nontriggered techniques.

20. Electrocardiographically Triggered High-Resolution Computed Tomography Electrocardiographically triggered HRCT may be used to reduce motion related artifacts , but has little effect on diagnosis. In a study using a helical scanner capable of 0.75-s gantry rotation, 500-ms HRCT scans, representing a 240-degree rotation of the gantry, were initiated at 50% of the R-R interval . Because of the shorter-than-routine scan time, images were reconstructed using a smoother algorithm than is usually used for HRCT.

21. Segmented Reconstruction Partial or segmented reconstruction of scan data can serve to reduce effective scan time and can result in a significant reduction in motion artifacts without increasing radiation dose, albeit at the expense of increased image noise. Arac et al. studied HRCT images obtained using a scanner capable of 1-s rotation and reconstruction using a full gantry rotation and a 225-degree rotation segment. Segmented reconstruction reduced cardiac motion artifacts.

22. Gantry Angulation When HRCT is obtained using interspaced images, angling the top of the CT gantry 20 degrees caudally with the patient supine (i.e., the gantry is angled toward the feet) improves visibility of the segmental and subsegmental bronchi, particularly in the middle lobe and lingula, by aligning them parallel to the plane of scan . This technique may be valuable in assessing patients with bronchiectasis . However, in the majority of patients with bronchiectasis, spaced HRCT images without gantry angulation are sufficient for diagnosis, and there would seem to be little use for this technique when volumetric HRCT is obtained.

23. Window Settings The window mean and width used for image display have a significant impact on the appearance of the lung parenchyma and the dimensions of visualized structures If the display technique used is not appropriate, normal structures can be made to look abnormal, or subtle abnormalities may be overlooked. The most important window setting to use in display is the so-called lung window.

24. Window level settings ranging from −600 to −700 HU and window widths of 1,000 to 1,500 HU are appropriate for a routine lung window . The use of an extended window width (i.e., 2,000 HU) reduces contrast between lung parenchymal structures, such as vessels, bronchi, and the air-containing alveoli, and may make interstitial structures appear less conspicuous or thinner than they actually are.

25. RADIATION DOSE The radiation dose associated with thoracic CT has received increased attention in recent years, as have attempts at CT dose reduction At the same time, the development of volumetric MD HRCT for diagnosing diffuse lung disease has resulted in an increased patient radiation dose, as compared to interspaced HRCT. This concern, however, has been tempered by the development of alternative image reconstruction techniques and other techniques that produce images with less noise, allowing for a reduction in the scanner parameters (such as the tube current) that contribute to the relatively high radiation doses.

26. HIGH-RESOLUTION COMPUTED TOMOGRAPHY ARTIFACTS Streak artifacts that radiate from the edges of or adjacent to sharply marginated, high-contrast structures such as bronchial walls, ribs, or vertebral bodies are common on HRCT. They are particularly prominent in patients with metallic implants such as spinal fixation hardware. On HRCT, streak artifacts are often visible as fine, linear, or netlike opacities that can be seen anywhere but are most commonly found overlying the posterior lung, paralleling the pleural surface and posterior chest wall (10)

27. Motion-Related Artifacts Pulsation or star artifacts are commonly visible, particularly at the left lung . The major fissure, usually on the left, or other parenchymal structures such as vessels and bronchi may be seen as double because of cardiac pulsation or respiration during the scan This appearance of doubling artifacts can mimic bronchiectasis.

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