Hrct i


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  • The abnormalities of subpleural interstitium is recognized over the costal surface and along fissuresNormal fissure is less than 1 mm thick, smooth and very thin opacities
  • Hrct i

    2. 2. HRCT?
    3. 3. HRCT?
    4. 4. HRCT?
    5. 5. HRCT?
    6. 6. High Resolution Computed Tomography Resolution :  A formal statement of a decision or expression of opinion put before or adopted by an assembly such as the U.S. Congress.????
    7. 7. High Resolution Computed Tomography Resolution :  Ability to resolve small object that are close together , as separate form.
    8. 8.  A scan performed using thin collimation and high- spatial frequency algorithm to accentuate the contrast between tissue of widely differing densities, e.g.., - air & vessels (lung) - air & bone (temporal & paranasal sinus)
    9. 9. The basic premise is simple, maximize spatial resolution by using the thinnest collimation and a high spatial frequency algorithm
    10. 10.  Narrow x-ray beam collimation: 0.5-1 mm vs. Conventional 3-10mm Cross sections are further apart: 10 mm intervals (random sampling) High- spatial frequency algorithm No i.v. Contrast needed  Inherent contrast  Mediastinum is not best studied
    11. 11. maximize spatial resolution byusing the thinnest collimation anda high spatial frequencyalgorithm
    12. 12.  Thin sections 0.5 – 1.5 mm is essential for optimal spatial resolution Thicker slices are prone for volume averaging and reduces ability to resolve smaller structure
    13. 13.  With thick collimation, for example, vessels that lie in the plane of scan look like vessels (i.e., they appear cylindrical or branching) and can be clearly identified as such. With thin collimation, vessels can appear nodular, because only short segments may lie in the plane of scan;
    14. 14. maximize spatial resolution byusing the thinnest collimation anda high spatial frequencyalgorithm
    15. 15.  Denotes the frequency at which the acquired scan data are recorded when creating the image. Using a high-resolution algorithm is critical element in performing HRCT.
    16. 16.  With conventional body CT, scan data are usually reconstructed with “standard” or “soft-tissue” algorithms, that smoothes the image, reduces visible image noise,. High spatial frequency or sharp algorithm (bone algorithm) is used which reduces image smoothing and better depicts normal and abnormal parenchymal interface
    17. 17.  Matrix size : Largest available matrix be used 512 x 512 Field of view : Smallest FOV that will encompass the patient is used as it will reduce the pixel size. (commonly 35 to 40) 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.
    18. 18.  Sharp reconstruction algorithm,  increase image detail,  increase the visibility of noise in the CT image .
    19. 19. Noise = 1/√ mAs X Kvp X scan timeit is inversely proportional to the square root of the product of the mA and scan time).
    20. 20.  Sharp reconstruction algorithm,  increase image detail,  increase the visibility of noise in the CT image . Much of this noise is quantum-related and thus decreases with  increased number of photons ,  increasing the mA or kV(p) used during scanning, or  increasing scan time,
    21. 21. Noise = 1/√ mAs X Kvp X scan timeit is inversely proportional to the square root of the product of the mA and scan time).
    22. 22.  In HRCT image, noise is more apparent than standard CT. Noise = 1/√ mAs X Kvp X scan time Scan Time is kept low as possible to minimize motion artifact, increasing scan time is not feasible, mAs and Kvp are INCREASED to reduce noise
    23. 23.  For routine technique – Kvp -- 120-140 mAs -- 200- 300 Scan Time : As low as possible (1-2 sec) to minimize motion artifact.
    24. 24.  For routine technique – Kvp -- 120-140 mAs -- 200- 300 Scan Time : As low as possible (1-2 sec) to minimize motion artifact. Increased patient and chest wall thickness are associated with increase image noise, may be reduced by increasing mAs and Kvp
    25. 25.  Effect of kV(p) and mA on image noise. HRCT scans obtained with kV(p)/mA settings of 120/100 (A) and 140/170 (B). Noise is most evident posteriorly and in the paravertebral regions. Although noise is greater in A, the difference is probably not significant clinically. Nonetheless, increasing the kV(p)/mA is optimal. Also note pulsation (“star”) artifacts in the left lung on both images and a “double” left major fissure.
    26. 26.  Collimation: thinnest available collimation (1.0-1.5 mm). Reconstruction algorithm: high-spatial frequency or “sharp” algorithm (i.e., GE “bone”). Scan time: as short as possible (1 sec or less). kV(p), 120-140; mA, 240. Matrix size: largest available (512 × 512). Optional kV(p)/mA: Increased kV(p)/mA (i.e., 140/340). Recommended in large patients. Otherwise optional. Targeted reconstruction: (15- to 25-cm field of view). Reduced mA (low-dose HRCT): 40-80 mA.
    27. 27.  At least one consistent lung window setting is necessary. Window mean/width values of -600 HU to - 700 HU/1,000 HU to 1,500 HU are appropriate. Good combinations are -700/1,000 HU or -600/1,500 HU. Soft-tissue windows of approximately 50/350 HU should also be used for the mediastinum, hila, and pleura. Windows: Windows may need to be customized; a low- window mean (-800 to -900 HU) is optimal for diagnosing emphysema. For viewing the mediastinum, 50/350 HU is recommended. For viewing pleuro- parenchymal disease, -600/2,000 HU is recommended.
    28. 28.  Slice thickness: 3-10 mm Scans a large volume, very quickly Volumetric scan - Covers the full lung +/- contrast
    29. 29.  Standard before helical CT Differs from helical CT technique in that slices are not contiguous Move the patient, stop, and scan All other parameters same Adv: lower radiation dose
    30. 30.  Transverse images of thin slices of lung (1-1.3 mm thick) are obtained at non-contiguous intervals, usually 1 to 2 cm apart, throughout the whole lung. The computer reconstructs the images to give high spatial resolution. This process results in images that show detail, but only 5 to 10% of the lung is sampled. This sampling is appropriate for evaluating diffuse lung disease, focal lesions may require more images.
    31. 31.  Obtained at 1cm intervals from lung apices to bases. In this manner, HRCT is intended to “sample” lung anatomy It is assumed that the findings seen at the levels scanned will be representative of what is present throughout the lungs Results in low radiation dose as the individual scans are widely placed 48
    32. 32. 1. Viewing of contagious slice for better delineation of lung abnormality 2. Complete imaging of lung and thorax 3. Reconstruction of scan data in any plane using MIPs or MinIPs. 4. Diagnosis of other lung abnormalitiesDisadvantage : greater radiation dose. It delivers 3-5 times greater radiation. 49
    33. 33.  Multidetector CT is equipped with a multiple row detector array Multiple images are acquired due to presence of multiple detectors Advantages : - shorter acquisition times and retrospective creation of both thinner and thicker sections from the same raw data Acquisition time is so short that whole-lung HRCT can be performed in one breath-hold. 50
    34. 34.  More coverage in a breath-hold  Chest, Vascular studies, trauma Reduced misregistration of slices  Improved MPR, 3D and MIP images Potentially less IV contrast required Gapless coverage Arbitrary slice positioning
    35. 35.  Various study shows the image quality of axial HRCT with multi-detector CT is equal to that with conventional single-detector CT. HRCT performed with spaced axial images results in low radiation dose as compared with MD-HRCT. Increased table speed may increase the volume- averaging artifact and may result in indistinctness of subtle pulmonary abnormalities. MDCT provides for better reconstruction in Z axis 55
    36. 36. • Low dose HRCT uses Kvp of 120- 140 and mA of 30-20 at 2 sec scan time.• Equivalent to conventional HRCT in 97 % of cases• Disadvantage : Fails to identify GGO in few cases and have more prominent streak artifact.• Not recommended for initial evaluation of patients with lung disease.• Indicated in following up patients with a known lung abnormality or in screening large populations at risk for lung ds. 56
    37. 37. Conventional-dose
    38. 38. Low-doseAlthough noise is much more obvious on the low-dose image, areas of ground-glass opacityand ill-defined nodules (arrows) are visible with both techniques
    39. 39. “The authors concluded that HRCT imagesacquired at 20 mA yield anatomic informationequivalent to that obtained with 200-mA scansin the majority of patients without significantloss of spatial resolution or image degradation”
    40. 40. In 16-slice and higher scanners, thecurrent protocol is to do a volumescan in 2-5 seconds and thenretrospectively reconstruct the imagesas 1mm at 0.5mm intervals and toreview the stack on the workstation
    41. 41.  Annual background radiation ----- --- 2.5 mSv PA CHEST Radiograph ----- ----- ----- 0.05 mSv Spaced axial HRCT (10mm space) ----- 0.7 mSv ( 14 X ray) Spaced axial HRCT (20 mm space) ------ 0.35 mSv ( 7 X ray) Low Dose Spaced axial HRCT -------- 0.02 mSV MD-HRCT ---- ------- 4 - 7 msv ( 60-80 x ray)Combining HRCT scan at 20 mm interval with low mAs scan (40 mAs) would result in radiation comparable to conventional X ray. 61
    42. 42. Decision should be tailored for individual cases
    43. 43.  Detect interstitial lung disease not seen on chest x- ray Abnormal pulmonary function tests Characterize lung disease seen on X-ray Determine disease activity Find a biopsy site
    44. 44.  Hemoptysis Diffusely abnormal CXR Normal CXR with abnormal PFT’s Baseline for pts with diffuse lung disease Solitary pulmonary nodules Reversible (active) vs. non-reversible (fibrotic) lung disease Lung biopsy guide F/U known lung disease Assess Rx response
    45. 45. Training Validation • Clinical 27% 29% • CXR 4% 9% • CT 49% 36% • Clinical & CXR 53% 77% • Clinical, CXR & CT 61% 90%Conclusion: HRCT both superior and additive to clinical and CXR data
    47. 47.  Useful to determine if there is small airway disease • Normal lung increases in density at endexpiration • Abnormal lung due to air trapping fails to increase in density on expiration
    48. 48.  Practically, these are the most important parameters to work with when performing HRCT scans
    49. 49.  If providing films is still important, then the filming should be done such that the pleural margins and ribs are seen with an optimum grey-sca
    50. 50.  maximum intensity projection (MIP) is a volume rendering method for 3D data that projects in the visualization plane thevoxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection
    51. 51. (a) Volume-rendered image provides clear definition of individual vessels. (b) MIP imagereconstructed from the same volume data shows all of the vessels, but their outlines merge; itis impossible to visualize the spatial relationships between the vessels or to delineate individualvessels on the MIP image.
    52. 52.  Maximum-intensity projection (MIP) image in a patient with small lung nodules obtained using a multidetector-row spiral CT scanner with 1.25-mm detector width and a pitch of 6. A: A single HRCT image shows two small nodules (arrows) that are difficult to distinguish from vessels. B: An MIP image consisting of eight contiguous HRCT images, including A, allows the two small nodules to be easily distinguished from surrounding vessels.
    53. 53.  first step in HRCT interpretation of diffuse lung diseases is a good quality scan
    54. 54. . Resolution and size or orientation of structures. The tissue plane, 1 mm thick, and theperpendicular cylinder, 0.2 mm in diameter, are visible on the HRCT scan because they extendthrough the thickness of the scan volume or voxel. The horizontal cylinder cannot be seen.
    55. 55.  Combined “routine” and HRCT studies  5 mm sections q 5 mm (separate lung and mediastinal reconstruction algorithms):  1 – 1.25 mm sections q 10 mm (lung algorithm) Optional image acquisitions  Supine and prone 1 – 1.25 mm sections  Inspiratory/expiratory 1 -1.25 mm sections  Low dose technique (mAs 40 – 80) Optional Reconstruction techniques  Sliding maximum and minimum intensity projection images (MIPs/ MINIPs): 5 mm’s q 5 mm
    56. 56.  Right lung is divided by major and minor fissure into 3 lobes and 10 bronchopulmonary segments Left lung is divided by major fissure into 2 lobes with a lingular lobe and 8 bronchopulmonary segments
    57. 57.  The trachea (windpipe) divides into left and the right mainstem bronchi, at the level of the sternal angle (carina). The right main bronchus is wider, shorter, and more vertical than the left main bronchus. The right main bronchus subdivides into three lobar bronchi, while the left main bronchus divides into two. The lobar bronchi divide into tertiary bronchi, also known as segmental bronchi, each of which supplies a bronchopulmonary segment.
    58. 58.  The segmental bronchi divide into many primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.
    59. 59.  Airways divide by dichotomous branching, with approximately 23 generations of branches from the trachea to the alveoli. The wall thickness of conducting bronchi and bronchioles is approximately proportional to their diameter. Bronchi with a wall thickness of less than 300 um is not visible on CT or HRCT. As a consequence, normal bronchi less than 2 mm in diameter or closer than 2 cm from pleural surfaces equivalent to seventh to ninth order airways are generally below the resolution even of high-resolution CT
    60. 60. There are approximately 23 generation ofdichotomous branchingFrom trachea to the alveolar sacHRCT can identify upto 8th order centralbronchioles
    61. 61.  Lung is supported by a network of connective tissue called interstitium Interstitium not visible on normal HRCT but visible once thickened. Interstitium is constituted by  AXIAL fibre system (peribronchovascular & centrilobular),  PERIPHERAL fibre system (subpleural & interlobular septa) and  SEPTAL fibre system (intralobular septa)
    62. 62. Lung interstitium Axial fiber Peripheral fiber system sysemPeribronchovascular Centrilobular Subpleural Interlobular interstitium interstitium interstitium septa
    63. 63.  The peribronchovascular interstitum invests the bronchi and pulmonary artery in the perihilar region. The centrilobular interstitium are associated with small centrilobular bronchioles and arteries The subpleural interstitium is located beneath the visceral pleura; envelops the lung into fibrous sac and sends connective tissue septa into lung parenchyma. Interlobular septa constitute the septas arising from the subpleural interstitium. 108
    64. 64.  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.
    65. 65.  Secondary lobulus  Most important structure  Smallest functional unit seen on CT Scans Interstitium Inter‐/intralobar septum Central artery Central bronchiolus
    66. 66.  Interlobular septa and contiguous subpleural interstitium, Centrilobular structures, and Lobular parenchyma and acini.
    67. 67. A group of terminal bronchioles
    68. 68. Accompanying pulmonary arterioles
    69. 69. Surrounded by lymph vessels
    70. 70. Pulmonary veins
    71. 71. Pulmonary lymphatics
    72. 72.  Pulmonary lymphaticsConnective Tissue Stroma
    73. 73.  Primary Lobule: Lung parenchyma associated with a single Alveolar duct. 4-5 Primary Lobules  Acinus
    74. 74. The normal pulmonary vein branches are seen marginating pulmonary lobules.The centrilobular artery branches are visible as a rounded dot
    76. 76. ESOPHAGUSRB1 LB1
    82. 82. LB2 RB2RB6 LB6RB9 LB10 RB10 RB7
    83. 83. RB1 LB1,2 ULRB2 UL ML LL LL LB9
    84. 84. RB1 joining RUL bronchus LB1,2 joining LUL bronchusRC2 LC2 CARINA RB8 LB8
    85. 85. RB1 LB3RB3 LB4 ML Bronchus RB8
    86. 86. RB3 LB3 LB5 RB4 RB5
    87. 87.  Internal diameter of bronchus and diameter of accompanying artery LEAST diameter is considered ,If obliquely cut section seen, Normal ratio is 0.65-0.70 B/A ratio increases with age .may exceed 1 in normal patients > 40 years.
    88. 88.  Wall thickness decreases as the airway divides. Wall thickness is proportionate to diameter T/D ratio approximates to 20% at any generation of airway. Assessment of bronchial wall thickness is quite subjective and is dependent on the window settings
    89. 89. •In an isolated lung, thesmallest bronchi visible(arrows) measures 2 to 3 mmin diameter.• Bronchi and bronchioles arenot visible within theperipheral 1 cm of lung.• Artery branches thataccompany these bronchi aresharply seen.
    90. 90. The diameters of vessels and bronchi are approximately equal.The outer walls of bronchi and vessels are smooth and sharp.Bronchi are invisible within the peripheral 2 cm of lung
    91. 91. . B: On an HRCT scan at the same level, interlobular septa can be seen marginating one ormore lobules. Pulmonary artery branches (arrows) can be seen extending into the centresof pulmonary lobules, but intralobular bronchioles are not visible. The last visiblebranching point of pulmonary arteries is approximately 1 cm from the pleural surface.Bronchi are invisible within 2 or 3 cm of the pleural surface
    92. 92. •Thin white line (large arrows).•Combined thickness of visceral ,parietal pleurae, pleural space,endothoracic fascia, and innermostintercostal muscle•Separated from the more externallayers of the intercostal muscles bylayer of intercostal fat.•Posteriorly, the intercostal stripe(small arrows) is visible anterior tothe lower edge of a rib.
    93. 93. •In the paravertebral regionsinnermost intercostal muscle isabsent,•At most, a very thin line (theparavertebral line) is present atthe lung-chest wall interface.•Represents the combinedthickness of the normal pleurallayers and endothoracic fascia(0.2 to0.4 mm)•As in this case, a distinct linemay not be seen