4 Computed Tomography

DR MUHAMMAD BIN ZULFIQARPGR III FCPS Services institute of Medical

Sciences/ Services Hospital LahoreGRAINGER & ALLISON’S DIAGNOSTIC RADIOLOGY

• FIGURE 4-1 CT performance. Performance ■doubled every two years. This trend has stopped around 2010.

• FIGURE 4-2 (A) Multidetector (MDCT) ■Principle. Modern multidetector CT machines use a third-generation setup with a rotating tube–detector combination. The detector is split into multiple thin parallel detector rows. (B) Dual-source CT. Two tube– detector units are combined in one machine. This requires only a quarter rotation to complete a 180° half-scan. (C) z-Flying focal spot (from Flohr et al.16). A z-FFS rapidly toggles the position of the focal spot in the z-direction. This produces twice the number of projections per rotation but does not increase detector width, image acquisition speed or number of available detector rows. In addition, the tissue covered by the two beams is separated towards the tube and overlaps towards the detector.

• FIGURE 4-3 Two pairs of images demonstrating the effect of ■using different reconstruction kernels. Images through the thorax on lung windows at the level of the aortic arch using (A) soft tissue and (B) lung algorithms show the latter to give a much sharper image of the focal lesion within the left upper lobe and of the background emphysema. Images through the upper lumbar spine on bone windows using (C) soft tissue and (D) bone algorithms show the latter to give a much sharper image of the focal metastatic deposit within the vertebral body.

• FIGURE 4-3 Two pairs of images demonstrating the effect of ■using different reconstruction kernels. Images through the thorax on lung windows at the level of the aortic arch using (A) soft tissue and (B) lung algorithms show the latter to give a much sharper image of the focal lesion within the left upper lobe and of the background emphysema. Images through the upper lumbar spine on bone windows using (C) soft tissue and (D) bone algorithms show the latter to give a much sharper image of the focal metastatic deposit within the vertebral body.

• FIGURE 4-4 Comparison of filtered back projection (FBP). ■(A) and (C) and iterative reconstructions (IR; (B) and (D)). If the filter settings of IR are set to maximum noise reduction, a plastic look with isolated noise pixels may arise (B). For appropriate settings, image quality can be substantially improved, especially in areas of high absorption, such as the shoulders, or high contrast, such as the lungs (D).

• FIGURE 4-4 Comparison of filtered back projection (FBP). ■(A) and (C) and iterative reconstructions (IR; (B) and (D)). If the filter settings of IR are set to maximum noise reduction, a plastic look with isolated noise pixels may arise (B). For appropriate settings, image quality can be substantially improved, especially in areas of high absorption, such as the shoulders, or high contrast, such as the lungs (D).

• FIGURE 4-5 Effect of imaging and reconstruction thickness. Sagittal ■images through the lumbar spine using (A) 1-mm imaging reconstructed in 1-mm-thick slices, (B) 1-mm-thick slices reconstructed in 5-mm-thick slices and (C) 5-mm-thick images reconstructed in 5-mm thick slices. Note the progressive deterioration in image detail, development of a step artefact and blurring of the images.

• FIGURE 4-5 Effect of ■imaging and reconstruction thickness. Sagittal images through the lumbar spine using (A) 1-mm imaging reconstructed in 1-mm-thick slices, (B) 1-mm-thick slices reconstructed in 5-mm-thick slices and (C) 5-mm-thick images reconstructed in 5-mm thick slices. Note the progressive deterioration in image detail, development of a step artefact and blurring of the images.

• FIGURE 4-6 CT-guided ■drainage procedure. The diagnostic study (A) demonstrates an abscess within the sigmoid mesentery following an episode of diverticulitis. Under CT guidance (B) a drainage catheter is inserted from the left side of the anterior abdominal wall and the abscess completely drained.

• FIGURE 4-7 ECG-synchronised CT acquisitions. Spiral CT with ■ retrospective gating (A) keeps the tube current constant throughout the entire data acquisition process while only a small portion (8–20%) of the data (grey) is used for image reconstruction. ECG dose modulation (B) reduces the tube current during systole and has a higher dose efficiency of 16–40%. Prospective triggering (C) with a stop-and-start technique uses 50–100% of the data, depending on the amount of padding. ‘Flash scanning’ (D) uses a very fast spiral data acquisition process with prospective triggering, and has a dose efficiency of nearly 100%.

• FIGURE 4-8 Effect of cardiac gating. Conventional ■axial CT image (A) through the left atrium in a patient with a history of ischaemic stroke shows extensive motion artefacts. However, retrospective gating of the images (B) clearly demonstrates thrombus within the left atrial appendage.

• FIGURE 4-9 (A) The Hounsfield units of different structures ■vary with the energy of the X-ray photons and, more importantly, so does the rate of change. Dual-energy imaging exploits this to identify individual chemical components within a structure. Imaging of a thoracic aortic dissection with (B) conventional unenhanced and (C) arterial phase imaging. A ‘virtual unenhanced’ image (D), created by subtracting the iodine from the arterial phase study, enhances the conspicuity of the dissection flap and removes the need for the unenhanced phase.

• FIGURE 4-9 (A) The Hounsfield units of different structures vary with ■the energy of the X-ray photons and, more importantly, so does the rate of change. Dual-energy imaging exploits this to identify individual chemical components within a structure. Imaging of a thoracic aortic dissection with (B) conventional unenhanced and (C) arterial phase imaging. A ‘virtual unenhanced’ image (D), created by subtracting the iodine from the arterial phase study, enhances the conspicuity of the dissection flap and removes the need for the unenhanced phase.

• FIGURE 4-10 Perfusion CT in left middle cerebral artery ■territory infarct. Mean transit time (MTT) map (A) shows an area of delayed MTT in the posterior part of the left middle cerebral artery territory. Cerebral blood flow (CBF) map (B) shows a larger area of reduced CBF indicating ischaemic and infarcted tissue. Cerebral blood volume CBV map (C) shows a small area of reduced CBV in the left parietal convexity corresponding to core infarct. The area of mismatch between the regions of reduced CBF and CBV is potentially salvageable ischaemic penumbra.

• FIGURE 4-10 Perfusion CT in left middle cerebral artery territory ■infarct. Mean transit time (MTT) map (A) shows an area of delayed MTT in the posterior part of the left middle cerebral artery territory. Cerebral blood flow (CBF) map (B) shows a larger area of reduced CBF indicating ischaemic and infarcted tissue. Cerebral blood volume CBV map (C) shows a small area of reduced CBV in the left parietal convexity corresponding to core infarct. The area of mismatch between the regions of reduced CBF and CBV is potentially salvageable ischaemic penumbra.