Fundamental Advantages of MDCT

The first advantage of MDCT is the ability to scan a lot faster while acquiring a dataset with similar slice thickness and image quality to that of SDCT. Faster table speeds are particularly advantageous in patients with reduced breath-holding capacity since table speeds on the order of 50 to 70 mm per second are achievable. Fast table speeds are often chosen in the setting of trauma, in patients with dyspnea, in young children or in those patients in whom there are communication difficulties such as a language barrier or deafness.

Mdct Advantages
Figure 17 Volume rendered projection CT angiogram of the abdominal aorta and iliac arteries revealing a large infrarenal aortic aneurysm. It is noted that the entire dataset was acquired during a single breath-hold on the order of 15 seconds.

The second advantage of MDCT is the ability to scan faster while acquiring a dataset with a similar slice thickness but better image quality, compared to SDCT. For example, this is the strategy used for routine imaging of the abdomen and pelvis in patients with average breath holding capacity. With these parameters, the entire abdomen and pelvis (350-400 mm of longitudinal coverage) can be scanned in a single breath-hold of 12 to 20 seconds in duration (Fig. 16). It is recommended that faster gantry rotation speeds (e.g., 0.5 seconds per rotation) should be selected whenever possible. Although images acquired using shorter gantry rotation times are reconstructed using fewer trajectories in the algorithm (i.e., undersampling), image quality is not significantly degraded (31). An exception might be encountered when scanning through the shoulders and bony pelvis or in large patients where aliasing in the form of streak artifacts may be encountered.

Figure 18 (A) Axial CT colonographic image acquired following insufflation of the entire colon with air. A moderate-sized polyp is noted in the cecum (arrow). It is noted that the internal attenuation of the polyp is similar to adjacent retroperitoneal fat, consistent with a lipoma. The lipoma is also well visualized on the coronal (B) and the sagittal (C) reformation and on a virtual endoscopic view (D).

Figure 18 (A) Axial CT colonographic image acquired following insufflation of the entire colon with air. A moderate-sized polyp is noted in the cecum (arrow). It is noted that the internal attenuation of the polyp is similar to adjacent retroperitoneal fat, consistent with a lipoma. The lipoma is also well visualized on the coronal (B) and the sagittal (C) reformation and on a virtual endoscopic view (D).

Figure 19 (A) An axial image through the upper abdomen reveals an approximately 2 cm hypoattenuating mass in the pancreatic head (arrow). The mass abuts the superior mesenteric vein but does not appear to invade the wall or lumen. (B) Antero-posterior volume rendered projection image reveals conventional anatomy as well as an unusual "hairpin" course of the proximal proper hepatic artery, without evidence of narrowing or irregularity (arrow). (C) Curved planar reformation through the main pancreatic duct reveals dilatation down to the head of the pancreas where there is a hypoattenuating mass (M). Curved planar reformations through the splenic artery (D), and superior mesenteric artery in both the coronal (E) and sagittal (F) planes reveal no evidence of tumor encasement or adjacent lymphadenopathy. (G) Curved planar reformation through the hepatic artery reveals circumferential tumor encasement at the level of the "hairpin" tortuosity (arrow). (Continued on next page).

Figure 19 (A) An axial image through the upper abdomen reveals an approximately 2 cm hypoattenuating mass in the pancreatic head (arrow). The mass abuts the superior mesenteric vein but does not appear to invade the wall or lumen. (B) Antero-posterior volume rendered projection image reveals conventional anatomy as well as an unusual "hairpin" course of the proximal proper hepatic artery, without evidence of narrowing or irregularity (arrow). (C) Curved planar reformation through the main pancreatic duct reveals dilatation down to the head of the pancreas where there is a hypoattenuating mass (M). Curved planar reformations through the splenic artery (D), and superior mesenteric artery in both the coronal (E) and sagittal (F) planes reveal no evidence of tumor encasement or adjacent lymphadenopathy. (G) Curved planar reformation through the hepatic artery reveals circumferential tumor encasement at the level of the "hairpin" tortuosity (arrow). (Continued on next page).

The second advantage of MDCT noted above is particularly useful for multiplanar reformations and CTA (36). When large areas of anatomy need to be scanned such as the abdominal aorta and iliac arteries (Fig. 17) or in CT-colonography (Fig. 18), faster table speeds are selected to complete the acquisition during a reasonable breath-hold and/or during peak arterial enhancement (37,38). On a four-slice scanner, the parameters for such a protocol would include a 4 x 2.50 mm detector configuration and a pitch of 1.5. This dataset will yield relatively high-resolution images for 3D and multiplanar reformations, particularly for larger blood vessels that are perpendicular to the axial plane.

The third advantage of MDCT includes the ability to scan during a similar time with either thinner slices and/or much better quality images, compared to

Blood Thinners Hold Times

SDCT. These datasets can be acquired during a comfortable breath-hold (10-20 seconds) with reasonable anatomic coverage (200-300 mm). For example, in the abdomen, this advantage is particularly applicable when evaluating single organs such as the liver, pancreas, or kidneys. As less anatomic coverage is needed to scan these organs, slower table speeds and thinner slices can be used to provide very high quality datasets for 3D and multiplanar reformations (28). Specific applications include the staging of pancreatic adenocarcinoma (Fig. 19), evaluation of the liver prior to hepatic tumor resection (Fig. 20) or transplantation, and the evaluation of the kidney prior to partial or donor nephrectomy.

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