(Radiographics. 2002;22:697-700.)
© RSNA, 2002
Special Focus Session
Multisection (Multidetector) CT: Applications in the Abdomen1
Pablo R. Ros, MD, MPH and
Hoon Ji, MD, PhD
1 From the Department of Radiology, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115. Received November 27, 2001; accepted December 12. Address correspondence to P.R.R. (e-mail: pros@partners.org).
Index Terms: Abdomen, CT, 70.12119 • Computed tomography (CT), multi–detector row, 70.12119
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Introduction
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In CT you can’t be too thin or too fast.—Modern radiology proverb, modified from the 1930s Hollywood proverb, "You can’t be too thin or too rich."
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Introduction
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The development of multisection or multidetector row computed tomography (CT) has prompted CT to regain its place as the most exciting technique in body imaging, a position lost to magnetic resonance imaging since the mid-1980s.
The Radiological Society of North America recognized the importance of multisection CT in body imaging, dedicating a special focus session to this topic at the 2000 annual meeting. I was given the opportunity to moderate this session on the applications of multisection CT in the chest and abdomen. The following article by W. Dennis Foley, MD, discusses the applications of multisection CT in the major abdominal viscera. Other topics discussed during the special focus session (vascular and chest applications) are not included to keep the focus in the abdomen. The following comments provide a foundation for Dr Foley’s article emphasizing the possibilities of multisection CT in the abdomen.
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Definition
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At the core of any CT unit capable of providing multiple sections with a single pass of the x-ray tube, there lie multiple rows of detectors instead of a single row of detectors. Multisection CT units have from eight to 32 physically available detector rows, although to date the maximum number of sections produced in clinical units has been only four. There are prototypes capable of generating eight images for each x-ray tube pass, and the first clinical units are scheduled to be deployed during 2002.
Because of their multiple detector rows, multisection CT units are much faster and have a higher z-axis resolution than single-section CT units.
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Advantages in Abdominal Imaging
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A key advantage of multisection CT is its speed. Combining a multirow detector array and a reduced gantry rotation time, multisection CT can be five to eight times faster than single-row spiral CT (1,2).
This, of course, allows scanning of a very large volume very quickly and therefore opens to CT many applications in the abdomen that were not possible with spiral CT. Typically, in scanning the abdomen with spiral CT, one trades resolution for space covered, since the area to be scanned is typically 40–60 cm from the lung bases to the pubis. Multisection CT can cover the entire abdomen with high-resolution images.
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Disadvantages
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Some of the disadvantages are related to the basic physical principles of multisection CT. Since the x rays are tilted in the outer rows of the detector array, a cone angle is created. In addition, since the x rays will wobble like a top in the circumferential rotation, cone artifacts are formed. In the outer rows, the cone beam artifact could be as high as 1 mm. However, in clinical practice, cone beam artifacts have not limited the applications of multisection CT in the abdomen.
Another drawback is the large data sets, typi-cally between 500 and 1,000 images. This of course makes mandatory the use of workstations rather than film to analyze abdominal multisection CT data.
Finally, with much higher speed, the timing of the enhancement is crucial.
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Clinical Applications
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Because of the fast speed of multisection CT, typically 1-mm-thick sections are possible in the abdomen, allowing isotropic volumetric imaging and high resolution. This is, of course, the key for three-dimensional display of abdominal organs. Therefore, for the first time in abdominal CT, one can perform multiplanar viewing, even of curved structures such as the pancreatic duct or the biliary tree (Fig 1).
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Figure 1a. Three-dimensional imaging of the pancreas. (a, b) Axial CT images show the planes used for curved coronal reconstruction. (c) Curved coronal reconstruction image shows the pancreatic duct. (d) Volume-rendered image shows the pancreas and its relationship to adjacent organs and vessels.
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Figure 1b. Three-dimensional imaging of the pancreas. (a, b) Axial CT images show the planes used for curved coronal reconstruction. (c) Curved coronal reconstruction image shows the pancreatic duct. (d) Volume-rendered image shows the pancreas and its relationship to adjacent organs and vessels.
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Figure 1c. Three-dimensional imaging of the pancreas. (a, b) Axial CT images show the planes used for curved coronal reconstruction. (c) Curved coronal reconstruction image shows the pancreatic duct. (d) Volume-rendered image shows the pancreas and its relationship to adjacent organs and vessels.
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Figure 1d. Three-dimensional imaging of the pancreas. (a, b) Axial CT images show the planes used for curved coronal reconstruction. (c) Curved coronal reconstruction image shows the pancreatic duct. (d) Volume-rendered image shows the pancreas and its relationship to adjacent organs and vessels.
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In solid organs, there is improved lesion detection because (a) thinner sections are routinely obtained; (b) motion artifacts are reduced; and (c) enhancement can now be multiphasic, with three phases of enhancement routinely imaged.
Finally, high-resolution, three-dimensional reconstruction is possible, providing the tools for virtual colonography, CT angiography (both arteriography and venography), and routine production of three-dimensional models for surgical planning.
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Scanning Parameters
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Multisection CT not only promises improved clinical usefulness in the abdomen but also provides a challenge for radiologists, since the quality of the images is determined by a number of scanning parameters that we can modify as levers to optimize our protocols. The appropriate collimation, pitch, reconstruction interval, time of scanning, and contrast material are crucial to produce high-quality images.
To improve spatial resolution, collimation is narrowed, but the trade-off is increased image noise and reduced length of coverage (3–6). Therefore, the choice of collimation in the abdomen depends in clinical practice on the purpose of the study and the specific structures being imaged. For example, to detect small pancreatic tumors, such as in cases of suspected insulinoma or ampulloma, a narrow collimation is preferred because of the small area of coverage and the de-sired higher spatial resolution (Fig 2). On the other hand, for imaging studies that require a long length of coverage, such as CT colonography (Fig 3) or CT angiography, maintaining a narrowcollimation may be possible only by increasing the pitch. This is the only way to overcome the otherwise reduced coverage encountered with narrow collimation. These examples demonstrate how the scanning parameters are interrelated and must be balanced to achieve the optimum result.
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Figure 2a. Ampulloma imaged with 1-mm and 2.5-mm collimation. (a) Axial CT image obtained with 1-mm collimation and 1.25-mm section thickness shows a low-attenuation mass (arrow) in the head of the pancreas. (b) Axial CT image obtained with 2.5-mm collimation and 3-mm section thickness shows the lesion indistinctly. Note the difference in image noise between the images.
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Figure 2b. Ampulloma imaged with 1-mm and 2.5-mm collimation. (a) Axial CT image obtained with 1-mm collimation and 1.25-mm section thickness shows a low-attenuation mass (arrow) in the head of the pancreas. (b) Axial CT image obtained with 2.5-mm collimation and 3-mm section thickness shows the lesion indistinctly. Note the difference in image noise between the images.
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Figure 3a. Virtual colonoscopy in a patient with a polyp. (a) Virtual colonogram shows a polyp (arrow). (b) Axial CT image obtained with a modified lung window shows a polyp (arrowhead) in the proximal transverse colon.
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Figure 3b. Virtual colonoscopy in a patient with a polyp. (a) Virtual colonogram shows a polyp (arrow). (b) Axial CT image obtained with a modified lung window shows a polyp (arrowhead) in the proximal transverse colon.
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For a given scan duration, increasing the pitch will allow a greater coverage area of scanning. However, maximizing the pitch results in a decrease in contrast resolution (4–7).
In multisection CT, there are two distinct versions of pitch, depending on whether the entire beam collimation is considered or just the detector collimation (beam divided by number of rows). For example, in a four–detector row scanner with a 10-mm beam collimation and a 10-mm table feed per rotation, the pitch could be 1 if beam collimation is considered or 2.5 if detector collimation is considered.
The most appropriate choice of pitch depends on the area or problem to be scanned. In CT angiography, maximizing the pitch is preferred because the loss of contrast resolution between high-attenuation vessels and the background is negligible. On the other hand, in the liver or the pancreas, the possible loss of contrast resolution with a high pitch should be taken into consideration and therefore the pitch should be increased only to cover the liver in its entirety.
In general, the smaller the reconstruction interval the greater the longitudinal (z-axis) resolution, with loss of coverage in the z axis (3). The benefit of overlapping reconstruction is greatest when nonaxial reconstruction is anticipated. In most cases, if nonaxial reconstruction is not anticipated, contiguous reconstruction is sufficient.
In general, administration of intravenous contrast media is essential for multisection CT applications in abdominal imaging. The amount of contrast medium has not been reduced with the advent of multisection CT. Intravenous contrast material administration is advocated not only for visceral applications but also for study of the intestine.
Use of oral contrast material has changed with multisection CT because of the higher frequency of nonaxial reconstruction. In these cases, high-attenuation oral contrast material degrades the reconstructed images. Likewise, when CT angiography is to be performed, as in cases of vascular disease or bowel ischemia as well as any other vascular reconstruction for visceral arteries or veins, water is preferred as the oral contrast agent.
The technique for abdominal multisection CT regarding scanning delays depends on the organ being studied. In general, multiple phases are imaged. The timing of scanning and the technique of contrast material administration are discussed in the following article by Dr Foley.
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Conclusions
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Multisection CT offers improved scanning speed and therefore the ability to image larger areas with thinner sections than have been previously available. These strengths, with the added benefit of improved scanning timing, improve the diagnosis and staging of abdominal diseases in both the solid viscera and the intestine. CT angiography opens new opportunities for CT evaluation of patients with suspected vascular diseases. Finally, the advent of volume rendering of multisection CT data sets in conjunction with improvements in three-dimensional software opens a new era of CT-based abdominal three-dimensional imaging.
We should consider that in the immediate future multisection CT would be able to image the abdomen independently of any planes. Once the raw data are acquired and sent to a workstation, the radiologist will use the CT data as he or she currently uses an ultrasound transducer, visualizing any plane of the abdominal anatomy.
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References
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Rydberg J, Buckwalter KA, Caldemeyer KS, et al. Multisection CT: scanning techniques and clinical applications. RadioGraphics 2000; 20:1787-1806.[Abstract/Free Full Text]
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Hu H, He HD, Foley WD, Fox SH. Four multidetector-row helical CT: image quality and volume coverage speed. Radiology 2000; 215:55-62.[Abstract/Free Full Text]
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Wang G, Vannier MW. The effect of pitch in multislice spiral/helical CT. Med Phys 1999; 26:2648-2653.[CrossRef][Medline]
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Introduction
Definition
