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Multi–Detector Row CT: Principles and Practice for Abdominal Applications¹

¹ From the Department of Radiology, Harvard Medical School and Massachusetts General Hospital, 32 Fruit St, Boston, MA 02114. Received June 24, 2003; revision requested August 29; revision received September 25; accepted October 28. Address correspondence to the author (e-mail: ssaini@partners.org).

	ABSTRACT

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
Abdominal imaging with multi–detector row computed tomography (CT) can be performed during short breath holds. On 16-channel multi–detector row CT scanners, the effective detector row thickness, depending on the manufacturer, is typically 1.0–1.5 mm, which results in a beam collimation of 16–24 mm. At a gantry rotation speed of 0.5 second and a pitch of 1, the table travel speed will be 32–48 mm/sec. At a smaller effective detector row thickness and a narrower beam collimation, a slightly higher pitch may be needed to obtain short–breath-hold CT scans. Typically, transverse scans are viewed at a reconstructed section thickness of 3–5 mm, with thinner sections used for CT angiography and whenever off-axial reformations are obtained. The radiologic technique should be optimized according to the transverse section thickness used for interpretation, and the contrast material administration protocol should be optimized according to the clinical problem, with the scanning triggered for enhancement of a target organ.

© RSNA, 2004

Index terms: Abdomen, CT, 78.1211 • Computed tomography (CT), multi–detector row, 78.1211 • Computed tomography (CT), technology, 78.1211 • Radiology and radiologists, How I Do It

	INTRODUCTION

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
In the era before the advent of spiral computed tomography (CT), the main parameters that radiologists considered when setting up abdominal CT scanning protocols were scanning area, or range; section thickness; and interscan delay. With the introduction of spiral techniques, pitch became an additional important parameter because it affects scanning time and section thickness. Now, with the availability of multi–detector row CT, scanning protocols have become even more complex owing to the larger number of interacting operator-defined parameters.

This article provides an overview of multi–detector row CT technology and a practical approach to setting up abdominal scanning protocols.

	TECHNOLOGY REVIEW

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
Detector Design
Only the very first CT scanners were built with a single detector (1). However, since the 1980s, third-generation CT scanners have been the most common. These scanners have several hundred approximately 0.5-mm-high detectors stacked in an approximately 50° arc opposite the x-ray tube in the x-y plane (Figure, part a). The x-y plane is the plane in which the tube rotates, and the z-axis is the direction of table travel. On single–detector row CT scanners, each detector is about 20 mm long in the z-axis and the section thickness is determined according to the collimation of the incident x-ray beam.

View larger version (56K): [in this window] [in a new window] [Download PPT slide]   Figure a. Simplified depictions of detector arrangements in CT gantries. (a) On single-detector row CT scanners, several hundred approximately 20-mm-long individual detectors are stacked opposite the x-ray tube. The section thickness is determined according to the collimation (shaded area) of the incident x-ray beam. (b) On multi-detector row CT scanners, the individual detectors are segmented into smaller elements along the z-axis. Hence, the detector arrangement has a gridlike configuration. Again, the shaded area represents the collimation of the incident x-ray beam; however, the section thickness is now determined according to the thickness of each detector row.

View larger version (55K): [in this window] [in a new window] [Download PPT slide]   Figure b. Simplified depictions of detector arrangements in CT gantries. (a) On single-detector row CT scanners, several hundred approximately 20-mm-long individual detectors are stacked opposite the x-ray tube. The section thickness is determined according to the collimation (shaded area) of the incident x-ray beam. (b) On multi-detector row CT scanners, the individual detectors are segmented into smaller elements along the z-axis. Hence, the detector arrangement has a gridlike configuration. Again, the shaded area represents the collimation of the incident x-ray beam; however, the section thickness is now determined according to the thickness of each detector row.

The detector designs of four-channel multi–detector row CT scanners, however, are more variable and include a hybrid-array design, as described above; a matrix-array design, in which all detector rows are of uniform thickness; and an adaptive-array design, in which the detector row thickness increases from the center to the outside.

Data Channels
The concept of multi–detector row CT scanning is not new. An early first-generation CT scanner that had two separate detectors along the z-axis was introduced in the 1970s. However, this technologic innovation, which was designed to reduce the scanning time, was quickly eclipsed by second- and third-generation CT scanners that facilitated shorter scanning times.

Currently, multi–detector row CT scanners that have four to 16 channels are commercially available (2). Although at first glance it may appear that the number of sections per rotation equals the number of detector rows on a scanner, in actuality, the number of sections that are acquired depends on the number of z-axis data channels on the CT scanner. Thus, a four-channel multi–detector row CT scanner has four z-axis data channels and enables the acquisition of up to four sections per rotation, even though many more detector rows are present. Similarly, 16-channel multi–detector row CT scanners have 16 z-axis data channels and enable the acquisition of 16 or fewer sections per rotation, even though 24–40 detector rows may be present. The limited number of data channels is related primarily to the more complex reconstruction algorithms because the number of sections per rotation increases from four to eight to 16.

Effective Detector Row Thickness
If the incident x-ray beam covers a greater number of detector rows than the number of z-axis data channels available on the CT scanner, then the signal from adjacent detectors will be combined and the detectors will function as a single unit. The effective detector row thickness is simply the sum of the widths of the contributing detector rows for each channel. In multi–detector row CT scan acquisitions, the effective detector row thickness of all channels must be identical. Thus, for example, on a 16-channel multi–detector row CT scanner with a 24–detector row hybrid-array design in which the detector has 16 0.75-mm-wide inner rows and eight 1.5-mm-wide outer rows, when a 24-mm-wide incident beam that covers all 24 rows is used, the effective detector thickness will be 1.5 mm where the central 16 rows have been paired to make up eight data channels and the eight outer rows will function as individual data channels. Similarly, on a four-channel multi–detector row CT scanner with a matrix-array design in which all 16 rows of the detector have a width of 1.25 mm, when a 20-mm-wide incident beam that covers all 16 rows is used, the effective detector thickness will be 5 mm where the 16 rows have been grouped in sets of four (4 · 1.25 mm = 5 mm) to yield the four data channels.

Effective detector row thickness is an important parameter because the reconstructed section thickness cannot be smaller than the effective detector row thickness. Thus, if diagnostic images are to be viewed on 2.5-mm-thick sections, then the effective detector row thickness must not be more than 2.5 mm. At a given effective detector row thickness, the specific options that can be used with a given thickness of reconstructed sections depend on the detector design and the reconstruction algorithm from the manufacturer.

Detector Configuration
The term detector configuration succinctly describes a given scan acquisition mode in terms of the number of z-axis data channels being used and the effective detector row thickness of each data channel. Thus, a detector configuration of 16 x 1.25 refers to a 16–data channel acquisition performed with use of an effective detector row thickness of 1.25 mm, and a detector configuration of 4 x 2.5 refers to a four–data channel acquisition performed with use of an effective detector row thickness of 2.5 mm.

Beam Collimation
The beam collimation is simply the product of the number of data channels being used and the effective detector row thickness. Thus, for a 16 x 0.5-mm detector configuration, the beam collimation will be 8 mm, whereas for an 8 x 1.25-mm detector configuration, the beam collimation will be 10 mm. The beam collimation critically influences the table speed, as described in the following text.

In addition to table speed, the degree of patient exposure to x-rays also is influenced by the beam collimation because of the "overbeaming," or penumbra effect (3). Even when the incident x-ray beam is collimated to the targeted detector rows, the beam is always slightly wider than the rows. At multi–detector row CT, the incident x-ray beam is about 2 mm wider than the selected detector configuration. Hence, the wider the incident beam, the smaller the percentage of "wasted" radiation due to overbeaming. Another way of looking at this phenomenon is as follows: Because unused radiation is delivered to the patient owing to overbeaming, when the beam collimation is consequently reduced, for a given z-axis coverage at a constant pitch, the number of rotations will need to be increased, contributing "wasted" radiation. Thus, for example, a 16 x 0.625-mm acquisition will require twice as many rotations as a 16 x 1.25-mm acquisition, and, therefore, an approximately 3% higher radiation dose will be delivered.

Pitch
For multi–detector row CT, pitch is generally defined as the table travel per rotation divided by the collimation of the x-ray beam (4). This beam-pitch definition is preferred over the section-pitch definition (ie, table travel per rotation divided by effective detector row thickness) that is used by some manufacturers because the interpretation of the pitch value is more intuitive. Thus, a beam-pitch of 1.0 facilitates an acquisition with no overlap or gap, a beam-pitch of less than 1.0 facilitates an overlapping acquisition, and a beam-pitch of greater than 1.0 facilitates an interspersed acquisition. Pitch has a smaller effect on image quality with use of multi–detector row CT scanners than it does with use of single–detector row CT scanners (5).

Table Travel Speed
Table travel speed is the most important parameter that radiologists have to manipulate when they are setting up scanning protocols. Beam collimation, pitch, and gantry rotation time define table speed according to the following relationship: Table speed equals beam collimation times pitch times number of gantry rotations per second. Hence, an acquisition performed with a detector configuration of 16 x 1.5 mm scanned at a pitch of 1.0 and at a 0.5-second gantry rotation time will result in a table speed of 48 mm/sec (16 data channels times 1.5-mm detector row thickness per data channel times pitch of 1.0 per rotation times two rotations per second). Similarly, an acquisition performed with a detector configuration of 4 x 2.5 mm scanned at a pitch of 1.5 and at a 0.5-second gantry rotation time will result in a table speed of 30 mm/sec (four data channels times 2.5-mm detector row thickness per data channel times pitch of 1.5 per rotation times two rotations per second).

In contrast, with single–detector row CT, a 5-mm-thick section acquired at a pitch of 2.0 and at a 0.5-second gantry rotation time will result in a table speed of 20 mm/sec (one data channel times 5-mm detector row thickness per data channel times pitch of 2.0 per rotation times two rotations per second). Thus, one can see that with a higher number of z-axis data channels it is possible to perform scanning faster and to reconstruct thinner sections.

	RADIATION DOSE CONSIDERATIONS

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
Radiation Dose Measurement
For patients undergoing CT, the biological effect of radiation exposure is referred to as the effective dose, which is expressed in millisieverts (10 mSv = 1 rem). This parameter incorporates the exposure of various organs to x-ray radiation and the unique susceptibility of organs to injury from this exposure (6). Although it is not possible to precisely measure a patient’s effective dose for a given CT examination, on CT scanners manufacturers are obligated to report a calculated absorbed dose for a U.S. Food and Drug Administration–designed phantom. This metric is referred to as the volume CT dose index, which is expressed in milligrays (1 rad = 10 mGy). The volume CT dose index can be used to compare the radiation doses delivered in scan acquisitions performed with different techniques. Although the volume CT dose index is the dose absorbed in a given plane, the dose length product, which is expressed in milligrays times centimeters, incorporates the z-axis scanning area and is also reported on the scanner when a scan is prescribed. The dose length product increases with longer scanning ranges and multiphase examinations. Estimates of effective dose can be made on the basis of dose length product calculations by using published conversion factors (7). At present, however, neither of these metrics is available to radiologists who review images.

Radiologic Technique
The effective dose is directly proportional to the tube current and thus increases linearly with tube current, which is expressed in milliamperes. Although increasing the pitch at a constant tube current will lower the effective dose, the images will be noisier. To keep the noise level constant, the tube current must be increased to compensate for the higher pitch, and this increase will make the two acquisitions dose equivalent. Thus, at constant noise levels, the effective dose will be independent of pitch; hence, the evolution of the term effective milliampere, which is the ratio of the milliampere value divided by the beam-pitch. The relationship between effective dose and tube potential (expressed in peak kilovoltage) is nonlinear and more complex because at higher peak kilovoltage values, more x-rays pass through the body, and, thus, there is less absorption. However, the low-contrast soft-tissue resolution and the density of iodinated contrast material are diminished at higher peak kilovoltage settings (8).

At constant image noise levels, the effective dose delivered to obtain a scan at 120 kVp with a relatively higher milliampere value is similar to that delivered to obtain a scan at 140 kVp with a relatively lower milliampere value. Thus, the choice of peak kilovoltage–milliampere combination is left to individual radiologists (9). At my institution, our approach is to use 140 kVp for routine adult examinations because the x-ray absorption is lower at higher photon energies. However, for CT angiographic examinations, 120 kVp or even 100 kVp for small subjects is used because the attenuation by iodine atoms—that is, the contrast enhancement–increases as the peak kilovoltage approaches 80, when the photon energy (approximately 50% of the peak kilovoltage value) is closer to the k edge of iodine, which is 33.2 keV (8). In the near future, automatic exposure control techniques will facilitate automatic modulation and optimization of tube current according to the size and density of the body part being scanned (10).

	PROTOCOL DESIGN

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
Identifying the CT Scanning Range
When planning a CT image acquisition, the first decision is that of choosing the z-axis scanning range. At body CT, the most common choices are the chest-abdomen-pelvis, abdomen-pelvis, and abdomen-only regions. Depending on the patient’s size, this selection will determine the centimeters of coverage needed.

Selecting the Scanning Duration
The second decision is that of determining how much time is available to scan the desired z-axis coverage area (11–14). This time may be defined on the basis of the duration of peak contrast enhancement or the length of time that the patient is able to maintain a breath hold. In general, for body CT, scanning times shorter than 15 seconds are desirable to avoid breathing-related motion artifacts.

Determining the Table Speed
The ratio of scanning area to scanning duration is used to determine the slowest table speed needed to image the selected anatomic range within the desired time. For example, chest-abdomen-pelvis CT of a 700-mm region that must be scanned within 20 seconds will require a table speed of 35 mm/sec. By manipulating the number of z-axis data channels available, the effective detector row thickness, the scanning pitch, and the gantry rotation speed, the radiologist should be able to achieve the desired table speed. In general, the highest number of z-axis data channels and the fastest gantry rotation speed should be used; however, for large patients, the gantry rotation time can be increased if the scanner is not able to generate the desired tube current (9). Hence, the key parameter used to determine table speed that is prescribed by the radiologist is effective detector row thickness.

As noted earlier, when choosing the effective detector row thickness, one must know the smallest section thickness that may be needed for transverse image viewing. Because, to maintain constant image noise levels, the radiation dose must be increased as the thickness of the reconstructed section decreases (6), for body CT it is uncommon to view submillimeter images. Once the effective detector row thickness has been selected, the pitch is manipulated to get the desired table speed.

With 16-channel multi–detector row CT scanners, an effective detector row thickness of 1.0–1.5 mm is used. In contrast, with four-channel multi–detector row CT scanners, an effective detector row thickness of 2.5 mm is used, because at a lower effective detector thickness, the table speed is insufficient to image the abdomen and pelvis in less than 20 seconds. However, a smaller effective detector thickness can be used to image smaller regions.

Selecting the Thickness of Reconstructed Image Sections
Next, the radiologist needs to establish the thickness of the reconstructed transverse images. In the abdomen, typically reconstructed sections are 3–5 mm thick. Thinner reconstructed sections are primarily used for off-axial reformations and not for viewing transverse images, with the exception of CT angiograms of the liver, kidneys, and pancreas (11–19).

Choosing the Radiologic Technique
Finally, the radiologic parameters must be optimized on the basis of the clinical problem and the typical viewing section thickness. In general, for 5-mm sections, abdomen-only multi–detector row CT of an average-sized subject yields a volume CT dose index of approximately 15 mGy and an estimated effective dose of 5 mSv (9).

	CONTRAST MATERIAL ADMINISTRATION

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
The short (<15-second) acquisition times associated with multi–detector row CT have made it necessary to revise iodinated contrast material administration techniques. Although the specific protocols used for different applications are still evolving, as a rule, contrast material administration should end at least 15 seconds before the end of an image acquisition so that the "tail" of the injection is not wasted. Because the start of scanning is timed according to the arrival of contrast material in the target organ, for most applications, because of the short scanning times associated with multi–detector row CT, the duration of the contrast material injection must be decreased. Although this offers the opportunity to lower contrast material volumes and doses for CT angiographic examinations, for liver imaging, the total iodine dose needs to be maintained to achieve adequate hepatic enhancement (15). Hence, with a fixed iodine dose, if the contrast material volume and concentration are kept constant, then the injection rate must be increased to shorten the duration of injection.

For routine abdominal CT examinations in which imaging is performed during the portal venous phase, approximately 150 mL of a 300 mg/mL concentration of contrast material is injected at 3 mL/sec for multi–detector row CT, as compared with 2 mL/sec injected for single–detector row CT. A higher injection rate is necessary because at 2 mL/sec, the injection duration will be 75 seconds and the tail of the injection will not be used. If, however, a higher concentration of contrast material is used—for example, 370 mg/mL—then the same amount of iodine can be administered in 120-mL doses and an injection rate of 2 mL/sec will be adequate.

Thus, for routine abdominal CT, the main reason to use a higher concentration of contrast material is so that a lower injection rate can be used. For CT angiographic examinations, because injection rates of 4–5 mL/sec are customary, the use of even higher injection rates dictated by shorter scanning times can be avoided by increasing the contrast material concentration. Indeed, for CT angiographic examinations, in which scanning is timed according to the arrival of contrast material in the aorta, study results support scanning with lower volumes and a lower iodine dose by using higher concentrations of contrast material (370–400 mg/mL) injected at 4–5 mL/sec (15). This approach is also applicable to arterial phase imaging of the parenchyma of the liver and pancreas (11,19).

For all applications, the coordination of contrast material arrival and initiation of scanning is critical with multi–detector row CT. For angiographic examinations, our approach is to use aortic enhancement as a trigger and scanning is initiated immediately (<5-second delay for breathing instruction) at aortic enhancement of 150 HU. Arterial phase parenchymal (liver and pancreas) CT examinations are triggered following a 10-second delay after aortic enhancement of 150 HU, and mesenteric vascular imaging is triggered following a 25-second delay after aortic enhancement of 150 HU. During the hepatic portal venous phase, imaging is triggered at 55-HU hepatic enhancement with a default delay of 90 seconds. For renal imaging, there are no changes to the timing parameters used at single–detector row CT scanning performed at 100 seconds after the injection of 100 mL of a 300 mg/mL concentration of contrast material at 2 mL/sec.

	CONCLUSION

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 
This article provides an overview of multi–detector row CT technology and perspectives about the application of this modality in the abdomen. However, the reader should understand that CT scanning techniques and contrast material injection protocols continue to evolve and acceptable alternatives to those outlined herein may be equally effective.

	REFERENCES

TOP ABSTRACT INTRODUCTION TECHNOLOGY REVIEW RADIATION DOSE CONSIDERATIONS PROTOCOL DESIGN CONTRAST MATERIAL ADMINISTRATION CONCLUSION REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) All Versions of this Article: 2332030994v1 233/2/323    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Saini, S. Search for Related Content PubMed PubMed Citation Articles by Saini, S.