Applied Radiology

The advent of spiral CT scanning has provided the opportunity for optimization of CT data sets by coupling fast scanning with rapid contrast delivery. Spiral CT scanning provides the ability to acquire volumetric CT data in a single breath-hold acquisition. Current scanners allow single acquisitions in the 40 to 50 second range, as well as the ability to acquire back-to-back spiral scans with minimal interscan delay time (generally less than 10 seconds). This rapid scanning stresses the importance of coordinating contrast administration and scanning to maximize the detection of pathology.1-3

Traditional methods for timing contrast delivery include the test bolus technique, which is time consuming, or the use of standard set scan delay intervals, which ignores individual variations in circulation time and contrast enhancement. There is increased need for easy to use programs that promise individualized control of the timing of contrast administration and data acquisition.

GE previously introduced SmartPrepTM (General Electric Medical Systems, Milwaukee, WI), an automated computer technique designed to tailor scans for the individual patients. Published results have been encouraging.4-6 We recently tested an even newer program called the C.A.R.E. Bolus (Combined Application to Reduce Exposure). This software program allows the physician to monitor an individual patient's contrast enhancement curve over a region of interest, and then it automatically initiates the spiral scan when a predetermined threshold enhancement level is reached. Herein, we report our initial experience with this new technique.

Materials and methods

C.A.R.E. Bolus is a commercially available software upgrade for the Siemens Plus CT Scanner. This program allows the operator to chose a contrast enhancement threshold for a region of interest, which will then automatically trigger the data acquisition once the preselected density threshold is reached.

The program is fairly straight forward. The operator first selects an appropriate level on the initial scout topogram (figure 1A) and obtains a single low-dose axial scan using mAs 90, kVp 120 and a collimation equal to the collimation for the subsequent spiral CT scan (typically 3 to 5 mm). A region of interest is then chosen (i.e. the aorta) for contrast flow and density monitoring (figure 1B). If monitoring of different vascular structures or organs is desired, a maximum of four regions of interest can be placed; however, only one of the regions of interest can be used to trigger the scan mode. Next, the user enters a threshold value (i.e. 150 HU) as the trigger for the initiation of the spiral scan (figure 1C). A sampling start time and rate, which specifies when and how often monitoring scans and contrast density plots will be displayed, also can be set for the area of interest. The frequency can be selected for up to 10 monitoring scans. Once the selected threshold is crossed, the system automatically begins scanning at the pre-selected start point (figure 1D). There is a delay of approximately 3 to 7 seconds from the time the threshold is crossed until the data acquisition starts. The three second delay is standard if the region of interest is selected as either the upper or lower border of the subsequent scan. If necessary, the trigger threshold can be adjusted, either higher or lower, during the examination.

In our initial protocol, we selected the aorta as the region of interest. Our sampling scans and plots were set to begin at 20 seconds after initiation of contrast injection and to continue at 5 second intervals until the threshold trigger was reached and the data acquisition automatically started. We discovered that an aortic threshold trigger of 150 Hounsfield units was most useful, as this represents a midpoint on the upward density curve and allows for the short delay (3 to 5 seconds) between the threshold trigger and the start of the scan. However, the sampling can occur as early as the specific clinical application might dictate.

Patients chosen for evaluation included those with thoracic or abdominal pathology where arterial phase imaging was desirable.

Discussion

Our initial experience with the C.A.R.E. Bolus has been promising. The program is easy to use, flexible, and allows each scan to be tailored to a patient's unique contrast enhancement profile. This program should be especially useful for CT angiography or in any study where arterial phase imaging is desired.7

More controlled studies are needed to determine the best utilization of these new computer programs in order to maximize the detection of pathology by individualizing the timing of rapid contrast administration and scanning. AR

References

1. Bluemke DA, Soyer P, Fishman EK: Helical (spiral) CT of the liver. Radiol Clin North Am 33(5): 863-886, 1995.

2. Oliver JH, Baron RL, Federle MP, Rockette HE, Jr.: Detecting hepatocellular carcinoma. Value of unenhanced or arterial phase CT imaging or both used in conjunction with conventional portal venous phase contrast enhanced CT imaging. AJR 167(1):71-77, 1996.

3. VanHoe L, Gryspeerdt S, Marchal G, et al: Helical CT for the preoperative localization of islet cell tumors of the pancreas; value of arterial and parenchymal phase images. AJR 165(6):1437-1439, 1995.

4. Silverman PM, Roberts SC, Ducic I, et al: Assessment of technology that permits individualized scan delays on helical hepatic CT: A technique to improve efficiency of contrast material. AJR 167:79-84, 1996.

5. Silverman PM, Brown B, Wray H, et al: Opti-

mal contrast enhancement of the liver using helical (spiral) CT of the liver: Value of Smart Prep. AJR

164:1169-1171, 1995.

6. Silverman PM, Roberts SC, Tefft MC, et al: Helical (spiral) CT of the liver; clinical application of an automated computer technique, Smart Prep, for obtaining images with optimal contrast enhancement. AJR 165:73-78, 1995.

7. Zeman RK, Silverman PM, Vieco PT, Costello P: CT angiography. AJR 165(5):1079-1088, 1995.

Dr. Horton, Dr. Urban, and Dr. Fishman are with Johns Hopkins Hospital in Baltimore, MD; Dr. Starr is with Siemens Medical Systems in Iselin, NJ, where Dr. Fishman is a consultant and a member of the CT Advisory Board.