|Stuart G. Silverman, MD
It is difficult to remember the days before spiral (helical) CT scanners. In the decade since their introduction, the advantages of those scanners (volumetric imaging, reduced motion artifacts, shorter scanning times, increased patient throughput, and the potential for new studies such as CT angiography) caused the previous generation of machines almost to disappear. The next hardware development was even more dramatic. In a 1-second rotation, a four-slice multidetector-array CT (MDCT) scanner can capture as much as eight times the volume available from a single-slice CT (SSCT) scanner.1 A virtual copy of a trauma patient can be created with a whole-body protocol in a room time of 23 minutes.2 Structures as small as 0.5 mm can be depicted. Complex anatomy can be reconstructed in any orientation. More areas of the body and types of pathology are accessible. The blood supply to a hepatic lesion can be depicted more rapidly and more accurately,3 and more hepatic lesions can be detected and characterized,4 improving the staging of many cancers. It is easier to determine whether a pancreatic cancer is resectable.5 The kidneys and ureters can be captured while the patient holds a single breath, identifying a ureteral stone as small as 1 mm6 or helping surgeons plan laparoscopic nephrectomy. Other applications are CT colography (virtual colonoscopy), pulmonary angiography,7 quantification of coronary calcium, and, in some patients, evaluation of coronary artery disease.8
|(a) Contrast enhanced 16-slice CT angiography of the abdominal aorta and its branches. Colored volume rendering technique. 16-slice CT’s unprecedented scan speed allows for thin slice acquisition (0.75 mm), increased z-axis coverage, and shorter scan times. These improvements allow for superior spatial resolution resulting in crisp and clear depiction of fine vascular details with visualization of even the smallest branches of the abdominal vessels in a rapid, non-invasive fashion using 16-slice MSCT angiography.
The four-slice MDCT scanners were only the beginning. With today’s 16-slice machines, a single scan can encompass as much as 157 cm of anatomy, and the total examination time is less than 10 minutes for many routine procedures. Temporal resolution is as fine as 100 to 500 milliseconds, and real-time reconstruction rates as high as six images per second are possible. Options such as fly throughs, perfusion imaging, image fusion, coronary calcium scoring, and bone-mineraldensity measurement are available.
Brigham and Women’s Hospital, Boston, has eight CT scanners. In June 2002, it installed a Siemens Somatom Sensation 16 and is now employing it for approximately 35 procedures per day. Stuart G. Silverman, MD, is director of abdominal imaging and intervention and director of CT service at the hospital and associate professor of radiology, Harvard Medical School. He says, “We are doing more CT colography studies and more CT angiograms, such as those of the pulmonary circulation, since we got the 16-slice scanner, but the main feature that has created enthusiasm for the new scanner is the potential to image the coronary arteries in a manner that will compete with catheter angiography.” The 16-slice scan’s speed reduces the artifacts caused by cardiac motion, and its high resolution may permit assessment of the main coronary vessels for stenosis. Silverman adds, “For years, MRI was touted as the answer to coronary imaging without catheter angiography, but I think that this scanner is going to challenge that idea. Defining their relative roles will take more studies. I suspect that CT will prove better for imaging the arteries and MRI, for studying ventricular function.”
|(b)16-slice MSCT angiography of lower-extremity run-off in a patient with bilateral aneurysms of the common iliac arteries. A range from the renal arteries to the ankles is covered with 16×1.5-mm collimation and 22 seconds total scan time. Colored volume rendered display of the entire scan range.
The MDCT machines, particularly the 16-slice models, are creating numerous challenges. One is creation of the appropriate protocols to obtain the desired information while minimizing the radiation dose. Another is finding time to examine all the images produced.9 A study performed using a scanner built in 1980 yielded about 40 images. An SSCT scanner produces about 150 images, whereas an MDCT study may produce more than 500. The sheer volume has rendered impossible the traditional examination of transaxial images and has called attention to the importance of state-of-the-art workstations.10
Adding to the radiologist’s workload is the greater number of examinations that can be performed by MDCT. A study11 performed at Massachusetts General Hospital, Boston, before and after the introduction of a four-slice scanner indicated a 13% increase in the number of scans, even as more complex studies were being done. “Scan times are no longer the issue in patient throughput,” Silverman explains. “The time that it takes to get the patient in and out of the scan room, and to send the images to the relay for distribution to the picture archiving and communications systemissues that are peripheral to the actual scanhave become the rate-limiting steps. Some of the slots on our examination schedule are as short as 15 minutes, and this is challenging all of us in CT to take care of these other things more quickly.”
|(c) Contrast enhanced 16-slice CT acquisition of a patient with Type B dissection of the descending thoracic aorta. The newest generation of 16-slice CT scanners offers simultaneous acquisition of up to 16 sub-millimeter slices and this way enables almost isotropic data acquisition. The high acquisition speed of this new technology with a gantry rotation time of 420 msec allows for effectively reducing artifacts from respiratory and patient motion and optimal contrast enhancement of vessels with decreased iodine load. It has been shown, that retrospective ECG-gating, which enables reconstruction of axial images at any time-point during the cardiac cycle, allows for effective reduction of cardiac pulsation artifacts that might interfere with the unambiguous evaluation of cardiac structures and the thoracic aorta. However, the spatial resolution that could be achieved with retrospectively ECG gated technique using the previous generation of 4-slice MDCT scanners was limited by the relatively long scan duration inherent to data oversampling. Thus, near isotropic spatial resolution could only be achieved for relatively small volumes, eg the coronary arterial tree, but not for extended coverage of the entire thoracic aorta. The advent of 16-slice CT scanners now effectively eliminates these previous trade-offs. The case shown here strikingly illustrates the technical prowess of this novel technology: With 16-slice CT not only the thoracic aorta was covered with retrospective ECG gating and in a single breath-hold, but the entire course of the aorta and its branches from arch to bifurcation. This way transmitted pulsation artifacts are effectively eliminated and potential sources of diagnostic pitfalls arising from cardiac motion should be effectively overcome. Image courtesy of U. Joseph Schoepf, MD, Brigham & Women’s Hospital.
Beginning with the first installation of a four-slice scanner, the increase in speed had an effect on staffing in the CT suite. “In the past, we had one technologist per scanner who did everything in series,” Silverman says. “Of course, it took longer, but the patient was on the table so long that it did not matter. This arrangement no longer makes sense. By having either technician aides or other technologists perform tasks in parallel, we have been able to improve CT efficiency. My estimate, from our early experience, is that one needs at least 1.5 technologists per scanner, and maybe as many as two, to maximize the efficiency of multislice CT.”
There is concern about the potential for higher radiation doses with MDCT, especially as the number of slices increases. Paulson et al12 have reported that, even after adjustment of the milliamperage, MDCT studies performed using SSCT protocols increased the organ dose twofold to threefold. Katz et al13 demonstrated that the radiation doses in children would be 70% higher for a study performed by MDCT than for one performed by SSCT, unless appropriate changes were made in scan parameters. Higher doses are not inevitable. Manufacturers are incorporating technology to suppress scatter radiation and to adapt the dose to the patient’s age, size, and anatomy. Moreover, Loose et al1 called attention to the fact that, for some areas of the body, MDCT will reduce the dose by capturing all of the desired anatomy in one scan, when, formerly, two scans would have been needed.
Silverman says, “It is how you use a scanner, not the scanner itself, that determines the radiation dose. If you keep the dose the same, but reduce the slice thickness, the noise will increase; to have a less noisy image, you will have to increase the radiation exposure.” His service is highly radiation conscious and has conducted several studies on dose. One solution to the problem has been to accept more noise. “Some of our images are grainier than in years past, but even though they may be less aesthetic, they are no less diagnostic,” Silverman says. All of the CT scanners at Brigham and Women’s Hospital are equipped with dose-modulation software that works during the scan.
THE FUTURE OF MSCT
Will any role remain for single-slice helical scanners? Only one of the CT scanners at Brigham and Women’s Hospital is a single-slice model. Eventually, this scanner also will be replaced with a multislice machine, but that upgrade is not considered urgent, as the machine is used for the guidance of interventional procedures. “You cannot do an interventional case every 15 minutes,” Silverman notes. “Generally, the maximum is about eight or 10 procedures a day, so there is no need to have a scanner capable of handling 35 or 40 patients during that time.”
Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.
- 1. Loose R, Oldendorf M, Deichen JT, Wucherer M. Multidector CT: what to do with all of the data? In: Reiser MF, Takahashi M, Modic M, Bruening R, eds. Multislice CT. Berlin: Springer Verlag; 2001:9-12.
- Ptak T, Rhea JT, Novelline RA. Experience with a continuous, single-pass whole-body multidetector CT protocol for trauma: the three-minute multiple trauma CT scan. Emergency Radiology. 2001;8:250-256.
- Funke M, Kopka L, Grabbe E. Biphasic contrast-enhanced multislice helical CT of the liver. In: Reiser MF, Takahashi M, Modic M, Bruening R, eds. Multislice CT. Berlin: Springer Verlag; 2001:35-38.
- Kulinna C, Helmberger T, Kessler M, Reiser M. Improvement in diagnosis of liver metastases with the multi-detector CT [in German]. Radiologe. 2001;41:16-23.
- Baum U, Lell M, Nömayr A, et al. Multislice CT in the diagnosis of pancreatic tumors. In: Reiser MF, Takahashi M, Modic M, Bruening R, eds. Multislice CT. Berlin: Springer Verlag; 2001:39-48.
- Mueller-Lisse UG, Mueller-Lisse UL, Oberneder R. Multi-detector CT of the kidney. In: Reiser MF, Takahashi M, Modic M, Bruening R, eds. Multislice CT. Berlin: Springer Verlag; 2001:23-34.
- Pitton MB, Kemmerich G, Herber S, Schweden F, Mayer E, Thelen M. Chronic thromboembolic pulmonary hypertension: diagnostic impact of multislice-CT and selective pulmonary-DSA [in German]. ROEFO Fortschritte auf dem Gebiete der Roentgenstrahlen und der Neuen Bildgebenden Verfahren. 2002;174:474-479.
- Treede H, Becker C, Reichenspurner H, et al. Multidetector computed tomography (MDCT) in coronary surgery: first experiences with a new tool for diagnosis of coronary artery disease. Ann Thorac Surg. 2002;74:S1398-S1402.
- Rubin GD. Data explosion: the challenge of multidetector-row CT. Eur J Radiol. 2000;36:74-80.
- Roos J, Desbiolles L, Marincek B, Hilfiker P. Multidetector row spiral CT: analysis of time management and workflow. Paper presented at: Radiological Society of North America annual meeting; November 26, 2001; Chicago.
- Jhaveri KS, Saini S, Levine LA, et al. Effect of multislice CT technology on scanner productivity. AJR Am J Roentgenol. 2001;177:769-772.
- Paulson E, Yoshizumi T, Frush D, Johnson G. Multi-detector vs. single-detector CT: the organ doses are higher than you think. Paper presented at: Radiological Society of North America annual meeting; November 27, 2001; Chicago.
- Katz S, Cuomo F, Katai F, Miller C, Brink J. Age-specific radiation dose associated with pediatric body CT: comparison of single and multidetector-row CT protocols. Paper presented at: Radiological Society of North America annual meeting; November 25, 2001; Chicago.