Many technical advances have occurred since the advent of CT in clinical medicine about 30 years ago. These advances include the development of helical or spiral CT scanners, which has resulted in significant reduction in image acquisition time; this reduction has impacted on improved image quality through the elimination of respiratory misregistration artifacts and minimization of motion artifacts. The use of multiple sets of parallel rings that rotate without electrical cables distinguishes helical CT from conventional CT scanners. This slip-ring technology allows simultaneous patient translation and x-ray exposure tracing a helical path around the patient. This projection of data is then mathematically interpolated, since no two data points are taken in exactly the same plane. The data are then reconstructed similar to conventional CT with the additional ability to produce slice shifting for multi-planar reformatting or three-dimensional imaging.
The latest breakthrough in CT technology is the recent development of multislice helical CT scanners that are capable of obtaining volumetric data sets in seconds. The multislice helical scanners differ from the single-slice helical scanners in the detector configuration. Multiple detectors are present and the collimator may be adjusted to expose only a few or all of the detectors. Although this has provided a major benefit in terms of speed, there is some measurable widening of the slice profile compared to standard monoslice units. Fortunately, this widening has not had a detrimental effect on clinical examinations. One vendor’s scanner features a matrix of detectors 20 mm in width (z-axis) with a number of detectors equal to that of conventional CT in the XY plane. However, the z-axis is subdivided into sixteen 1.25-mm detectors, each with a separate response pathway. When operated in the helical mode, four contiguous detectors can be sampled ranging from 5 mm (4 x 1.25 mm) to 20 mm (4 x 5.0 mm). Various permutations can result in slices of 1.25, 2.5, 3.75, and 5 mm. The definition of pitch on standard helical scanners is:
Table speed per gantry rotation
where beam collimation = image thickness.
On a multislice, multidetector scanner, collimation is wider than image thickness. Thus, the definition becomes:
Table motion per gantry rotation
minimum slice thickness
for a particular detector configuration.
For four 5-mm-thick slices with a table motion of 15 mm per rotation, the pitch is 3 (15/5 = pitch 3). With a subsecond gantry rotation of 0.8 seconds, the same pitch affords 18.75 mm of coverage. In the case of this scanner, certain sweet spots have been identified at pitches of 3:1 Hi-Quality (HQ) and 6:1 Hi-Speed (HS). In fact, in certain situations, HQ operating modes can result in benefits including: decreased slice profile, increased coverage per unit time, and decreased technique mAs. Trade-offs do exist between these modes with HS, resulting in greater coverage at the expense of image quality. The radiation dose of the multislice scanners is higher than that of a comparable scan done on a single-slice unit since it is necessary to collimate for multiple detectors. The dose may be increased by a factor of approximately 1.5 as compared to a single-slice unit with comparable scan parameters. Higher table speed and narrower collimation would decrease this effect. The higher dose would be of greatest concern in young patients and in those who may require multiple studies. However, this issue may be a limited clinical concern as new tracking software has been developed to decrease the radiation dose to levels that are comparable to or, in some cases, even less than those of the single-slice helical scanners.
For head and neck imaging, rapid scanning techniques have essentially eliminated artifacts related to swallowing and respiratory motion. Improved vascular opacification not only helps to distinguish nodes or masses from vessels, but also allows evaluation of intrinsic vascular disease and may be competitive with MRI and sonography in the evaluation of carotid artery disease. Multislice imaging provides exquisite 3-D images.
When imaging the thorax, helical CT virtually eliminates respiratory motion and misregistration artifacts due to the speed of acquisition. Conventional CT of the chest often runs into difficulty at the lung bases where diaphragmatic motion may cause significant misregistration. Helical CT has been shown to be superior to conventional CT in the evaluation of pulmonary nodules and allows the use of smaller volumes of contrast, decreasing cost. Excellent vascular enhancement provides the opportunity to evaluate in a relatively noninvasive way for pulmonary emboli in both proximal and segmental branches of the pulmonary arterial system. Multislice scanning offers the ability and flexibility of using thinner sections yet provides greater coverage, optimizing image quality and reducing examination times in sick patients.
In the abdomen and pelvis, helical CT can optimize evaluation of both solid viscera, such as the liver or pancreas, as well as the hollow gastrointestinal tract. With multislice helical CT, there are added advantages over the single-slice helical technique. The same volume of tissue can be imaged in less time when the same collimation is used. For example, it would take 30-40 seconds to cover the abdomen and pelvis using 5-mm collimation on a single- slice helical unit. The same anatomic area can be covered in 15 to 20 seconds with a multislice unit. If narrower collimation is required, the multislice scanner can cover the same area using thinner slices in the same amount of time or less as the single- slice scanner using wider collimation. Such images with very narrow collimation may be used for exquisite 3-D rendering of anatomy.
EVALUATION OF LIVER, PANCREAS
In evaluating the liver and pancreas, these scanners allow true triphasic imaging to be accomplished. For screening, the liver is optimally imaged during the nonequilibrium phase following intravenous contrast administration. Peak liver enhancement occurs during the portal-venous phase. Lesion to liver contrast for most lesions (hypovascular) would therefore be optimal at this time and rapid scanning to cover the entire liver during the peak enhancement phase would be optimal for lesion detection. With conventional CT, such rapid scanning is not possible since it requires 1.5 to 2.5 minutes to cover the entire liver. Multislice helical CT affords a significant advantage in that the entire liver can usually be scanned in approximately 10 seconds or less, capturing the early arterial phase for vascular anatomy, the late arterial phase for hypervascular lesions, and the portal-venous phase for identifying hypovascular lesions and providing coverage of the abdomen and pelvis for extrahepatic disease. It is known that the detection of hypervascular lesions from primaries, such as hepatocellular carcinoma, renal cell carcinoma, breast carcinoma, melanoma/sarcoma, and neuroendocrine tumors, is much improved during the very early arterial dominant phase of contrast enhancement, which occurs shortly after intravenous contrast administration.
The posttherapy follow-up of previously detected malignant disease may also be more reliable with multidetector helical CT because section intervals can be more closely spaced and it may therefore be easier to determine the presence and degree of change of focal liver lesions on sequential studies. Multiphasic CT can allow improved characterization of liver lesions, such as some cavernous hemangiomas that characteristically demonstrate early, peripheral nodular enhancement, and gradual filling in of the cavernous spaces with contrast. With conventional CT, a solitary hemangioma or lesions not far removed spatially may be accurately characterized, but multiple lesions in several lobes or segments of the liver are better evaluated with a faster scanner that has better temporal resolution.
Other solid organs in the abdomen and pelvis are also better imaged by helical CT due to the much shorter imaging times. Pancreatic adenocarcinoma can be accurately staged before surgery, especially with regard to vascular involvement, which is critical information for the surgeon. Ideally, preoperative imaging should evaluate local disease as well as any metastatic disease, and multislice helical CT allows true triple-phase imaging of the pancreas and liver to optimize evaluation of the vascular structures about the pancreas, as well as the extent of locoregional and distant disease. Hypervascular neoplasms of the pancreas, such as neuroendocrine tumors, are best visualized during the arterial dominant phase of contrast enhancement and quickly wash out during later phases, making them virtually invisible on later images.
CT evaluation of the kidneys is performed for both benign and malignant disease. When contrast is used to try to characterize renal lesions, the phase of contrast enhancement is critical. The very early corticomedullary phase may identify a hypervascular cortical lesion but is suboptimal for evaluation of medullary or collecting system lesions that are better evaluated later after intravenous contrast administration. Noncontrast CT of the kidneys and ureters is being used more and more frequently for evaluating renal stone disease. Radiographic evaluation of a patient in severe pain with renal colic is best accomplished with the most rapid imaging.
Imaging of vascular anatomy and pathology is easily accomplished with multislice helical CT. In the head and neck, maximum-intensity projection (MIP) images permit visualization of calcified carotid plaques and are very effective in evaluating vascular stenoses noninvasively. The renal vasculature may be studied with two-dimensional and three-dimensional reformations, which provide great promise in assessing for renal artery stenosis and other vascular anomalies of the kidneys. The benefit of adding together sections is that artifacts can be minimized, while the use of narrow collimation can assist in visualizing small vascular anomalies with multislice helical CT. The use of three-dimensional and multi-planar reformatted images affords the surgeon a better means of surgical planning. In the case of imaging the abdominal or thoracic aorta and its branches, multislice imaging affords, for the first time, full coverage.
BENEFITS FOR ABDOMINAL IMAGING
The hollow viscera of the abdomen and pelvis can also be studied with helical CT. Virtual colonoscopy is a new and exciting frontier for radiologists, who may be able to impact upon patient survival through the early diagnosis of colonic polyps, which are known risk factors for the development of colorectal carcinoma. CT evaluation of the colon requires that the patient arrest respiration during imaging so that misregistration artifacts do not occur and obscure colonic lesions. Given that the normal colon usually extends from the level of the diaphragm to the level of the ischial tuberosities, this large volume of anatomy must be scanned within a breath-hold to optimize the raw images used for reconstruction. Also, narrow collimation is necessary to detect small colonic lesions since it has been shown that polyps 10 mm or greater are considered suspect for the development of carcinoma. The multislice helical scanner provides narrowly collimated images in the span of a normal breath-hold. For example, the abdomen and pelvis, which encompass the entire colon, can be scanned in 2-3-mm contiguous slices in 20 to 25 seconds, a relatively comfortable breath-hold time for most ambulatory patients.
The advantage of multislice helical CT in the setting of trauma or in pediatric or critical care patients is obvious. Rapid diagnosis is of the essence in the emergency department setting where the more rapid the intervention, the greater the likelihood of survival.
For example, aortic injuries, which are not uncommon in motor vehicle accidents, must be accurately and quickly assessed for rapid surgical intervention. Head injuries are also very difficult to assess in a patient who may be uncooperative due to altered mental status. The more rapid the scan, the less chance of motion and misregistration and therefore the more accurate the diagnosis. Similarly, intensive care patients require rapid imaging as they are frequently difficult to scan. Pediatric patients also require sedation, and again the rapidity of the scan is essential to preserve scan quality.
Rapid scanning of patients is clearly advantageous for clinical diagnosis of various disease processes. Through-put is also impacted in a positive way by allowing more patients to be scanned and creating shorter delays in the event of emergent add-on cases, which must be performed in a timely fashion without delaying routinely scheduled patients. The even shorter acquisition time required by multislice helical CT lends an opportunity for providing improved care and increased flexibility in a busy radiology practice. Multislice helical CT clearly leads the way to a new era in diagnostic imaging as we enter the next millennium.
Revathy B. Iyer, MD, is assistant professor of radiology, and Paul M. Silverman, MD, is professor of radiology and chief, section of abdominal imaging (CT, MRI, ultrasound, and gastrointestinal and genitourinary imaging) at MD Anderson Cancer Center, Houston, and a Decisions in Axis Imaging News editorial advisory board member.