Spiral CT Offers Better Detection of Disease, More Accurate Staging

BALTIMORE-The advent of spiral CT offers advantages over regularCT scans, Elliot K. Fishman, MD, said at a conference sponsoredby the Johns Hopkins School of Medicine, where he is professorof radiology and oncology.

With spiral CT, Dr. Fishman said, a scan can be completed in asingle breathhold, minimizing inconsistencies. By keeping thex-ray turned on and moving the patient, the entire abdomen canbe scanned in only 32 to 40 seconds. This faster throughput improvespatient cooperation and compliance.

More important, spiral CT creates images with close interscanspacing. Data accumulate as volumes, not slices, so radiologistscan sample data at any interval. "Compared to standard dynamicCT," he said, "spiral CT offers increasedsensitivity,more accurate staging, and an increased role in patient care."

Recent advances in spiral CT scanning have highlighted the liver(Figures ), he said. With CT, optimal lesion detection requiresseparate scans of both arterial and venous phases, to increasethe chance of detecting both hyper- and hypovascular tumors. Now,a single spiral CT scan can do both at the same time.

Equipped with a Silicon Graphics work station, Dr. Fishman hasused real-time 3D rendering to create vascular maps in less than10 minutes. "Spiral CT costs only $500 to $600 per scan,compared to $3,000 to $4,000 for an angiogram," he said."It provides the same information but is far less invasive."

While standard CT usually maps data at 8 mm intervals, spiralCT can go down to 4 mm without additional radiation dose or scantime. This has increased detection by 10% and accuracy by almosta third in both the liver and lung.

Once the lesion is found, Dr. Fishman said, the objective of scanningshifts to patient management. "You can create 3D imagingmaps to assess resectability, plan surgery, or simulate surgeryin advance."

Spiral CT is proving useful in other disease sites. Research fromJapan into gastric carcinomas, for example, indicates that thetechnique can look at the depth of invasion of the gastric wall.

Spiral CT has also been used to help maximize cortical-medulladifferentiation in kidney tumors. For pancreatic cancer, it hasproved to be better than 95% accurate in detecting lesions, and85% accurate for staging. "Overall," he said, "spiralCT offers a tremendous opportunity for surgical planning, monitoring,and rapid tumor volumetrics."

Spiral vs Standard CT

Standard CT scanning and spiral CT differ in the time it takesto acquire the data. With standard dynamic CT, an image is typicallyobtained in 1 to 2 seconds; this is followed by a 4- to 5-secondperiod in which the table moves to the next positiona. Thus, approximately8 to 12 scans can be obtained per minute, depending on the scannerused.

With spiral CT, the patient is placed in the scanner and movesat a constant speed through the scanning gantry. Instead of thex-ray tube coming on and off at select intervals, the x-ray tubestays on during the entire process, allowing data to be acquiredin a continuous stream.

With spiral CT, the patient needs to hold his or her breath onlyone time, whereas with dynamic CT, 30 to 50 individual breath-holdsmay be needed per study, depending on the area scanned.