MDCT makes inroads in urology, biopsy guidance, and oncology imaging

Jun 18, 2006

As multidetector CT has progressed from four- to 64-slice technology, the ability to obtain thinner slices and more detailed information is prompting more physicians and clinicians to abandon tried-and-true techniques with other modalities in favor of MDCT.

In presentations at last week's International Symposium on Multidetector-Row CT in San Francisco, MDCT had increasingly prominent roles in a number of areas, including urography, oncology imaging, and biopsy guidance.

At Stanford University in Stanford, CA, MDCT has surpassed intravenous pyelograms (IVPs) in urography. "There are many lesions I see now on CTU (CT urography) that I would not have seen on an IVP," said Dr. F. Graham Sommer, professor of radiology at Stanford.

Three basic covenants remain the same in MDCT urography. The mandatory precontrast scan by clinicians must identify the presence of renal calculi, calcifications, and masses by imaging from the abdomen to the pelvis. Second, Sommer emphasized that the nephographic phase should begin approximately 90 seconds after the start of contrast injection to optimize imaging results. Finally, the excretory phase -- with excretion of contrast into the renal collecting structures and ureters -- requires adequate patient hydration with either water or IV saline.

The main indication for MDCT urography is hematuria and suspicion of a solid renal mass, added Dr. W. Dennis Foley, professor of radiology and chief of digital imaging at the Medical College of Wisconsin in Milwaukee. Additional indications may include suspected urinary tract anomaly and the evaluation of urinary diversion procedures following cystectomy.

The group's MDCTU technique begins with a precontrast low-dose study of the abdomen and pelvis to evaluate urolithiasis, followed by a split-bolus contrast injection and acquisition of a nephropyelogram. The CT image is reviewed in conventional axial mode, multiplanar reformation, 5-cm slab maximum intensity pixel and average intensity pixel projections of the kidney and urinary tract, and one to three digital radiographs. The digital radiographs are a scanned projection obtained with the CT scanner utilizing increased radiation dose.

The combination "provides the objective of urography -- that is, a satisfactory display of all segments of the urinary tract with a good clinical diagnosis," Foley said. "It does not require patient hydration. In fact, patient hydration we prefer not to do in order to have an adequate display with a digital radiogram."

MDCT-guided biopsies

CT-guided procedures also have benefited from MDCT, which has facilitated the localization of instruments. In the case of biopsies, the simultaneous display of multiple slices improves assessment of the needle orientation and location. Dr. John R. Haaga, chairman emeritus at Case Western Reserve University in Cleveland, said that MDCT provides "superlative visualization of small structures and vessels making trajectory planning more accurate."

With the issue of radiation dose so pertinent today, Haaga said that dose can be lower in concert with "a high-contrast situation with a needle" and proper planning. "It is indefensible to scan all patients with very thin slices," he added, "because it will increase the radiation dose."

Most devices feature automatic adjustment technology based on a patient's body size to minimize radiation dose without diminishing image quality.

He also favors 3D reconstructions, which can be very helpful for standard biopsies to identify the appropriate site, and worthwhile for abscess drainage. "It is very important to visualize the complete extent of the abscess and determine where it is amenable to precutaneous drainage or to help plan it," he said.

MDCT's best views

While pancreatic carcinoma is a highly lethal disease with "few, if any, long-time survivors," Dr. Brooke Jeffrey, professor of radiology and chief of abdominal imaging at Stanford, outlined how MDCT and 2D and 3D displays are benefiting detection and diagnosis.

The only treatment that has been shown to have a modest benefit is surgery with negative margins, Jeffrey noted. Without surgery, the survival rate is eight to 12 months, compared with 18 to 24 months with surgery.

"The key to management is patient selection. In order to do that, we must have accurate local staging, and the key to that is vascular encasement," he said. "This is where 2D and 3D displays are most helpful in patient selection for surgery."

Using MDCT, the value of negative contrast for the evaluation of the stomach and duodenum is helpful in detecting ampullary carcinoma, which has a survival rate of five years in 25% to 30% of cases. The key to diagnosis is distending the duodenum with water.

Pancreatic carcinoma image displays provide unique diagnostic information unobtainable with axial images alone, Jeffrey said, and are "highly accurate for local staging." Curved planar reformation offers 2D display of tubular structures, such as ducts or vessels, traversing 3D volume.

"The advantage of curved planar reformation is that it preserves original grayscale contrast of the original data," Jeffrey said. "It is much easier to see soft-tissue abnormalities around blood vessels, (and) very hard to see the mass in the volume-rendered image."

In addition, MinIP -- minimum intensity projection -- displays are exceptional at imaging low attenuation ducts and soft-tissue invasion.

Jeffrey noted that Stanford has done some work with MDCT biphasic imaging and found that a subset of iso-attenuating lesions "can be easily missed unless you do this type of reformation."

By Wayne Forrest
AuntMinnie.com staff writer
June 19, 2006

Zero to 64: CT urography zooms ahead with more detector rows, March 18, 2005

CT gains ground in urology, June 18, 2004

MRI, MRCP and 3-D MRA separate pancreatitis from cancer, Nov. 26, 2001