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A changing image of health

Scanning techniques have become more sophisticated and reliable as screening tools

October 03, 2001|By Connie Lauerman, Tribune staff reporter.

In the Chicago area, Rush-Presbyterian St. Luke's Medical Center has been a leader in PET imaging, the state of the art in nuclear medicine. Rush has been using PET clinically since 1994, scanning some 6,000 patients, most of them with cancer.

Short for positron-emission tomography, PET scans involve injecting glucose with a radioactive substance to image metabolic activity in cells. Cancer cells use much more glucose than normal cells because they're growing.

"We do the whole body," said Dr. Amjad Ali, associate professor of diagnostic radiology and nuclear medicine at Rush Medical College. "We can detect [cancer] metastases."

Indicating an image with a dark area on a computer screen in the hospital's nuclear medicine section, Ali said, "This is a patient who came with a liver mass on a CT scan. We can tell it's a tumor. It is dark. It is picking up a lot of the glucose we injected."

The latest advancement, Ali noted, is the ability to merge CT images with PET images. The combined image, showing both function and anatomy, "will make it easy for a surgeon to open the body and take out the tumor."

"People keep talking about combining PET and CT," said Loyola's Henkin. "But actually MRI and PET are very valuable to integrate because they're both functional imaging modalities and they contain a lot of information."

The latest advance in MRI technology is the functional MRI, a modification of traditional MRI.

"Traditional MRI looks at structures, so that if there is a stroke it will show the part of the brain that's damaged. Or if there's cancer it will show the growth in the brain," said Dr. Marsel Mesulam, director of the cognitive neurology and Alzheimer's center at Northwestern University Medical School.

"Functional MRI looks at the function of brain cells by measuring changes in blood flow. When brain cells work harder they need more blood. Functional MRI can detect changes as small as 1 1/2 to 2 percent."

During functional imaging, he said, subjects are given a certain task to do in order to determine exactly what parts of the brain are active. Blood flow emits a telltale signal in response to the magnetic field.

A practical technique

There's a range of practical applications for fMRI, including early detection of Alzheimer's disease and providing a "map" for neurosurgeons operating on patients with intractable epilepsy or arteriovenous malformations. Armed with specific information about how a patient's brain is organized, surgeons can avoid damaging its language and movement centers.

Cardiac fMRI studies are also being done at Northwestern to detect changes in the heart muscle that indicate previous small or large heart attacks.

"It's still an emerging application, but it is used clinically to screen coronary-bypass-surgery candidates," said Robert Judd, a biomedical engineer and assistant professor of medicine at Northwestern.

"You can look at the ECG, you can look at abnormal contraction by echocardiography, and you can see that potions of the heart are not contracting well, and you can look at nuclear scans, but they detect infarctions that are large, on the order of 20 percent of the heart.

"Cardiac fMRI allows you to detect the very small infarctions that are being missed by the traditional techniques."

The idea behind coronary-bypass surgery is that if blood supply is restored, the portion of the heart that does not contract will begin to contract again, Judd said.

"But if they're not contracting because they've already had cell death [from heart attack], they're not going to benefit from the [surgical] intervention."

The Food and Drug Administration recently approved the use of digital mammography, a technology available at Northwestern.

"A film image is fixed. The digital image can be made lighter or darker," said Dr. Yolanda Adler, senior attending radiologist at Northwestern Memorial Hospital's Lynn Sage Comprehensive Breast Center.

"We can magnify something that looks suspicious. It has the potential for helping us find things a little easier. It reduces the number of repeats to get additional views. The obstacle now is that the cost is still very high."

Doctors at the University of Chicago are offering some patients a "virtual colonoscopy," a CT scan done in a special way, in lieu of the standard colonoscopy, a more invasive test that requires sedation and involves more risk, including a drop in blood pressure and perforation of the colon.

Virtual colonoscopies, however, do require the preparation on the patient's part. That means a day of bland food and a dinner of laxatives.

After an injection to relax the colon, it is inflated with air, resulting in minor discomfort. But the whole test takes only 15 minutes and involves less radiation than a regular CT scan.

Its viewing cameras can pick up growths hidden in folds of the colon that a colonoscopy might miss, as well as tumors in the kidneys and liver.

"Colonoscopy is not perfect, even though we think of it as the gold standard," said Dr. Abraham Dachman, director of computed tomography at the University of Chicago. "Some studies show a 6 percent incidence of missing things that hide behind folds of the colon.

"We think most of the time we will be able to pick up something inside the colon that's important and if we do, then you need a colonoscopy."