Scanning the scans

As modern technology has revolutionised the practice of medicine, perhaps no field has changed more rapidly than diagnostic imaging. Healthcare Middle East looks at the respective merits of CRT, MRI and PET technologies.

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By  Harvard Medical School Published  January 8, 2006

|~||~||~|As modern technology has revolutionised the practice of medicine, perhaps no field has changed more rapidly than diagnostic imaging. MRIs and CT scans have made it possible to peer deep inside the body without a scalpel or stitch, and to diagnose diseases with remarkable precision and at an earlier stage. And increasingly, a promising new player, the PET scan, is joining the arsenal. How do these new and rapidly evolving scans stack up?

Humble beginnings: X-rays

It all started in 1895, when Wilhelm Roentgen discovered x-rays. Six years later, he earned the first Nobel Prize in physics; by then, doctors were already using x-rays to diagnose problems, and the collaboration between physicists and physicians was under way.

Although they are still called roentgenograms, x-rays have improved dramatically over the past century. New digital techniques have made x-rays sharper and clearer than ever. Digital imagery also allows x-rays to be viewed and stored electronically, eliminating bulky film, and allowing the images to be transmitted to physicians anywhere in the world in an instant. The new scanning techniques share these advantages.

Digital technology, though, can’t overcome the intrinsic limitations of x-rays; they can’t produce images of tissues that are not dense. Healthy lungs, for example, appear uniformly black because they are filled with air, but if lungs fill up with fluid (congestive heart failure) or pus (pneumonia), the abnormal area looks white on the film or screen. By using radio-dense contrast materials, doctors can obtain images of tissues that otherwise allow x-rays to pass right through: examples include angiography for blood vessels, barium swallows and enemas for the gastrointestinal tract, and intravenous pyelography for the kidneys and urinary tract (but not the prostate).

X-rays carry energy, and that energy can damage tissues if the dose is too high. Many of the x-ray pioneers, both physicists and physicians, paid a steep price for their discoveries, but in the process they learned how to make x-rays safer. A 2004 study, for example, estimated that diagnostic x-rays account for less than 1% of all malignancies in the United States.

Interventional radiology

In the beginning, radiologists were simply diagnosticians, performing and interpreting the x-rays ordered by a doctor, then reporting the results to him. Before long, radiation oncologists began to use x-rays to treat cancer. Today, radiation therapy can be administered externally (3-dimensional conformal radiotherapy) or by placing seeds right into the gland to cure early prostate cancer (brachytherapy).

And now, thanks largely to new techniques, diagnostic radiologists and radiation oncologists have been joined by a new breed of interventional radiologists. The developments have made medicine much more specialized than ever — but also much better. Interventional radiologists treat a wide variety of problems. Vascular specialists, for example, can perform balloon angioplasties to open narrowed arteries and then insert stents to hold them open.

They can place a stent graft in a bulging aortic aneurysm to shore up its walls, preventing the disaster of a rupture. On the other hand, if a leaking vessel is the problem, interventional radiologists may be able to staunch the bleeding by placing small beads in the artery to block it (embolisation).

Vascular diseases produced the first triumphs for interventional radiology, but there have been many other success stories. Interventionists can place stents in narrowed ureters, preventing kidney failure. Guided by ultrasound or CT, they can biopsy suspicious tissues, including lymph nodes and the prostate. They can place the special IV lines that allow many patients to be treated at home. And radiologists can drain pockets of pus from the abdomen and chest, saving patients from open operations that were previously needed to treat abscesses.

A leap forward: CT scanning

Computed tomography (CT scanning, sometimes called computer-assisted tomography or CAT scanning) arrived in the early 1970s, altering the landscape forever. And rapidly evolving developments are making CT better than ever.
CTs use x-rays to image the body. In the first-generation CT scan (the axial CT), the patient lies on a table that moves through a doughnut-shaped tube containing the x-ray generator (see figure below). The generator rotates around the patient, beaming x-rays through his body. X-rays that pass through are collected by detectors that channel the electrical signals into computers, where they are reconstructed into detailed images of the body.

In first-generation CTs, the table moves the patient for a short distance then stops while an image is obtained. The process is repeated until the scan is complete. But in second-generation scans, spiral (or helical) CTs, the process is a bit different: The patient holds his breath and the table moves him through the scanner without stopping while the tube rotates around him continuously in a spiral fashion (see figure below). The computer compensates for the effects of motion, constructing more detailed images than were formerly possible. Spiral CTs don’t expose the patient to any more radiation than ordinary CTs, and they are also much faster.

Spiral CTs can provide remarkably detailed images of the nervous system, chest, and abdomen. In the urinary tract, for example, they have replaced the older methods of looking for kidney stones. Unfortunately, they cannot be used to picture the prostate, though they can detect prostate cancers that have spread to lymph nodes in the abdomen.

Even as many hospitals are proudly employing their shiny new spiral scanners, other centers are shunting them aside to install third-generation multislice (also called ultrafast or multidetector) CTs. First- and second-generation CTs use a single row of detectors to pick up the x-rays that pass through the patient; multislice scanners have up to eight rows, along with improved computer software (see figure below). As a result, they are remarkably fast, able to scan a patient’s entire chest, abdomen, and pelvis while he holds his breath for just 20 seconds. By focusing on a slightly smaller area of the body for the same time period, the CT can be used to obtain much finer slices, which provide remarkably detailed images of the body in 1-mm segments.

High-resolution CT images are a major advance. For example, they make it possible to detect tiny clots in the lungs, tumors that are too small to show up with other methods, and small flecks of calcium in a coronary artery. It’s a great achievement — but now comes the hard part, learning how to apply the possibilities of physics to the practicalities of patient care.

CT scanning is a remarkable tool. And by administering contrast material to blood vessels (CT angiography) or the intestinal tract (virtual colonoscopy), doctors can expand its use. However, CTs are much more expensive than ordinary x-rays. In addition, some experts are concerned that because they are so very good, they are used more often than really necessary.

The evolution of CT scans 1. First-generation CT scans have the patient lie on a table that moves through the x-ray generator in short steps, obtaining an image at each stop.2. For a spiral or helical CT scan, the table moves the patient through the scanner without stopping. At the same time, the x-ray generator moves around the patient in a spiral fashion.3. Multislice or ultrafast CT scans use multiple rows of x-ray scanners and more sophisticated software to create highly detailed pictures from the 1-mm–thick image segments.

Next up: MRIs

CTs have not replaced x-rays, and MRIs won’t push CT scanners from hospitals to museums. Still, magnetic resonance imaging can offer major advantages, at least for certain patients.

Instead of relying on radiation, MRIs use the body’s natural magnetic properties to produce detailed images of any part of the body. Because hydrogen is so abundant in the body, especially in water and fat, it is used as the target. Like the planet Earth, the hydrogen nucleus — just a single proton particle — rotates on its axis. Also like Earth, every hydrogen proton behaves like a bar magnet, with north and south poles.

Under normal conditions, the magnetic poles are randomly aligned and don’t generate enough energy to produce images. But when the body is placed in a strong magnetic field, the proton “magnets” line up along an axis, like so many parallel compass needles. If the protons are then hit with a short burst of precisely tuned radio waves, they will momentarily turn around. Then, in the process of returning to their original orientation, they emit a brief radio signal of their own.

To obtain an MRI, a technician places the patient in a long tube that produces an intense magnetic field and generates pulses of radio waves. The tissues’ hydrogen protons resonate, emitting radio signals that are captured by detectors and processed by a computer into detailed images.

By alternating the sequence of pulses, doctors can use MRIs to obtain images of tissues anywhere in the body. And by using a contrast agent (gadolinium), doctors can further enhance the MRI. Even so, some organs still manage to hide their secrets from MRIs. The prostate is an example, but new endorectal coil MRIs, in which the pulse generator is placed in the rectum right behind the prostate, may (or may not) change that.

MRIs sound scary, but since they don’t use radiation, they are safe for all of the body’s tissues. Still, they frighten some patients, who develop claustrophobia when they are confined to a narrow, noisy tube for 30 minutes or more. Sedatives can help, and new open scanners are less threatening (if a bit less accurate). Patients who have metal in their bodies (such as pacemakers, inner ear implants, clips on brain aneurysms, some artificial joints, and shrapnel) cannot be given MRIs (though they can have CT scans). MRIs are also very expensive.

A new player: PET scanning

Positron emission tomography (PET) may be the new kid on the block, but it will never replace the older techniques. That’s because its value appears to be confined to detecting certain cancers and, possibly, some infections. Still, PET scanning may help many patients.

PET scanning has more in common with older nuclear imaging techniques than it does with CTs and MRIs. Nuclear medicine obtains images by administering tiny doses of radioactive isotopes tagged to chemicals that are taken up by specific body tissues. The chemical emits radiation, which is picked up by a gamma camera, which produces images of the tissue to detect abnormalities. For example, the bone scan is an old standby that remains the best way for doctors to detect prostate cancers that have spread to bones. Other important examples include nuclear imaging stress tests and SPECT scans for the heart.

PET scanning depends on the fact that malignant cells have faster metabolisms than normal cells; it’s why the cancers grow so rapidly and why chemotherapy can destroy cancer cells without producing as much damage to normal cells. Patients who undergo PET scans receive injections of radioactive F-fluorodeoxyglucose (FDG), which is metabolized just like glucose (sugar). Malignant cells need more sugar to fuel their souped-up metabolism, so they take up more FDG, and so emit radiation detected by the PET scan. Because normal tissues take up some FDG, they can sometimes light up on the scan, producing false positive results. It’s particularly true of the white blood cells that enter tissues to fight infections; for now, that’s a problem for doctors hunting for cancer, but it may turn into an asset if hidden infections become the target.

Although PET scans are new, doctors have already demonstrated that they can detect tiny lung cancers, both those that are confined to the lung (when surgery may be curative) and those that have spread to the lymph nodes (which cannot be cured surgically).

PET scanning can also help in other malignancies, including head and neck cancers, gastrointestinal cancers, and lymphomas. And in the latest advance, PET and CT scans can be integrated, offering the potential advantages of each in a single session. To date, however, PET scanning has been a disappointment in detecting prostate cancer, in part because most prostate cancer cells metabolize glucose and divide slowly. As a research technique, PET scans have produced valuable insights into why some patients with normal coronary angiograms can still get angina (Syndrome X). Because PET scanning is so new, it is not yet widely available. And it is very expensive (perhaps $2,000 vs. insurance payments of about $600 for a typical MRI and $300 for a CT scan).

Above all, though, it is a powerful and promising tool but doctors have still not learned when and how to use it to best advantage. For example, more research is needed to compare PET scans with multislice CTs in detecting early lung cancer —and to learn whether either technique is truly lifesaving as a screening test.

Compared to the CT, MRI, and PET scans, ultrasound may seem like kid stuff. But in this case, at least, appearances are deceiving. In fact, ultrasound is a painless, entirely safe, relatively fast, and inexpensive way to view the body. For some organs, like the heart and the prostate, ultrasound provides valuable information that CTs and MRIs cannot.

For other problems, like gallstones, ultrasounds are preferred to CTs because they can provide as much (or more) information with less expense and hassle. And interventional radiologists often use ultrasound to guide their therapy of many disorders (see box above).Instead of using radiation, ultrasounds (also known as sonograms or echoes) rely on high-frequency sound waves that are beamed into the body from a probe.

The sound waves that echo back from the target organ are captured and processed by a computer, then projected onto a video screen and preserved in digital images.In most cases, the ultrasound probe is positioned on the skin, but it can be placed in a body cavity to get a closer look. For example, a probe can be put on the chest wall to picture the heart, but for more detailed and accurate images, it can be passed down the esophagus (“food pipe”) and positioned just behind the heart.

Doctors began using ultrasound to examine the prostate more than 30 years ago, but didn’t get very far until they learned to place the probe in the rectum, right behind the gland. Even in the CT-MRI era, transrectal ultrasonography (TRUS) is the best way to picture the prostate. It’s still not good enough to detect small prostate cancers, but it does make it possible to diagnose the disease with biopsies and to treat it with radioactive seeds.

Which scan?

Ultimately, it depends on the circumstances. Plain old x-rays are fine for many problems, from pneumonia and heart failure to fractures and dislocations.
CT is the current champ for mobile areas of the body, including the lungs and abdomen. Because they are faster than MRIs, CTs are usually preferred for trauma patients, even when the brain is involved.
Aside from trauma, though, MRIs are generally preferred over CTs for precise images of the brain and nervous system. MRIs also shine for diseases of the joints, muscles, and possibly for the pelvic organs. Although new research may soon extend its reach, PET scanning is currently approved only for the diagnosis of lung cancer and certain other malignancies. Used wisely, it can be a great asset. But PET scans don’t work well on all organs; for example, a much simpler technique, ultrasound, is still the best way to picture the prostate.

This article is provided courtesy of Harvard Medical International.


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