Meetings present strategies for lowering dose loads

August 1, 2006

Growing interest in dose reduction strategies took a prominent role in scientific meetings held this spring. In this roundup, Diagnostic Imaging highlights noteworthy presentations.

Growing interest in dose reduction strategies took a prominent role in scientific meetings held this spring. In this roundup, Diagnostic Imaging highlights noteworthy presentations.

- Attend to all patient groups. Pediatric dose has received a fair amount of attention in the literature and in practice, but appreciation for needs of patients in other age groups-particularly young adults-is still lacking, according to Dr. Lincoln Berland, chief of body CT at the University of Alabama in Birmingham. His lecture, "Essentials of radiation and risk with MDCT," was presented at the Society of Computed Body Tomography and Magnetic Resonance (SCBT/MR) meeting in April.

Performing multiphase scans with multislice CT is easy and common. But in patients at substantially higher risk for radiation exposure (generally children and young adults without a terminal illness), a single-phase scan may be more appropriate, according to Berland.

In a young woman scanned for a liver abnormality, for example, a single-phase contrast-enhanced scan may be perfectly satisfactory for making a diagnosis. Chest CT should be avoided if possible in young women, he said, adding that CT studies are used too indiscriminately to rule out pulmonary embolism in patients with modest chest discomfort.

"Unfortunately, we do not have a lot of control over requests ordered. It's up to us to educate clinicians in referral patterns," Berland said. "What we can do is protocol the exam differently. For example, we can avoid doing multiphase scans when they are not necessary."

- Shield the breast. Using anthropomorphic breast phantoms, researchers in Virginia measured radiation exposure in chest CT for a PE protocol and concluded that dose ranged from 14 to 20 mGy, equivalent to 100 to 400 chest x-rays or 25 to 50 mammograms.

"It's a very high dose-more than referring physicians and patients realize," said Dr. Mark Parker, an associate professor of radiology at Virginia Commonwealth University, at the 2006 American Roentgen Ray Society meeting.

In conjunction with a private technology innovation company, Parker and colleagues developed a custom-made tungsten and antimony breast shield that reduced dose 56% to 61% in phantom tests. They are seeking funding for a new phantom study to see if the shield can be used without losing the ability to detect pathology in the lungs and mediastinum. If so, it could be routinely used to protect women undergoing chest CT, Parker said.

- 'Stop the madness' in PE. Alarmed by the high rate of negative CT angiograms to rule out pulmonary embolism, researchers at the Oregon Health and Science University in Portland analyzed appropriateness in a retrospective study, with a particular focus on use of CTA in women aged 35 years or younger.

Of 580 CTAs performed over 14 months, 25% were performed on people under the age of 40, and 13% of the patients were women aged 35 or younger. Researchers reviewed clinical records and, where available, the results of D-dimer tests, which are used to diagnose thrombosis. Results were presented in an ARRS session called "CTA to rule out PE: Stop the madness."

Researchers noted that 8.9% of CTAs were positive, with a similar positive rate in male and female patients aged 40 and under, according to Dr. Mary Costantino, a radiology resident at OSHU. In women aged 35 or under, the positive rate was very low at 3.9%. And in women 30 years and under, who accounted for 10% of the study group, just 1.7% were positive.

Using established clinical criteria, clinicians can get an indication of the likelihood that a patient has PE. In the retrospective study, however, these criteria indicated that 51% of patients who underwent CTA actually had a low probability for PE, 48% had intermediate probability, and 0.5% had a high probability. They noted that six women under 35 had high D-dimer results, but all nevertheless had negative CTAs.

The results suggest the established clinical criteria were not being applied carefully enough. Staff are developing strategies to ensure more appropriate utilization.

- Adjust tube current. Automatic tube current modulation (ATCM), a feature that adjusts tube current and dose depending on the patient's anatomy, helps reduce dose substantially in children, according to a study from the Children's Hospital of Philadelphia presented at the ARRS meeting.

ATCM is an optional or standard feature available on most later model MSCT scanners. In larger patients, greater mAs is required to achieve a desired signal-to-noise ratio. If the patient is smaller, tube current can be lower, resulting in lower radiation dose. Another way to reduce dose when scanning children is to adjust for weight, rather than using a single current for all patients.

In a study of 74 children undergoing abdominal CT, ATCM helped reduce dose by an average of 15% through weight adjustment alone. ATCM and weight adjustment together resulted in a dose 60% lower than that required when a single standard current had been used.

- Control 64-slice parameters. Researchers at the University of Washington examined the use of the operator-selected noise level on the GE VCT 64-slice scanner in combination with the ATCM feature.

If an operator reduces the noise level to improve image quality, ATCM automatically increases the tube current (within operator-selected limits), raising radiation dose.

The need for image quality to aid diagnosis must be balanced against the need to protect patients from unnecessary radiation exposure, according to lead author Kalpana Kanal, Ph.D., an assistant professor in diagnostic physics.

UW physicists created a table that allows operators to look up a particular scan and set parameters for reconstruction slice thickness and noise at the lowest dose possible. If the operator wants to change any of the parameters to decrease image noise, he or she can use the table to see how the dose will be affected and whether the increase is justified, based on the improvement in quality.

The table is specific to the GE model scanner, but Kanal says that similar resources could be created for scanners from different manufacturers. The group's research won the cum laude prize for excellence at the SBCT/MR meeting.

- Reduce triple rule-out dose. Interest and enthusiasm are growing for the triple-rule-out MSCT study for emergency room patients with chest pain to check for a cardiac event, PE, and acute aortic syndrome.

Triple rule-out studies are done with gating and require fine detail, however. Consequently, radiation dose can be very high. In patients with higher heart rates, a larger dose is needed for coverage during ECG-gated reconstruction.

Researchers at the University of Washington found they could reduce the radiation dose with an ECG dose modulation feature on their GE MSCT scanner. In cardiac CT, radiologists want to image the heart at a particular point in the ECG cycle, but in the past, the radiation dose was administered throughout the cycle. With the dose modulation feature, dose is turned down but not completely shut off at points when images are not required, sparing the patient radiation exposure.

Reducing the patient's heart rate with beta blockers to the 40 to 75 bpm range is also useful, as this allows imaging in a single scan and makes better use of the ECG dose modulation feature. With these techniques, it's possible to reduce dose by up to 50% in patients, according to the study, which was presented at the ARRS meeting.

- Stick to the region of interest. In abdominal and pelvic CT, technologists commonly take additional images above and below the intended area of anatomic interest. Researchers at Emory University looked at scout and extra images for 65 consecutive abdominal/pelvic CTs performed on a 16-slice scanner.

The additional area beyond the region of anatomic interest accounted for 20% of the length of the scout images. Out of 6142 images, 719 (11%) were outside the intended anatomic area of interest, resulting in a 4% addition in radiation dose. Five findings were found in additional images and not on the CT images for the area of interest, but these were all incidental (Figure 1).

Cutting the extra images is an important measure for reducing cumulative dose, said Dr. Saravanan Namasivayam, a research fellow who presented results at the ARRS meeting.

"Appropriate selection of scanning areas can improve CT practice by enabling radiation dose reduction without compromising diagnostic accuracy," Namasivayam said.

- Try ultralow-dose interventions. Ultralow-dose (30-mAs) CT-guided percutaneous interventions are proving both safe and successful. Researchers at Boston University examined the use of the low-dose technique on 291 CT-guided interventional procedures, including percutaneous biopsy (two-thirds of cases) and catheter placements.

Prior to the interventional procedures, all patients underwent imaging with the standard radiation dose (from 175 to 250 mAs), as diagnostic-quality images are required for planning the procedure. But radiologists attempted to perform all subsequent imaging, such as needle and catheter placements, needle readjustment, and postprocedure imaging, with a 30-mAs dose. They had the option to increase dose, and in some cases this was necessary.

Overall, the technical success rates were comparable to those achieved with standard radiation doses.

"The low-radiation-dose technique using 30 mAs does result in technical procedure success," said Dr. Brian Lucey, who presented study results at the ARRS meeting. "Many interventional patients are young, and it [dose reduction] is worth doing."

- Physician, protect thyself. Physicians who perform angiography over the course of decades have a higher cancer risk relative to the rest of the population. A new shield and bar technique can help cut down on this risk, according to Dr. Martin Magram, an assistant professor of radiology at the University of Maryland Medical Center in Baltimore.

Magram developed a body-length floor-mounted lead plastic shield to reduce exposure during most fluoroscopy and all cine angiography procedures. The shield is used with an extension bar attached to the angiography table, allowing the operator to control the table for panning during angiography.

"This technique can save considerable radiation exposure to lead-protected and unprotected parts of body and does not adversely influence the conduct of procedure," Magram said.