CHICAGO – Dual-energy computed tomography may have a role in clearing patients with embedded bullets and other metallic objects for magnetic resonance imaging who would otherwise be excluded because of safety concerns.

A Swiss analysis of bullets and shotgun pellets found that dual-energy CT had a high discriminatory power to distinguish between projectiles with and without ferromagnetic properties. MR safety depends on whether a metallic object has ferromagnetic properties – something that most patients don’t know or may be unable to communicate during an emergency evaluation, study author Dr. Sebastian Winklhofer said at the annual meeting of the Radiological Society of North America.

Dr. Sebastian Winklhofer was awarded the RSNA Trainee Research prize for his study.

"Dual-energy measurements may contribute to MR safety and allow for MR imaging of patients with retained projectiles," he said.

Retained metallic objects such as bullets, shrapnel, and medical devices are frequently contraindicated for MRI, because they might move, dislodge, or accelerate at dangerously high velocities toward the scanner’s magnet. This has led to patient injury, and in some cases death, if the object is located near or in a critical anatomic structure.

Session comoderator Dr. Seppo Koskinen of Helsinki University Central Hospital, Finland, said he recently had to forgo MR imaging in a patient with a bullet lodged in the spinal canal because the patient didn’t know if it was lead or steel.

"We didn’t do an MR, but maybe with this system we could have," he said in an interview. "So it has a real clinical impact."

Unsafe exams and misinformed refusals to refer or scan a patient are also occurring as a result of confusion over the ever increasing number of metallic orthopedic and cardiovascular implants. For example, a device that is known to be safe on a 1.5 tesla scanner may be contraindicated on a more powerful 3T scanner or on other scanners using different settings.

When the American College of Radiology penned its Safe MR Practices guidelines in 2007, no cardiac pacemakers or implantable defibrillators were labeled as safe or conditionally safe for MR imaging, prompting the college to recommend that routine MRI is inadvisable in patients with these devices.

In its scientific statement on the controversial topic, the American Heart Association says data are available to support MR imaging in patients with cardiovascular devices, but recommends a series of precautions, including careful patient screening and an accurate determination of the device and its properties (Circulation 2007:116:2878-91).

Dual-energy CT has been used in research and clinical settings to assess the composition of various objects, said Dr. Winklhofer of University Hospital Zurich, Switzerland. He highlighted one study showing that dual-energy CT had a 98% specificity in differentiating between urinary stones that did or did not contain uric acid (Invest. Radiology 2010;45:1-6).

In the current study, nine bullets and two shotgun pellets were scanned in an ex vivo chest phantom using second-generation dual-source CT (SOMATOM Definition Flash, Siemens Healthcare), with tube voltages set at 80, 100, 120, and 140 kVp. Two readers, blinded to the ferromagnetic properties of the projectiles, independently assessed CT numbers on 44 images reconstructed using the extended CT scale technique, which helps overcome artifacts arising from metallic objects. A CT number is the density assigned to a voxel in a CT scan.

Dual-energy indices (DEI) were calculated from 80/140 kVP and 100/140 kVP pairs, and receiver operating characteristics analyses were fitted to predict ferromagnetic properties by means of DEI and CT numbers.

The bullets/pellets ranged in diameter from 2 mm to 14 mm; five were ferromagnetic and six nonferromagnetic.

Intrareader agreement was significantly correlated with mean CT number measurements (P less than .001), with excellent intraclass correlation coefficient agreement (Reader 1: ICC = 0.998; Reader 2: ICC = 0.963), Dr. Winklhofer said.

Interreader agreement was also significantly correlated (P less than .001), and again, there was excellent ICC agreement (both readers ICC = 0.988).

In contrast, when the same projectiles were scanned with single-energy CT, no significant differences in CT numbers for ferromagnetic vs. nonferromagnetic projectiles were observed, said Dr. Winklhofer, who received the RSNA Trainee Research prize for his work.

"Single-energy does not allow for differentiation between those types of projectiles, whereas the dual-energy results show it is possible to differentiate between ferromagnetic and nonferromagnetic projectiles," he said.

Finally, the dual-energy indices of ferromagnetic projectiles were significantly higher than were those of nonferromagnetic projectiles for both the 80/140 kVP and 100/140 kVP energy pairs (at a P value less than .10).

Before dual-energy CT is ready for prime time, however, additional studies are needed to prove that the technique is valid in phantoms of other body regions and in humans, Dr. Winklhofer said in an interview. The prospective study is currently underway.

Dr. Winklhofer and Dr. Koskinen reported no conflicts of interest.

p.wendling@elsevier.com

Bullets no match for dual-energy CT scrutiny