Cardiac CT Studies Demonstrate Low Radiation Risk

New studies presented at RSNA 2008 challenge reports of high radiation risk from cardiac CT and offer ways to even further reduce the risk.

U. Joseph Schoepf, M.D. Medical University of South Carolina

Sebastian Leschka, M.D. University Hospital Zurich, Switzerland

U. Joseph Schoepf, M.D., a professor of radiology and medicine at the Medical University of South Carolina in Charleston, took aim at studies published in 2007 in The New England Journal of Medicine and The Journal of the American Medical Association on lifetime cancer risks from radiation received during computed tomography coronary angiography (CTCA).

There is nothing wrong with the conduct and findings of the NEJM and JAMA studies using the Biological Effect of Ionizing Radiation (BEIR) VII Phase 2, said Dr. Schoepf. "However, for these studies there wasn't a single, actual patient scan performed," he said. "The results were extrapolated from the atom bomb radiation survivors to radiation received at CTCA, which is not necessarily a valid approach, considering the differences of these radiation sources," he said.

Working with Walter Huda, Ph.D., Dr. Schoepf studied 104 patients undergoing 64-slice CTCA.

Researchers found a 0.12 percent lifetime risk for developing cancer from radiation received during the scan—dramatically lower than the one in 114 risk reported by the press from the journal articles. "Our assessment is more reflective of the real world," said Dr. Schoepf.

The highest radiation dose risk was found to be in the breast and lungs. "This is a very theoretical risk," said Dr. Schoepf. "However, CT can diagnose life-threatening conditions in a non-invasive way. There is a much greater real risk of dying from complications during invasive work-up or if cardiac conditions go undiagnosed if a patient refuses a CT scan for fear of radiation."

A multicenter study reported in the Feb. 4, 2009, issue of JAMA demonstrated that while the radiation dose of 12 mSV for CTCA is comparable with other diagnostic procedures, the dose varies significantly between study sites and CT systems. Authors of the JAMA article call for improved education of physicians, noting that radiation exposure could be reduced substantially with uniform application of dose reduction techniques that are currently used only infrequently.

The JAMA article is important because it provides for the first time a global and objective benchmark of radiation exposure associated with cardiac CT, said Dr. Schoepf.

"This article tells the story of learning and learning curves in the adaptation of new technology," he said. "We see that it takes time for novel developments to penetrate clinical practice, but also that we can be optimistic about their eventual implementation."

Effects of Dose Reduction on Image Quality Examined

Another study presented at RSNA 2008 investigated the effect of low kilovoltage dual-source CTCA on qualitative and quantitative image quality parameters and the radiation dose.

"CTCA represents a rapidly evolving imaging modality for the assessment of coronary artery morphology and stenosis," said Sebastian Leschka, M.D., a staff radiologist at the Institute of Diagnostic Radiology at the University Hospital Zurich in Switzerland. "An increase in radiation dose has paralleled advances in spatial and temporal resolution CT and improvements in diagnostic accuracy of CTCA."

The goal for every examination using CTCA should be a reduction in radiation dose, said Dr. Leschka. For his study, dual-source CTCA was performed in 80 total patients of normal weight with suspected coronary artery disease. Forty patients were scanned with CTCA standard protocol, 120 kilovolts (kV), 330 mAs. Twenty patients were examined at 100 kV/330 mAs and another 20 at 100 kV/220 mAs. Two observers independently assessed each coronary segment. Diagnostic image quality was obtained in 99 percent of all coronary segments without significant differences among the three protocols.

"The image noise was higher in both 100 kV CTCA protocols," said Dr. Leschka. "Radiation dose savings of up to 50 percent could be achieved when reducing both tube voltage and tube current in CTCA without impairing the subjective image quality."

There were some surprises when comparing both 100 kV CTCA protocols, he said. "The increase in image noise and decrease in the contrast-to-noise ratio were not significant when the tube current was lowered," he said. "This might indicate an overexposure of radiation in the protocol with 100 kV and higher tube current because image quality parameters are only improved to a minor extent."

Since completion of the study, Dr. Leschka said all CTCA exams at the University Hospital Zurich in patients of normal weight are routinely performed with a 100 kV and tube current time product of 220 mAs.

A follow-up study has determined the radiation doses and image quality of different dual source CTCA protocols tailored to patient-specific parameters, such as heart rate and body mass index in a large number of consecutive patients, Dr. Leschka added. "The combined application of low kilovoltage in normal weighted patients and prospectively ECG-gated CTCA in patients with a heart rate below 70 beats per minute resulted in a dose saving of up to 86 percent while maintaining the diagnostic image quality of the exam," he said.

Pulmonary Embolism a Focus of Chest Series Presentations

Other studies presented at RSNA 2008 explored the use of CT venography (CTV) and CT pulmonary angiography (CTPA) in cases of suspected pulmonary embolism (PE).

Dameon Duncan, M.D., M.B.A. Long Island Jewish Medical Center

Michael Lu, M.D. University of California, San Francisco

CTV should not be routinely performed in patients receiving CTPA for suspected PE, unless there are associated risk factors for PE or deep vein thrombosis (DVT), said Dameon Duncan, M.D., M.B.A., who presented his study during the chest series course.

Dr. Duncan and colleagues performed a retrospective analysis of combined CTV and CTPA (CTVPA) reports between May 2003 and June 2006 at Long Island Jewish Medical Center in New Hyde Park, N.Y. The study focused on 1,315 patients undergoing diagnostic CTVPA, dividing patients into groups according to combinations of PE and DVT findings. Data was then analyzed to further categorize patients by reported risk factors for DVT.

Results showed that 51.9 percent of all patients had risk factors for DVT, while 48.1 percent did not. Of the 638 patients with risk factors for DVT, 13.6 percent had DVT and 86.4 percent did not; 2.1 percent of all patients did not have risk factors and were positive for both PE and DVT; and 0.8 percent of all diagnostic CTVPAs did not have risk factors, were negative for PE and positive for DVT.

Another study looked at whether the interval increase in RV/LV diameter ratio between any prior contrast-enhanced chest CT (of up to 5mm slice thickness) and a positive CTPA (of 1mm slice thickness) for acute PE improves prediction of 30-day mortality compared to the diameter ratio from the positive CTPA alone.

The study retrospectively examined 100 consecutive patients with a CTPA diagnosis of acute PE and prior contrast-enhanced chest CT. Of those patients, 27 underwent CTPA, 2 had angiograms for suspected aortic dissection, and 71 had 5 mm slice thickness contrast-enhanced chest CTs.

Results showed that "an interval increase in the size of the right ventricle is significantly more accurate overall than right ventricular size alone for predicting mortality after acute PE," said author Michael Lu, M.D., of the University of California, San Francisco.