Filters Lower Radiation Dose in Adult, Pediatric CT

Technology continues to help reconcile radiology's dual needs for image quality and low CT radiation dose, according to new research from Massachusetts General Hospital (MGH) in Boston.

Sarabjeet Singh, M.B.B.S., M.M.S.T. Massachusetts General Hospital

One study presented at RSNA 2008 showed that radiation dose can be reduced by as much as 75 percent in adults who require a chest CT with the use of 2D non-linear adaptive filters (2D-NLAF) that effectively decrease image noise in low-dose CT scans without compromising image quality.
A second study presented at RSNA 2008 showed a reduction of 25 to 30 percent in radiation dose for pediatric chest and abdominal CT scans with the use of those filters. Both studies were conducted between April 2007 and April 2008.

"2D-NLAF post-processing of low radiation dose CT images increases the diagnostic acceptability of low-dose images without compromising the diagnostic information," said lead author of both studies, Sarabjeet Singh, M.B.B.S., M.M.S.T., a cardiac imaging research fellow at MGH.

Radiologists Asked How Low They Can Go

Such research not only reinforces continued efforts by radiologists to lower radiation dose but also helps steer new technology from the research lab to the exam room, experts said. With continued media scrutiny and a rapidly climbing number of CT scans each year, the pressure remains on radiologists to show how low they can go in terms of dosage—especially in children, experts said.

"Studies like these show that the radiology community is stepping up to the challenge and continuing to lower the dosage," said Cynthia H. McCollough, Ph.D., director of the CT Clinical Innovation Center and a professor of radiological physics at the Mayo Clinic in Rochester, Minn. Dr. McCollough presented a refresher course on radiation dose for CT angiography at RSNA 2008. "We're doing a lot of CTs but the dosage from each exam gets lower and lower," she said. "Images now are being done at about half the dose used in the 80s."

Algorithms Resemble Human Vision System

Radiation dose can be reduced by as much as 75 percent in adults requiring a chest CT with the use of 2D non-linear adaptive filters (2D-NLAF), according to a study presented at RSNA 2008. These low-dose, filtered adult chest CT images show lower image noise for both 50 and 75 percent levels.

Adaptive filtering has proven to reduce noise and enhance CT images without losing edge, boundary or feature details. The 2D-NLAF software used by Dr. Singh and colleagues is based on algorithms that resemble the human vision system. The software analyzes each part of the picture, removing random noise without affecting image quality, lesion detection, conspicuity or reader confidence, said Dr. Singh.

"Our studies point out some important features of the algorithm, like preserved Hounsfield Units (HU) and sharpness of small structures without creation of artifacts," he said.

In the first half of a two-part study, Dr. Singh and colleagues took 35 limited chest multidetector CT scans at both standard (144-224 mAs) and one-half (72-112 mAs) radiation doses. Low-dose images were processed through 2D-NLAF filters at two settings to reduce noise and increase sharpness. Standard and original and filtered one-half dose images were randomized and reviewed by two radiologists for image noise, contrast, visibility of small structures and presence of artifacts.

In the study's second part, 18 standard dose (150-190 mAs) chest multidetector CT scans with subtle lesions were post-processed using noise projection software to obtain 50 percent (70-95 mAs) and 75 percent (40-50 mAs) simulated low-dose images, which were then processed using the same filters. In addition to the image criteria, images were reviewed for lesion size, number, conspicuity, likely diagnoses and diagnostic acceptability.

In both study parts, all low-dose scans showed significant improvement in subjective and quantitative image noise with 2D-NLAF filters, without showing any loss of contrast or visibility of small structures, Dr. Singh said. All 26 lesions were seen on standard- and low-dose original and filtered images, and there was no significant difference in CT number, lesion conspicuity, characterization and reader confidence between filtered 75 percent low-dose and standard-dose images.

2D non-linear adaptive filters (2D-NLAF) can help reduce radiation dose for pediatric chest and abdominal CT by improving low-dose follow-up CT imaging, according to a study presented at RSNA 2008. Here, low-dose post-processed images of a 7-year-old male with history of liver transplant indicate lower noise. Images courtesy of Sarabjeet Singh, M.B.B.S., M.M.S.T.

"Results show that 2D filters can help reduce CT radiation dose for chest examinations by as much as 75 percent without compromising diagnostic acceptability or lesion detection," Dr. Singh said.

Filters Lower Dose in Pediatric Follow-up Imaging

A second study showed that 2D-NLAF can help reduce radiation dose for pediatric chest and abdominal CT by improving low-dose follow-up CT imaging.

Dr. Singh and colleagues examined 26 children who underwent follow-up chest and abdominal CT after initial exams. Follow-up CT exams were performed at 25 to 30 percent lower dose compared to the initial CT study with use of automatic exposure control and post-processed with 2D-NLAF. Results showed significant improvement in qualitative and objective noise after 2D-NLAF when compared to unfiltered low dose images, but no difference in HU value, visibility of small structures, lesion conspicuity and diagnostic confidence.

"Using 2D-NLAF can help reduce radiation dose for pediatric chest and abdominal CT by increasing the acceptability of low-dose, follow-up CT imaging," said Dr. Singh.

Larger Study Under Way

Further research into noise reduction tools like 2D-NLAF filters will move the technology closer to broad-scale implementation, said Dr. Singh, who is conducting a larger study. "We are acquiring low radiation dose CT images in 400 patients and will have more data after processing these cases with 2D filters," he said.

Ultimately, the goal is to get the most information from a CT scan using the least radiation, said Dr. McCollough. "It's like squeezing water from a sponge," she said.