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Radiation dose in CT scans and image quality

Since its introduction in 1973, x-ray Computed Tomography (CT) has established itself as a primary diagnostic imaging modality. CT scans are one of the most frequently-used imaging tools in medicine. In fact, more than 72 million are performed, in the U.S. only, each year, to diagnose various medical conditions. CT allows physicians to diagnose injuries and disease more quickly, safely and accurately than alternative more invasive or less sensitive imaging techniques. But concerns persist about radiation dose and exposure from these tests, especially when given to children and young adults.

A Public Health Concern

CT scans help diagnose medical conditions including broken bones, cancers, internal bleeding or signs of heart disease. Yet, despite its prevalent use, medical societies and federal agencies have been increasingly trying to reduce the number of unnecessary scans due to radiation’s link to an increased risk for cancer. The quantity most relevant for assessing the risk of cancer detriment from a CT procedure is the “effective dose”. The unit of measurement for effective dose is millisieverts (mSv). On average, the organ studied in a CT scan of an adult receives around 15 mSv of radiation, compared with roughly 3.1 mSv of radiation exposure from natural sources each year.

In orthopedic trauma, CT scans help diagnose injuries around a joint, particularly if the fracture involves the joint’s surface. The scans also help clinicians assess joint displacement and aid in surgical planning to put the joint back in position. An x-ray, while a good initial screening tool, does not provide the same level of detail.

CT scanning

Image quality and radiation dose

Radiation dose is one of the most significant factors determining CT image quality and thereby the diagnostic accuracy and the outcome of a CT examination. Radiation dose should only be reduced under the condition that the diagnostic image quality is not sacrificed. Therefore, to understand how the radiation dose in CT can be reduced, it is necessary to be familiar with the relationship between image quality and radiation dose.

Two important characteristics of the computed tomographic (CT) image that affect the ability to visualize anatomic structures and pathologic features are blur and noise. Increased blurring reduces the visibility of small objects; increased visual noise reduces the visibility of low-contrast objects. Sources of blurring in CT include the size of the detectors (sampling aperture), the size of the voxels, and the reconstruction filter selected. Noise is caused by the variation in attenuation coefficients between voxels. Use of small voxels and edge-enhancing filters helps reduce blurring and improve visibility of fine details. However, small voxels absorb fewer photons and therefore result in increased noise. Noise can be reduced by using large voxels, increasing radiation dose, or using a smoothing filter, but this filter increases blurring. An optimized protocol for a specific clinical study must take these physical principles into account and be adjusted to give proper balance among detail, low noise, and patient exposure.

Coronal reformatted slice of 3D helical X-ray CT scan of the abdomen/pelvis

For these thin-slice images, the right scan shows much lower noise than the left image. (Patient identifiers were removed.)

The goal of dose reduction can be approached from the following two perspectives. The first perspective is to appropriately define the target image quality for each specific diagnostic task, not requiring lower noise or higher spatial resolution than necessary. For example, CT colonography mainly involves the detection of soft-tissue-like polyps from a background consisting of air and contrast-tagged stool (i.e., a high-contrast situation), so the noise level is allowed to be high and the dose can be relatively low without sacrificing the diagnostic confidence.The second perspective on dose reduction is to improve some aspects of image quality, such as reducing image noise, which can then be implemented in order to allow radiation dose reduction. This task can be accomplished by optimizing the CT system and scanning techniques, and improving the image reconstruction and data processing.

Advancements in CT Scanner Technology Mean Less X-Ray Radiation for Patients

X-ray machines emit radiation during every single use, but recent technological advances in CT scans could drastically lower the amount emitted and diminish the risks associated with them.

The newest CT scanning systems have seen decreases in radiation production, meaning doctors will no longer have to potentially risk their patient’s long term health for short term medical gains. The significant decrease is thanks, in part, to not only an increase in the number of detectors in the machines, which will require less passes of the scanner over a particular organ, but also faster moving x-ray components which means the patient will be under the scanner for less time.


Ultra-Low Dose CT Scans Successfully Detect Fractures

Researchers at NYU Langone Medical Center are reporting in a new study that they successfully performed CT scans for joint fractures with one-fourteenth the amount of normal CT radiation, without compromising image quality or a surgeon’s ability to effectively diagnose an injury. Specifically, the researchers reduced the average amount of radiation from 0.43 msV to 0.03 msV, or down to the average dose given in a routine chest x-ray.

“We have taken a frequently used and necessary imaging test and made it safer,” says lead study author Sanjit R. Konda, M.D., assistant professor of orthopedic surgery at NYU Langone and director of orthopaedic trauma at Jamaica Hospital Medical Center. “Providing patients with a CT scan with 14-times less radiation could have significant implications from a public health and safety standpoint.”

Konda’s team worked with radiologists from NYU Langone to reduce the amount of CT radiation while maintaining image quality. Together, they developed a protocol called REDUCTION (Reduced Effective Dose Using Computed Tomography In Orthopaedic Injury).

Previously, the group applied this protocol to examine air around a knee joint where infections easily could develop. Its application was so successful that the researchers set out to use the protocol to reduce radiation for traumatic joint fractures.

50 patients showing clinical symptoms of joint fractures received ultra-low dose radiation CT scans. Images from these ultra-low dose CT scans were compared to a sample of age-matched, similar fracture injuries where patients were evaluated with a standard CT scan.

This is an ultra-low radiation dose CT scan of a fracture of the tibial plateau (left) compared to a conventional dose CT scan. Credit: NYU Langone MedicalCenter

Ultra-low dose radiation CT scan of a fracture of the tibial plateau (left) compared to a conventional dose CT scan. Credit: NYU Langone MedicalCenter

According to results, researchers achieved 98% sensitivity and 89% specificity with the ultra-low dose CT scans.  Equally important, these findings were comparable to the conventional CT-scans (98% sensitivity and 85% specificity with occult fractures removed). Image quality was rated moderate to near perfect by the orthopedic surgeons.

The ability to perform ultra-low dose radiation CT scans without compromising image quality demonstrates the comprehensive capabilities of this protocol.

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