Q&A | The Dawn of the Sub-mSv CT Era

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 - James H. Thrall, MD
James H. Thrall, MD

CT dose has emerged as the leading differentiator in the CT market and a top of mind issue for radiologists, clinicians and patients. The National Council on Radiation Protection and Measurements (NCRP) Report 160, published in 2009, helped sound the collective alarm by measuring medical radiation at the natural background level.

However, dose is a rapidly moving (and falling) target, and radiology has multiple arrows in the quiver, explains James Thrall, MD, chief radiologist, Massachusetts General Hospital (MGH of Boston) and president of the American College of Radiology (ACR). The sub-millisievert (mSv) era, in fact, is just around the corner.

How does the current state translate into the sub-mSv framework?

Thrall: NCRP Report 160 documented the average effective dose for CT at about 7.1 mSv in 2006. Aggregate exposure is likely below that point today because dose has come down substantially since 2006.

If we reduce the average dose to 1 mSv, that's about an 85 percent per scan reduction. If we reduce it to 0.5 mSv, that's close to a 95 percent reduction. Somewhere in that range is the sub-mSv goal.

How close are we to that mark?

Thrall: Actually, we are doing pretty well. Using new technology, we are already in the sub-mSv range for most pediatric applications. For pediatric cardiac imaging, we're seeing doses as low as 0.2 mSv, which represents astonishing progress. However, we cannot claim that we are in the sub-mSv range among obese pediatric patients.

With respect to adult imaging, an MGH survey demonstrated that we are imaging at about 30 to 50 percent below the effective doses reported by the NCRP. We've achieved this without a wholesale upgrade to new technology, which is going to be one of the keys to the sub-mSv era.

What technical components facilitate sub-mSv scanning?

Thrall: Every component of the CT scanner is under active improvement, and all of these improvements contribute to reduced dose.  

There are new, more efficient detector materials that require less radiation to collect a statistically valid sample of photons from the patient. Manufacturers are adopting iterative reconstruction methods, which appear to have an advantage of 20 to 50 percent in dose reduction.

Are there changes at the practice or department level that complement technical developments?

Thrall: We've found that we can use a multi-phasic injection of contrast material with multidetector CT. In the past, we might have done one scan to look at the arterial phase and a second scan to look at the venous phase. By staging the contrast injection, we can image the arterial and venous phases simultaneously, which results in a 50 percent dose reduction.

At MGH, we've applied appropriateness criteria to the ordering process and reduced CT utilization (and dose) by 15 to 30 percent.

Can you describe the pathway to sub-mSv scanning?

Thrall: We can start with the installed base of equipment by optimizing protocols, so that even the oldest CT scanners can deliver reduced dose. Then, the next most widely applied technology will be iterative reconstruction. Following that, we will see creative new detector materials and new designs such as multi-tube scanners.

What is the timeline for achieving the sub-mSv goal?

Thrall: We are rapidly moving in the right direction. We need the installed base of older equipment to be replaced, which will not happen overnight. The educational process underpinning protocol optimization and appropriate utilization takes time and repetition.

I would be surprised if we achieved the sub-mSv goal in less than five years across the country. Individual departments will achieve it in a couple of years, and I expect sub-mSv CT to be prevalent in the seven- to 10-year time frame.

How do you recommend radiologists and hospitals proceed?

Thrall: Radiologists should be leaders in the application of appropriateness criteria and decision support in the ordering process to eliminate studies that are truly not indicated. On top of that, we need to be meticulous in applying all of the different tricks of the trade to reduce dose. My advice to hospitals? Modernize your installed base of equipment.