Low-dose CT Techniques & Applications

CT is the workhorse of imaging, but as utilization surged in the new millennium, so did concerns about radiation dose. Work on refining reconstruction and acquisition techniques and driving doses down started a number of years ago but recent refinements are pushing dose lower than ever. Meanwhile, low-dose CT’s role in screening has taken center stage.

The rapid expansion of CT was propelled by the fact it had become a critical component of quality healthcare, says William P. Shuman, MD, vice chair and medical director of the Department of Radiology at University of Washington Medical Center in Seattle. “CT is very valuable in the process of getting accurate diagnosis and without good diagnosis, there is no good therapy.”

Still, concerns about dose both within medical literature and in the popular media have propelled efforts to advance low-dose CT technique. While the risks of medical radiation are debatable—and miniscule compared with the risks of not receiving a scan for most indications—patient safety is always paramount. There used to be a trade-off between image noise and dose, Shuman notes. Now, the noise index can be increased to lower dose, and advanced reconstruction can help clear the noise.

“We end up with the best of both worlds—extremely low dose and good images,” he says.

For decades, CT images were reconstructed from raw data using filtered back projection (FBP). FBP algorithms were limited by assumptions that simplified CT geometry to balance reconstruction speed and image noise, but more complex techniques, such as adaptive statistical iterative reconstruction (ASIR) were used to reduce noise while preserving resolution. There is a minimal trade-off in terms of speed, with raw datasets reconstructed with FBP at 15 images per second vs. 10 images per second with ASIR. This is a minor concern as it still takes longer to move the images to a workstation for viewing than it does to convert raw data to images using simple iterative reconstruction.

The next generation of reconstruction is a still more complex algorithm, model-based iterative reconstruction (MBIR), which uses detailed models of radiation and CT equipment. MBIR is much more demanding, requiring multiple computers working for 30 minutes or more to create a complete scan. But the result is quality images at radiation doses as much as 80 percent lower than standard reconstruction. Shuman says he thought physicians in areas like the emergency department, where urgency is king, might not be interested because of the delay. Not so, they are happy to wait in some cases, such as with pediatric patients who are more sensitive to radiation.

Shuman and colleagues compared FBP, ASIR and MBIR in routine-dose clinical CTs of the liver in a study published in the May 2013 issue of the American Journal of Roentgenology. A total of 36 patients with known liver lesions were scanned, and randomized images were presented to a pair of independent blinded reviewers who categorized the appearance of lesions and scored lesion conspicuity.

For the 51 focal lesions in the scans, the reviewers found no statistical difference in lesion detection among the reconstruction algorithms. There was, however, significant difference in background noise. With MBIR, background noise in air was 76 percent and 80 percent lower compared with ASIR and FBP, respectively. Likewise, background noise in subcutaneous fat was 42 percent lower with MBIR compared with ASIR and 54 percent lower with MBIR compared with FBP.

This kind of promising research with MBIR opens the door to still further reduce dose.

Dose falling through the gaps

In addition to complex reconstruction algorithms, other techniques can be used to reduce CT. One technique, called sparse sampling, was on display at the 2013 annual meeting of the Radiological Society of North America. There, Ranish Deedar Ali Khawaja, MD, a radiology research fellow at Massachusetts General Hospital (MGH) and Harvard Medical School, presented work by himself and colleagues, which was conducted as a collaboration between MGH and Philips Healthcare. Sparse sampling is a data sampling technique that can generate quality CT lung screening scans at doses as low as 0.25 mSv. For comparison, the average effective dose of a typical standard-dose chest CT is around 7 mSv, while the low-dose scans used in the National Lung Screening Trial (NLST) were close to 2 mSv.

The basic idea behind sparse sampling is that noncontinuous data could be acquired with angular gaps between projections as opposed to conventional continuous scanning, Khawaja explains. Proposed technologies would take angular views every fourth projection. 

“Taking less angular views is just one method of taking fewer x-rays,” he says. “Another is reducing the dose in each view. The advantage of sparse sampling is that—so far in preliminary results—we have shown that it gives improved image quality at the same dose as simply going to very low doses in each view.”

While Khawaja and colleagues’ study was a simulation—there are no FDA-approved scanners with sparse sampling yet available—it did demonstrate the implication of such technology down the road. Images were generated by reconstructing 25 percent of the angular projections and the processed with iterative reconstruction. In nine of the 10 patients studied, diagnostic confidence with sparse sampling was equivalent to that of standard-dose FBP.

Under surveillance

Low-dose CT lung screening has been making headlines recently, from continued analysis of the NLST to the debate over whether Medicare should cover low-dose screening for those at high risk for lung cancer.

Beyond broad screening efforts, though, one study presented at the 2013 American Association for Thoracic Surgery (AATS) annual meeting evaluated the use of ultralow-dose CT specifically for patients undergoing surveillance after lung cancer surgery.

What the researchers were looking for was a different way to diagnose new or recurrent lung cancer in these patients aside from standard chest radiography, explains Waël C. Hanna, MD, MBA, assistant professor of surgery at McMaster University, who was a fellow at the University of Toronto at the time of research.

“We thought screening those individuals for a recurrent lung cancer would be useful for prolonging their survival, but when we looked at the data, we found that x-rays, which were the standard of care and still are standard of care in many hospitals, had a very low sensitivity for detecting new or recurrent lung cancer,” says Hanna.

Lung cancer survivors are at a particularly high risk of developing a new lung cancer, he explains. These patients have a 1 to 2 percent chance per year of developing a new or recurrent cancer. For comparison, the high-risk population in the NLST had 0.3 to 0.5 percent chance per year.

To test ultralow-dose CT vs. chest radiography for post-surgery surveillance, Hanna and colleagues enrolled 311 patients who had undergone curative lung cancer resection from 2007 through 2012. Each patient was scanned with both modalities for up to five years, initially every three months, with increasingly longer follow-up intervals as the study progressed.

Results showed CT was more than four times as sensitive as chest x-ray for diagnosing new and recurrent lung cancer, and it also had a slightly higher negative predictive value compared with x-ray. While Hanna says the trial wasn’t designed to show survival, a vast majority of the cancers were detected only at CT within two years—in time to perform repeat resections to stop the spread of the disease.

Hanna says the results are enough to suggest using low-dose CT as the sole method of surveillance. and notes that the American Thoracic Society and the National Comprehensive Cancer Network also have recommended low-dose CT in their guidelines.

“Low dose CT is something that is going to be integral in the management of lung cancer, either via screening…or for surveillance.”