As recently as 2005, Boston Children’s Hospital’s Department of Radiology performed 25 to 30 CT studies daily to check ventricular shunts–devices placed in children with hydrocephalus and other conditions to drain fluid from the brain’s ventricles. Today, the volume of these CT scans has fallen to one exam every few days. Richard Robertson, MD, radiologist-in-chief at Boston Children’s, thinks this 77 percent drop is great news.
Neuro-imaging exams are essential for children with ventricular shunts presenting with new neurologic symptoms to help determine whether the shunt is working properly or has become blocked or disconnected. “Kids who have shunt catheters can have a large number of CT studies, in some patients up to 50 or 60 over their lifetimes. A child with an infection or shunt malfunction may have many studies even in a single month,” says Robertson.
Although the exams are necessary, exposure to ionizing radiation from even a single CT exam carries a slightly increased risk for cancer that rises with each subsequent exam. There is no known threshold below which exposure is considered safe.
During the 2005 meeting of the American Society of Neuroradiology, one of Robertson’s colleagues gave a presentation about single-slice acquisition MRI, a limited, two-minute exam that provides the basic information needed to assess the size of the cerebral ventricles. Unlike a conventional MR scan, which has a long acquisition time, this short exam wouldn’t require most young patients to be sedated.
Robertson realized the exam also offered a way to avoid exposing some ventricular shunt patients to ionizing radiation. But a single-acquisition MRI exam, which utilizes a fraction of the scanner’s tools, goes against the grain of trying to utilize the full capabilities of MR technology for every patient.
The challenge in switching to MRI
During the last two decades, MR scanners have acquired many new capabilities, including diffusion imaging, which maps water diffusion; spectroscopy, which uses hydrogen signals to display metabolic changes in brain disease; and FLAIR sequences, which suppress fluid signals to improve visualization of lesions. A conventional MR scan consists of multiple MR sequences, each designed to bring out a different tissue characteristic.
“Twenty years ago, we had three sequences, each of which might take from 10 to 15 minutes to complete, and an MRI scan took an hour,” says Roberts. “Today, even though acquisition time has dropped and each sequence can be performed in less than a couple of minutes, a standard scan still takes an hour or longer to finish due to the advances in sequence options.”
Were all these technical features really needed for ventricular shunt patients? “Our MR imaging protocols are designed to address a broad range of clinical concerns, but if we focus on the specific clinical question, we can eliminate sequences that don’t add significant value,” says Robertson. “We need to determine what’s truly needed to answer the clinical question and structure the scan to answer that specific question.”
Robertson, together with the neuroradiologists and the MR team, embarked on a plan to provide a limited MRI exam that would answer this question: Are this patient’s symptoms possibly related to increased intracranial pressure from shunt malfunction?
They needed to overcome several hurdles to change from assessing ventricular size with CT to using MR. Most MRI appointments are booked for 45 minutes to one hour, making it difficult to fit shunt patients, who arrive primarily via the emergency department, into the MRI schedule. Accommodating unscheduled, albeit ultra-fast, two-minute scans, required MRI staff to adopt a new way of doing business. Robertson’s team also worked to convince the emergency and neurosurgery clinicians to consider MR instead of CT in patients with suspected shunt malfunction. “Physicians worried that the MRI process would delay care,” he says.
Another concern was cost. The single-acquisition MRI exam required approximately the same amount of time and resources as a CT study. A typical CT exam billed for several hundred dollars, while the bill for an MRI exam is significantly higher. But since single-acquisition MRI uses less time and fewer resources than other MR exams, Robertson convinced Boston Children’s finance department to bill the MRI vent check as a limited exam, priced comparably to a head CT study, to eliminate the financial disincentive.
With an implementation plan in place, the program went live in 2007.
Despite considerable enthusiasm, substitution of single-acquisition MR for CT exams was slow for the first two to three years. However, as the risks of pediatric exposure to ionizing radiation became front-page news, stakeholders recommitted to the single-slice MRI model.
“We’ve seen an enormous ramp-up over the last three years,” Robertson says. “We’ve gone from doing just an occasional study to performing more than 85 percent of vent check imaging exams with MRI rather than CT.”
The Radiology Department is now eyeing other limited MRI applications, such as using MR rather than CT to evaluate patients with chronic sinus symptoms. For this application, radiologists need to visualize fluid and mucosal thickening. These changes can be readily observed via a limited MR exam (although the bone detail available with CT is not currently possible with MR).
“As pediatric imagers, we need to ensure that imaging is done thoughtfully, maximizing the information gained and minimizing patient risk. It’s more labor-intensive, but safer, than a shotgun approach to radiology exams,” says Robertson.
Learn more about Boston Children’s radiology services.