Critical Blind Spot in MRI
As patient preferences move toward non-invasive treatments, so does the call for greater accuracy in radiation-based methods. Clearer imaging of the tumour’s size, shape and location lets doctors accurately determine dosage – and aim radioactive beams at the right spot.
“Of all imaging modalities, MRI has the best soft tissue contrast, so it’s the modality of choice by radiation oncologists for target delineation in radiation therapy treatment planning,” explains Nosrati. There’s just one shortfall: “The only thing missing in MRI is to show up implanted metallic devices.”
Devices such as gynecological catheters or prostate brachytherapy seeds are indispensable components of certain cancer treatments. Identifying the existence and location of these metal-based devices is critical in treatment planning and delivery, Yet, MRI shows up the devices as mysterious voids – difficult to distinguish from calcium or empty cavities.
Removing CT from the Workflow
The current clinical workflow requires a computed tomography (CT) scan for its superior positive contrast in identifying “MR-invisible” metallic devices. After an MRI, patients are transferred to another room for a CT scan. The images are later registered together into one.
The problem: “The patient is never in exactly the same position during the two scans,” Nosrati explains. “Errors in image registration and dose calculations can happen.”
The two-step MR-CT based workflow does the job, but with a host of other drawbacks – time, cost and increased secondary cancer risk from radiation administered for CT scans.
The solution: Combine the strengths of both into a single, MR-only process. Ideal concept, but actualizing it has been a moving target – various techniques coming close, but with logistical complications, inordinate expense or other encumbrances.
Nosrati proposed a new method: Create positive contrast, and automate the identification of metal devices by using magnetic susceptibility as the source of positive contrast, and thus eliminate CT from the current workflow.
The method: Quantitative Susceptibility Mapping (QSM). Metal has positive magnetic susceptibility whereas biological tissues are negative. QSM is a relatively novel MR post-processing technique that quantifies magnetic susceptibility of different substances using MR images acquired by a specific protocol.
