From the October 2017 issue of HealthCare Business News magazine
By Michael Friebe
The unparalleled soft tissue contrast and excellent image quality of MR have made it the imaging modality of choice especially for neuro and oncological applications.
The obtained MR images are then frequently used as relevant preoperative images for surgery planning, or as a visual reference directly in the surgery suite.
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MR systems directly employed for therapeutic guidance or surgical monitoring are rarely available. The standard imaging systems in the surgery room are typically X-ray, ultrasound or endoscopy (video) performing the patient monitoring, controlling and device guiding functions.
Figure 1 (click image in upper right corner): Typical horizontal field 3T system with 70cm bore diameter. Actual therapy is difficult for the surgeon and only very little vertical space is available. A holding arm could be very helpful (shown in black), but must be very small and flexible to be of use.
To use these preoperative MR images also for therapy purposes usually requires fusion with the available imaging data in the surgery room. That is either not done, or happens only in the head of the clinician, so called cognitive fusion, or is accomplished by more or less accurate and complicated special software tools in combination with optical and/or electromagnetic device tracking systems.
For this fusion via software-based image co-registration the images are manually or semi-automatically registered to each other. A preoperative MR image, as the wording implies, was taken before the surgery, however, sometimes even on a bed with different curvature than the surgery table or in a different patient position.
For prostate MR, for example, the imaging is typically done with the patient in a straight supine position, while the actual biopsy or therapeutic treatment is performed prone or supine with the legs angled toward the torso. And for breast MR, the patient typically lies in an elevated prone position while surgery is performed on a surgical table in supine position or on the side. You easily recognize the difficulties using these preoperative MR images for accurate image fusion with the available imaging modalities available in the therapy or surgery room.
Real, precise registration is not possible that way with typical image registration errors in the order of 3-7mm, and in case of missing fiducial marker structures sometimes even significantly larger. That may be sufficient for actual therapies using an additional imaging system or for coarse orientation, but is too large for tissue biopsies and other precise therapies on small structures.
Using real time MR as a guidance device for intraoperative or interventional therapies (eg. biopsies and treatment of prostate cancer, liver laser-/radio-frequency-/ or cryo-ablations, brain interventions) could be very beneficial and would obviously eliminate or greatly reduce these registration errors.