Imaging technique measures tumour stiffness to aid surgical planning

Important steps in planning tumour surgery include identifying borders between tumour and healthy tissue and assessing the tumour stiffness, e.g. hard and calcified or soft and pliant. For decades, tumours near the surface of the body have been evaluated for stiffness by simple palpation-the physician pressing on the tissue. Because tumours within the skull cannot be palpated, researchers used Magnetic Resonance Elastography (MRE) to assess pituitary tumour stiffness by measuring waves transmitted through the skull into pituitary macroadenomas (PMAs). MRE reliably identified tumours that were soft enough for removal with a minimally-invasive suction technique versus harder tumours requiring more invasive surgery.
‘The group developed brain MRE several years ago and is now successfully applying it to clinical diagnosis and treatment,’ explained Guoying Liu, Ph.D., Director of the NIBIB Program in Magnetic Resonance Imaging. ‘This development of a new imaging technique followed by its practical application in surgical planning for better patient outcomes is an outstanding example of one of the main objectives of NIBIB-funded research.’
MRE is a special magnetic resonance imaging technique that captures snapshots of shear waves that move through the tissue and create elastograms-images that show tissue stiffness. John Huston III, M.D., Professor of Radiology at the Mayo Clinic in Rochester, MN, and senior author of the study, explains how MRE works. ‘MRE is similar to a drop of water hitting a still pond to create the ripples that move out in all directions. We generate tiny, harmless ripples, or shear waves, that travel through the brain of the patient. Our instruments measure how the ripples change as they move through the brain and those changes give us an extremely accurate measure–and a coloUr-coded picture–of the stiffness of the tissue.’
Ninety percent of PMAs are soft-nearly the consistency of toothpaste. Therefore, without MRE, surgeons would routinely plan for a procedure called transphenoidal resection that employs very thin instruments that are threaded through the nasal cavity to the pituitary gland at the base of the skull, where suction is used to remove the tumour. However, in about 10percent of the cases, the surgeon will encounter a hard tumour. At that point an attempt is made to break-up the tumour-essentially chipping away at it with sharp instruments. If that is not successful, the surgeon must perform a fully-invasive craniotomy that involves removing a piece of the skull bone in order to fully expose the tumour.
The more extensive procedure means added risk and discomfort for patients, and up to a week-long recovery in the hospital compared to the transphenoidal approach that allows patients to leave the hospital in a day or two. Using MRE, hard PMAs can be identified and the more extensive craniotomy can be planned before starting the surgery, which makes the more invasive procedure less taxing for both the surgeon and patient. Similarly, MRE showing a soft PMA gives surgeons confidence that the nasal entry and removal by suction will be successful-eliminating the likelihood that the surgeon may need to perform a second fully-invasive craniotomy.

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