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An imaging technique for assessing the elasticity of human tissue — could help physicians improve neurosurgical operations and reduce complications.
A stiffness map of a slice through the human brain.
Neurological imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), allow physicians to detect brain abnormalities, such as tumours, non-invasively. But unlike breast or other tumours beneath the skin that doctors can palpate to determine their size, firmness and consistency, it’s impossible to do the same with the brain because it is encased in the skull.
However, a new technique — called magnetic resonance elastography (MRE) — will allow neurosurgeons to “palpate by imaging” to determine the texture and stiffness of a brain tumour before treating it.
Developed at the Mayo Clinic in Rochester, Minnesota, MRE measures the wavelengths of vibrations transmitted through tissues and has been used primarily to diagnose liver disease. However, NRC researchers in Winnipeg have adapted this imaging technology to help characterize the elasticity of healthy and diseased brain tissue.
The MRE project is part of an NRC Genomics and Health Initiative program to establish virtual reality (VR)-based training systems for neurosurgeons. VR neurosurgery training centres are currently located in Halifax, Montreal, Ottawa, Toronto, London, Winnipeg and Calgary.
An MRI uses a magnetic field and radio waves to create cross-sectional images of the brain that provide information on the size, shape and location of a tumour. In contrast, an MRE can determine the stiffness or hardness of a tumour by gauging the elasticity of tissue as it vibrates in response to sound waves. Since different tumours vary in stiffness, MRE can help physicians make a precise diagnosis. The information it provides is not available using other imaging techniques.
“This is important for neurosurgeons to know in advance,” says Dr. Marco Gruwel of the NRC Institute for Biodiagnostics. “The brain is very delicate and a tumour is difficult to remove. One wrong cut can have serious consequences.”
Surgical planning tool
He adds that knowledge of the consistency of a particular tumour could help doctors plan a surgical strategy and avoid multistage surgical procedures. By giving surgeons information on a tumour’s consistency and stiffness, an MRE will help reduce complications during complex and intricate brain surgery. Such information can reduce surgical time and discomfort to the patient, and also reduce the risk that damage could occur to surrounding tissues, such as functional areas, nerves and blood vessels — or that cancer could recur if a tumour is not completely removed.
Some of the gadgetry behind the innovative technology is quite simple — and creative. Dr. Gruwel, Dr. Peter Latta and their colleagues designed a homemade MRE system that uses two acoustic actuators (stereo subwoofers or bass loudspeakers), two black sump-pump hoses and a no-name-brand plastic liquid honey bottle. The loudspeakers, which produce low-amplitude sound waves in the range of 50 to 100 Hertz (Hz), are located in a separate room from the MR scanner to prevent interference with its image-gathering magnet. They are both hooked up to a hard-walled hose made of semi-flexible material, which transmits sound waves into a honey bottle placed near the head of a patient.
The MRE system uses two bass loudspeakers to generate sound waves.
When sound enters the bottle, it gently vibrates the patient’s head, generating acoustic waves inside the brain that change in frequency, depending on the elastic properties of the tissues they pass through. For instance, diseased tissue is stiffer than healthy tissue. The resulting data are captured as images (elastograms) that distinguish between healthy and diseased tissues.
Besides precisely measuring tissue stiffness, MRE can also evaluate an organ’s health, non-invasively. Therefore, it could eventually replace the need for painful biopsies that may only sample a healthy section of tissue and miss diseased areas — or result in bleeding and even death due to complications.
Virtual reality neurosurgical simulator
Combining the new imaging technology with a virtual reality-based neurosurgical simulator, developed by NRC, could also help neurosurgeons hone their skills and improve patient care by enabling them to practise procedures on a virtual brain. High-definition haptic hardware — mimicking the surgical tools used in the operating room — allows surgeons to touch and move parts of a simulation of a patient’s brain. When combined with MRE data from a patient, the virtual tissues that the surgeon touches in the simulator would feel the same way that they will in the actual surgery.
Two images from a virtual reality system for neurosurgical simulation: a stiffness map obtained from MRE (left) and the actual photo-realistic effects that the user sees (right).
“The stiffness indicates the density of tumour tissue and informs the surgeon how much pressure to exert when removing a tumour so as not to affect other parts of the brain,” says Dr. Gruwel. He adds that MRE could be used to examine other organs — beyond the brain and liver — including the breasts, kidneys, lungs, prostate, muscle, connective tissue, blood vessels and cartilage. 
