Biomedical Engineering Advances Improve MRI Sensitivity
Magnetic Resonance Imaging (MRI) is one of the most common and useful medical imaging modalities used in hospitals today. Unlike an X-Ray, an MRI allows physicians and researchers to image soft tissue in the body, allowing them to probe for anatomical anomalies such as tumor masses or torn ligaments. In addition, it is a relatively non-invasive technique that does not use ionizing radiation or require the addition of any labelling or radioactive agents to form an image. An MRI works by combining a very strong magnetic field and radio frequency waves to act on tissue in the body. The magnetic field allows hydrogen atoms in the body to absorb and emit the radio waves, which can be detected by a coil placed very close to the patient. There is an abundance of hydrogen atoms in the human body, particularly in water and fat, enabling the MRI to generate detailed images. By varying the pulses of radio waves transmitted, clinicians can achieve different contrasts in the image.
One of the most important factors in any kind of imaging is the signal-to-noise ratio (SNR). This is essentially a measure of how strong the actual signal is as compared to the background. Improving this parameter in a technique such as MRI is a goal of biomedical engineering researcher. An augmented SNR has more than just technical implications. Practically, it can mean cutting down on imaging time and amplitude of radio waves, which can translate into a more comfortable patient experience and a lower energetic burden on the instrument. One of the most promising platforms for improving SNR in MRI are metamaterials. Made from a combination of elements, these are engineered materials which have properties that are not found in nature, such as the way they interact with and affect electromagnetic radiation. These properties are due to a very small scale repeating 2D or 3D pattern, which allows them to block, absorb, enhance or bend waves in unique ways.
Pilot studies using metamaterials in conjunction with MRI have been carried out as proof of concept. However, these applications have revealed several drawbacks, such as a large size and lack of flexibility when integrated with the MRI. Since the signal detector must be placed very close to the patient’s body, any device intended for practical use would have to be compact and easy to implement with existing instruments. A team of physicists and radiologists from the Netherlands, Russia and Australia have reported the development of such a material in a recent issue of Nature’s Scientific Reports. This hybrid metamaterial is made of a combination of copper metallic strips (the surface) and a mineral called calcium titanate (the substrate). The substrate is only 8 mm thick, and the material is designed to fit between the patient and the radio frequency receive coil array (signal detector). The metamaterial improves MRI imaging by acting on both the transmission and receiving functions of the instrument. It focuses the magnetic field in a specific area, allowing higher efficiency of the transmitted radio frequency field, and in turn improves the image SNR due to a higher receive sensitivity.