Magnetic resonance imaging (MRI) has revolutionized medical diagnostics and research by providing a non-invasive way to examine internal tissue structures and processes on a millimeter scale, often with much better soft tissue contrast than X-rays and others. methods. MRI is unique in that the contrast depends on many different tissue properties, including water density, water movement, local chemical environment, blood oxygen levels, fat fraction, cellularity, and vascularization.
The value of MRI can be drastically increased, making its output more quantitative, so that the images become maps of important tissue properties. Quantitative imaging further ensures that measurements are reproducible, data from one image system can be confidently compared to data from another, and performance stability can be observed over time.
To help achieve these goals, the Physical Measurement Laboratory (PML) of the National Institute of Standards and Technology in collaboration with the National Institute of Biomedical Imaging and Bioengineering (NIBIB) at the National Institutes of Health (NIH) recently launched a unique service that provides MRI research centers and clinics with access to NIST-traceable reference standards.
Almost all conventional NMR the results are quality – that is, they show differences in contrast between adjacent areas, but do not measure the specific properties or signal intensities that caused these differences. To generate such a species reliable quantitative datathe scanner should be calibrated by comparing its measurements to standard reference materials (which mimic different specific tissues or dynamic processes such as fluid diffusion) with the same measurements made by a SI-traceable metrology system.
In medical imaging, such calibration objects are called phantoms. PML’s Magnetic Imaging Group, a long-time world leader in the creation of MRI phantoms for quantitative imaging, now offers a “borrowing library” of phantoms to calibrate entire MRI systems, as well as units designed to image the breast, heart, knee and the brain. Phantoms contain a wide variety of structures to determine geometric accuracy, image uniformity, and signal-to-noise ratio, along with many different types of tissue facial expressions that have well-defined properties such as water diffusion, proton rotation relaxation, neurochemical composition, and content. fat.
Many of the phantoms in the library were developed by NIST in partnership with organizations such as the National Cancer Institute, the Radiological Society of North America and the International Society for Magnetic Resonance Imaging in Medicine. Others have been developed by universities, including University College London and the University of Michigan, and phantom commercial manufacturers; NIST has provided a path for SI-calibration and phantom curation. The phantom borrowing library is an open resource: users can both check phantoms and add new phantoms to the library.
The Lending Library provides easy access to clinical and research sites, scanner manufacturers and medical device developers to calibrated phantoms, along with related reference data and public domain analysis software. Phantoms are used in a variety of applications such as clinical trials, medical device development, disease research, scanner quality assurance and scanner monitoring.
“The loan library meets the important need to have easy access to medical phantoms, as well as a convenient route for obtaining calibration with SI-tracking and curating phantoms. As in vivo image-based metrology becomes more complex and widely used, there is a critical need for these measurements to be well defined and validated, ”said Stephen Rusek, Project Physics Project Manager.
“We recently included two new phantoms for brain imaging that assess the accuracy of local measurements of neurotransmitters and the stability of a functional MRI scanner of brain activity,” he said. “We look forward to expanding the loan library to include biomimetic phantoms that are multimodal in nature. We will soon include a triple modal abdominal phantom with realistic kidney and liver lesions that can be imaged with ultrasound, computed tomography and MRI.
To date, institutions that have borrowed from the library include: Washington University in St. Louis; University of Illinois; Cornell Medical School; University of California, San Diego; Sloan Catering Memorial Cancer Center; Case Western Reserve University; University of Southern California; Cleveland Clinic; Boston University; University of Washington; and Massachusetts General Hospital.
“The loan library is a great complement to our service for measuring biomarkers, and they reinforce each other,” said Quantitative MRI Project scientist Karl Stupic. “As new phantoms are added to the library, the measurement service needs to develop a methodology to support these phantoms – expanding the opportunities that NIST can provide to the MRI community. The loan library allowed customers to conduct research on sites and in various systems, exploring system variations. These studies include comparisons of new MRI systems, different stress fields and systems undergoing upgrades.
To obtain MRI images, the patient’s body is placed in a strong magnetic field. The protons (hydrogen nuclei) in the body’s water molecules have a quantum property called spin, which causes each proton to act somewhat like a strip magnet with north and south poles, which are initially randomly oriented. The applied magnetic field forces the rotations of the protons to be arranged in the same direction.
The tissue is then irradiated with pulses of radio frequency waves that reverse proton rotations beyond alignment. Between pulses, the protons “relax” to their original aligned states, emitting their own radio frequency waves, functioning as a natural nanoscale RFID tag. The imaging system detects and locates spatially those signals that have different properties depending on the type of tissue and its chemical environment.
To depict the diffusion of water molecules, MRI uses proton rotations as labels on water molecules and measures the speed at which water moves by observing the motion and relaxation of rotation. Water diffusion is an important measure of local tissue structure. For example, hypercellular tumors will have limited water diffusion, while necrotic tissue may have increased water diffusion. In nerve tissue, which is highly anisotropic, water diffusion can measure the direction of neural bundles as well as the potential damage caused by traumatic brain injury.
Diffusion imaging becomes important in breast imaging because it can highlight tumors without the need for injected contrast agents. “Recent studies of breast cancer patients have shown that diffusion can measure the response to chemotherapy very quickly, within the first day of treatment,” said Catherine Keenan, chief project officer for quantitative MRI, chief developer of NIST’s breast phantom. “It’s a really exciting development. It is possible to see if a person will respond to treatment after the first dose. The breast phantom, which is part of the borrowing library, can be used by magnetic resonance imaging to ensure that they perform accurate diffusion measurements in the breast coil.
“Our team borrowed phantoms from various academic groups and commercial users in an ad hoc way. I want to thank Stephen for formalizing this effort and NIBIB for helping to fund it, “Keenan said. “We are excited to see that new phantoms are contributing and developing an even larger consumer group.”
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