As a clinical technology, MRI offers unsurpassed flexibility to look inside the human body.
In 1977, inspired by the observation that cancerous and healthy tissues produced different nuclear magnetic resonance signals, Raymond Damadian, Michael Goldsmith, and Lawrence Minkoff performed the first MRI scan of a live human body. In the early days of clinical MRI, scans took hours and provided low spatial resolution, but they have become essential for distinguishing between healthy and diseased tissues. By its 40th anniversary, MRI was a must-have tool in hospitals and clinics of all sizes. And it has found applications in image-guided interventions and surgeries, radiation therapy, and focused ultrasound. Advances in technology, meanwhile, have pushed the envelope of scanner performance with improvements to speed and spatial resolution.
At the frontiers of MRI development, work is focused on fast, quantitative imaging. Clinical needs increasingly demand functional information—on heart-muscle contractions, brain activity,3 chemical concentrations in tumors,4 and blood flow in and out of tissue5—in addition to anatomical structures. New approaches must also maintain a patient’s comfort and safety; MRI is well-known for sparing patients any exposure to ionizing radiation, yet it is not without hazards.
HOW MRI WORKS
When biological tissue is placed in a magnetic field, nuclei with magnetic moments become magnetized. RF pulses are then applied that match the resonance, or Larmor, frequency of the nuclei, causing them to tip out of alignment with the external magnetic field and precess about it. The precessing nuclei, in turn, induce oscillating magnetic fields at the Larmor frequency; those oscillations are detected via Faraday induction of an electromotive force in a nearby coil of wire.
In practice, many nuclei must precess in phase with each other to produce a detectable signal. The loss of phase coherence among precessing nuclei over time is called T2 relaxation. And the orientations of the nuclei eventually return to their equilibrium orientation in the external magnetic field—a process called T1 relaxation. Early NMR experiments revealed that various tissues have distinct T1 and T2 relaxation times. For certain diseases, including cancer, changes in either time can distinguish between diseased and healthy tissue. That feature is useful in the case of lesions whose absorption of x rays is similar to that of surrounding healthy tissue, which makes them difficult to detect using radiography or x-ray computed tomography.
In NMR measurements, the timing of the applied RF pulse and of the RF readout signal from the tissue can be chosen so that the strongest signal is produced by tissue with the shortest T1 relaxation time. A measurement whose timing is chosen that way is called a T1-weighted measurement. Alternatively, the sequence timing can be chosen so that the strongest signal comes from tissue with the longest T2 relaxation time, a T2-weighted result.
In tissue, the hydrogen nucleus is the most abundant magnetizable nucleus. Its gyromagnetic ratio is 42.56 MHz/T, which results in operating at Larmor frequencies of roughly 64 MHz and 128 MHz for 1.5 T and 3 T MRI scanners, respectively. Magnets with a range between 0.2 T and 7 T are used for clinical scanning, and human scanning up to 10.5 T is currently available in research settings.
SPATIAL ENCODING
Using the NMR signals from tissue for clinical diagnosis requires that they be localized in three dimensions to form images. Three sets of electromagnetic gradient coils in the MRI scanner accomplish that task. Each produces a linearly varying magnetic field along one of three orthogonal axes. And each gradient can be switched on and off to produce different strengths depending on the current applied; the gradient fields are superimposed on the main magnetic field—usually 1.5 T or 3 T—to create a spatially dependent variation in Larmor frequency. If a gradient is applied for some time and then turned off, all signals have the same frequency, but their relative phase shifts, accumulated while the gradient was on, vary according to position along the gradient axis…..
Read more at https://physicstoday.scitation.org/doi/10.1063/PT.3.4408
