The basic process
The way MR images are generated is complicated and is much harder to understand than plain radiography, CT and ultrasound. It has strong underpinnings in physics which must be understood before any real sense of 'how it works' is gained.
What follows is a very abbreviated, 'broad strokes' description of the process. Essentially, the process can be broken down into four parts: 1) Preparation, 2) Excitation, 3) Spatial Encoding and 4) Signal Acquisition. For a more detailed description of each part of the process, please refer to the links scattered throughout this introduction and at the bottom of the page.
The patient is placed in a static magnetic field. Hydrogen protons within the patient's body align to the magnetic field. Additional preparation sequences can also be performed to manipulate the contrast in the image, e.g. inversion prep.
An RF pulse (radio frequency pulse) is emitted from the scanner, tuned to a specific range of frequencies at which hydrogen protons precess. This results in some of the hydrogen protons being "knocked" 180o out of alignment with the static magnetic field and being forced into phase with other hydrogen protons. As the energy from the RF pulse is dissipated, the hydrogen protons will return to alignment with the static magnetic field. The MRI signal is derived from the hydrogen protons as they move back into alignment with the magnetic field, and fall out of "phase" with each other. (The actual process is much more complicated, broken down into T1 relaxation, and T2 decay).
Spatial encoding of the MRI signal is accomplished through the use of gradients (smaller magnetic fields) which perturb the main magnetic field, and cause hydrogen protons in different locations to precess at slightly different rates. The portion of the gradient coils and the associated current that is perpendicular to the main magnetic field cause a force (Lorentz force) on the coils. The gradients are turned on and off very quickly in this process causing them to vibrate producing the majority of the noise associated with the MRI environment. This occurs even though they are embedded in epoxy.
As the protons undergo relaxation, the change in the local magnetic fields creates currents in the receive coils. These currents can be detected as a change in voltage. The signal must then be sampled, converting the analog signal to digital, and then is stored for processing. The MRI signal is then broken down and spatially located to produce images.
Multiple image sets are obtained in the standard exam (which varies from facility to facility). Exam times vary according to the part of the anatomy being studied, pathology expected, and radiologist preferences. Occasionally, a contrast medium may be used to enhance images. Typically, exams are ordered without and with contrast for comparison purposes. Very rarely, and only in certain circumstances are exams ordered with contrast only. After the exam the patient is removed from the scanner and given post-procedure instructions (information about contrast medium if used, sedation if used, and time when to expect a report from the examination).
- Mansfield P, Chapman BL, Bowtell R et-al. Active acoustic screening: reduction of noise in gradient coils by Lorentz force balancing. Magn Reson Med. 1995;33 (2): 276-81. Pubmed citation
- MRI (introduction)
- MRI physics
- MRI hardware
- signal processing
MRI pulse sequences (basics | abbreviations | parameters)
- spin echo sequences
- inversion recovery sequences
- gradient echo sequences
- fat-suppressed imaging sequences
- diffusion weighted sequences (DWI)
- derived values
- CSF flow studies
- susceptibility weighted imaging (SWI)
- saturation recovery sequences
- echo-planar pulse sequences
- metal artifact reduction sequence
- T1 rho
- spiral pulse sequences
- MR angiography (and venography)
MR spectroscopy (MRS)
- Hunter's angle
- lactate peak: resonates at 1.3 ppm
- lipids peak: resonate at 1.3 ppm
- alanine peak: resonates at 1.48 ppm
- N-acetylaspartate (NAA) peak: resonates at 2.0
- glutamine-glutamate peak: resonate at 2.2-2.4 ppm
- gamma-aminobutyric acid (GABA) peak: resonate at 2.2-2.4 ppm
- 2-hydroxyglutarate peak: resonates at 2.25 ppm
- citrate peak: resonates 2.6 ppm
- creatine peak: resonates at 3.0 ppm
- choline peak: resonates at 3.2 ppm
- myo-inositol peak: resonates at 3.5 ppm
- functional MRI
- MR fingerprinting
- MR hardware and room shielding
- MR software
- patient and physiologic motion
- tissue heterogeneity and foreign bodies
- Fourier transform and Nyqvist sampling theorem
MRI contrast agents
- gadolinium ion
- extracellular MRI contrast agents
- hepatobiliary MRI contrast agents
- intravascular (blood pool) MRI contrast agents
- gastrointestinal MRI contrast agents
- tumor-specific MRI contrast agents
- reticuloendothelial MRI contrast agents
- contrast agent safety
- MRI safety