How MRI Images Are Generated

An MRI system consists of five major components: a magnet, a magnetic gradient system, a radio frequency (RF) coil system, a receiver, and a computer system. During MRI scanning, the patient is placed in a strong magnetic field. The strengths of the magnetic field employed in typical MRI machines range from 1.0 to 3.0 Tesla (T). In comparison, the strength of the Earth’s magnetic field is 5 × 10⁻⁵ T. Exposure to the field causes the magnetic moments of hydrogen atom nuclei (protons) in water and lipid molecules in the body to snap into alignment with the magnetic field much like a compass needle aligns with the Earth’s magnetic field. The alignment can be either parallel or anti parallel to the magnetic field. Parallel alignment is a low-energy state, while the anti parallel alignment is a high energy state; the distribution of protons among these two energy states is proportional to the strength of the magnetic field (the higher the strength of the magnetic field, the greater the number of protons that acquire parallel alignment). During an MRI scan, an RF transmitter produces an electromagnetic pulse perpendicular to the magnetic field, with a frequency that causes the magnetic moments of the aligned protons to transition to the higher energy state. As the RF transmitter pulses off, the protons return to the low-energy state, radiating the difference in energy between the two states as photons. These photons comprise the signal that the MRI scanner detects.

Additional magnetic fields can be applied to generate gradients of magnetic field strength, in effect varying the composite field strength across the patient’s body and thus allowing for spatial localization. In addition, as histologically distinct tissues (as well as healthy and pathologic forms of the same tissue) contain different concentrations of hydrogen atoms, their respective radio frequency emissions are unique. In combination, these effects, following appropriate transformation of their signals as collected by the MRI scanner, allow for the production of diagnostically useful images.^9–11

Growth Patterns of MRI Technology

MRI diagnostic technologies represent a rapidly growing field, with continuous increases both in the number of installed scanners (in the United States and worldwide) and in the number of scans performed. Based on data from the Organization for Economic Co-operation and Development (OECD), the United States is a world leader both in the availability (number of scanners per million population; second only to Japan) and utilization (number of MRI scans per year per 1,000 population; highest in the world) of MRI scans.¹² Figure 1 demonstrates the annual growth in the number of MRI units per million population and Figure 2 the growth in MRI exams performed per 1,000 population for all OECD countries that have reported data for at least two time points (U.S. data are the oldest and most frequently updated). Unfortunately, similar estimates are not available by type of scanner; however, available data can be considered indicative of an increasing trend.

Figure 1

Growth patterns in the availability of MRI scanners in selected OECD countries. MRI scanners per million population in selected OECD countries with data available for at least 2 separate years. The graph was generated by the authors based on OECD Health (more...)

Figure 2

Growth patterns in the use of MRI in selected OECD countries. MRI scans per 1,000 population in hospitals and ambulatory care (total) for selected OECD countries with data available for at least two separate years. The graph was generated by the authors (more...)

Emerging Stress-Loading MRI Technologies

Due to the high disease burden, the development of imaging technologies to facilitate the diagnosis and management of musculoskeletal conditions is an active area of research. Often, new technologies are adopted early in their development in the hopes of improving patient outcomes, and therefore sometimes have not yet been rigorously evaluated. ^13,14 Multiple studies have identified rapid increases in the use of stress-loading imaging technologies, for spine imaging in particular.^14–16 As discussed in the previous section, this appears to be a worldwide trend. In the United States, the increase in spine imaging has been accompanied by an increase in spinal surgery, which has also been documented by multiple studies. However, it is not certain whether increased utilization of advanced imaging and surgical interventions have improved patient outcomes. This may be due to the limited ability of MRI to discriminate between patients who require intervention and those that do not, or the limited therapeutic effect of the available interventions.^13,17 An additional concern stems from the high frequency of positive MRI exams on clinically a symptomatic patients, which has been reported to exceed 50 percent in some studies.^18–22 These limitations, along with the relatively high cost of obtaining MRI scans, have spurred research to modify existing devices and develop novel technologies to improve diagnostic accuracy with an aim to improving patient-relevant outcomes.

One area of possible modification is the physiologic conditions under which the MRI scan is performed. The standard clinical MRI scanner configuration includes a large, cylindrical magnet, in which the patient is placed lying flat, either prone or supine. The patient is required to remain motionless during the imaging period, which can range from a few seconds to several minutes, depending on the exam. The typical closed-bore MRI allows for limited movement and can induce claustrophobia or anxiety in some patients. ²³ Furthermore, due to limited space in closed-bore MRI, it may not serve the needs of obese patients requiring less physiologically constraining imaging systems. In response to the limitations of conventional MRI in imaging musculoskeletal conditions, engineers and scientists have attempted to develop new MRI techniques that better mimic actual physiologic conditions, such as weight-bearing, upright, or other physiologic positions, on the theory that images taken under more natural conditions would be better at capturing pathology and therefore result in more accurate diagnoses and better patient outcomes. Open MRI systems have been designed to allow greater flexibility in patient positioning and may alleviate claustrophobia. In such systems, the bore is open, typically laterally, and may be of shorter length so that only the body part of interest is placed under the magnet. Devices have been developed that enable imaging in weight-bearing positions or simulate gravity (for example through axial loading, a technique that compresses the body along the joint of interest) in open, semi-open, or conventional scanners. Other devices or placement techniques allow imaging of the patient in postures other than the typical supine position, such as placing the spine or joint in the position of pain or anatomic abnormality through flexion or extension.

Despite the progress in developing new MRI techniques, considerable uncertainty remains as to the optimal imaging approach for most musculoskeletal conditions, the specific indications for MRI, and the relative benefits and harms of different MRI configurations (weight-bearing or not, open or closed, neutral positioning or flexion/extension). Specific indications for the use of weight-bearing or stress-loading MRI lack consensus and need further evaluation. A technology assessment conducted on behalf of the State of Washington in 2007 did not reveal adequate data to determine the diagnostic validity or accuracy for upright, multi positional MRI (one specific type of stress-loading MRI technology). Additionally, the technology assessment could not determine whether technologies that allow positional imaging (for example, flexion and extension views) provide additional diagnostic information, despite the acquisition of non-neutral views being associated with additional costs.²⁴ Since then, other studies assessing the diagnostic utility of stress-loading MRI have been published, but no systematic evidence reviews have been published.^25–28

Systematic assessment of the available imaging modalities is also necessitated by their substantial cost. A 2005 study of Florida hospitals analyzing financial data from fiscal year 2002 found the mean operating expense and charge per procedure for MRI was $165 and $2,048, respectively.²⁹ However, costs and charges associated with MRI can vary depending on type of MRI used (e.g., standard vs. open), the anatomic localization of the medical condition (e.g., knee vs. spine) or location of the MRI facility (e.g., city, state, country), and it is unclear how positional MRI technologies may affect the overall health care cost-burden.

The objectives of this Technical Brief are to describe the current state of use of stress-loading MRI technologies, enumerate their potential benefits and harms for the diagnosis and management of patients with musculoskeletal disorders for whom this diagnostic test may be considered, and to describe the evidence available to date that supports these applications.