Magnetic Resonance Imaging: Techniques & Methods

Magnetic Resonance Imaging: Techniques & Methods

Diagnostic imaging is critical in healthcare because scanning a patient's internal organs provides physicians and radiologists with valuable information about the patient's health.

And while most imaging techniques are non-invasive, they often require the use of X-rays or ionizing radiation that, in large enough doses, can be detrimental to a patient's health. CT scans are a typical example of an imaging technique that requires X-rays to produce an image.

However, magnetic resonance imaging (MRI) employs large magnets, strong magnetic fields, magnetic field gradients, and radio waves to generate an image instead of using potentially harmful X-rays or ionizing radiation. This imaging technique produces detailed pictures of a subject's anatomy, including bones, joints, muscles, tendons, and soft tissues such as cartilage.

MRI scans are essential in radiology because of their ability to detect tumors, inflammation, various neurological conditions, and heart and blood vessel abnormalities.

Though much safer than other medical imaging techniques, the enclosed space that the patient is put into and the loud noises that the machine makes may lead to feelings of claustrophobia. However, recent developments have allowed for more "open" designs which accommodate patients who may feel overwhelmed by the procedure.

For noise level issues, a radiologic technologist will provide patients undergoing MRI scans with earplugs or headphones containing no metal whatsoever that help mitigate the loudness of the scan. (Some machines are capable of performing quieter scans. In this case, earplugs are not necessary.)

Additionally, due to the powerful magnets used in MRI machines, patients with certain implants, such as pacemakers and insulin pumps, must undergo different medical imaging procedures or remove the implant, if possible. Additionally, patients with cochlear implants must take a series of precautionary steps to avoid the risks MRI scans pose when a patient has these implants, according to the FDA.

Originally referred to as nuclear magnetic resonance imaging, MRI scans expose atomic nuclei to strong magnetic fields. These nuclei then absorb this energy and emit radiofrequency (RF) energy which a receiver can pick up. Hydrogen atoms are specifically targeted due to their abundance in the human body.

The contrast in the resulting image is due to the differing rates at which the tissues' nuclei are excited, return to their equilibrium state, and release their energy. This process of returning from an excited state to equilibrium is referred to as the relaxation process.

Additionally, a more pronounced contrast can be achieved by using MRI contrast mediums or agents, which possess specific electromagnetic properties that allow for more detailed MRI images. Gadolinium-based contrast materials are the most common agents used.

MRI System Techniques & Methods

MRI machines may differ slightly from model to model. Still, all of them consist of a strong magnet, shim coils for correcting shifts in the magnetic field, a gradient system that localizes the magnetic resonance signal, and an RF system.

Most MRI systems operate at 1.5 Tesla (T), but some commercial systems function at a range of intensities from .2 to 7 T. The superconducting magnets used in clinical MRI machines generate so much heat that they need to be cooled with liquid helium.

MRIs with lower magnetic field strengths, however, use permanent magnets and do not require such cooling. These open MRI machines are designed to be more comfortable and are used for patients with claustrophobia when in enclosed spaces. Ultra-low field MRI systems that operate at the microtesla-to-millitesla range exist and use sensitive superconducting quantum interference devices (SQUIDs).

Depending on their function and what they are scanning or scanning for, MRI devices can vary the radiation pulse settings to create specific images. To learn more about these differences, read on! As well, if you're interested in the various accreditations available for medical imaging, the American College of Radiology offers many resources on several modalities.

Similarly, the Radiological Society of North America provides high-quality educational resources, education credits towards certification maintenance, and five peer-reviewed journals, as well as grant funding dedicated to providing young radiologists the support needed while still in training.

Magnetic Resonance Angiography (MRA)

Imaging blood vessels pose issues due to their size and delicate makeup. Magnetic resonance angiography, or MRA, describes a group of non-invasive techniques used to look at blood vessels in the brain, spinal cord, and other parts of the body.

MRA techniques can be classified as either being flow-dependent or flow-independent. Flow-independent angiography involves non-contrast-enhanced methods that do not require high blood flow rates. Utilizing the differences in the relaxation rates and chemical shifts can generate an accurate image without contrast agents.

Flow-dependent techniques are all based on the flow of blood through the body. Since most tissue in the body is static and blood flows around and through the static structures, an image can be constructed that distinguishes the blood vessels from the surrounding tissue. Flow-dependent techniques can be separated into two main categories:

  • Phase-contrast: Also called PC-MRA, phase-contrast is used to determine flow velocities. Subatomic particles have magnetic properties called spin. When stationary, the spin of nuclei has no measurable phase shift. However, when moving, there is a measurable net phase shift. PC-MRA utilizes this to distinguish flowing blood from static tissue.
  • Time-of-flight: As tissue is exposed to excessive RF, it can become magnetically saturated, affecting the radiofrequency that is emitted. Since blood is continuously flowing, it does not become as magnetically saturated as the surrounding tissue. This means that the flowing blood emits more radiofrequency waves, distinguishing it from the static tissue. It is alternatively known as inflow angiography.

Cardiovascular Magnetic Resonance Imaging

Due to the sensitive nature of the tissue around the heart, non-invasive imaging techniques are essential to determining the heart's health.

Cardiac MRI uses are similar to conventional MRI. Still, by utilizing echocardiography gating and high temporal resolution, it has been specifically tuned to image the heart and the myocardium, the heart's muscular tissue.

This method is used primarily for diagnostic purposes and surgical planning of congenital heart disease. It is also used for various other heart-related issues, and some of the ongoing areas of study include assessing myocardial ischemia, myocarditis, and cardiomyopathies. Within cardiac MRI, different techniques are used to look at specific functions within the heart.

Cine imaging

To properly assess the health of a heart, observing it while it is beating is Important. A single image can tell us much about the heart, but a video can tell us more. Cine imaging or cine-cardiac motion studies provide this by taking multiple images during the cardiac cycle and stitching them together to create a video. Using balanced steady-state free precession (bSSFP), cine sequences can obtain high contrast cardiac images for a very detailed look at the heart while beating.

Late gadolinium enhancement

By injecting a gadolinium-based contrast agent into the patient intravenously, higher contrast between normal and infarcted or dead myocardium can be achieved. The rate that the tissue or muscles of the heart accumulate and wash out the gadolinium depends on myocardial blood supply, hematocrit, renal function, and what type of disease is present in the tissue.


Also known as cardiac MRI perfusion or stress cardiac magnetic resonance perfusion, perfusion is an imaging test used to detect coronary artery diseases.

It consists of using the stress/rest protocol, which calls for the patient to be put under some stress and observe them as they come down from that stress. Normally this is done by putting the patient on a bicycle or treadmill, however, this is not possible in an MRI.

For perfusion, adenosine is used as the stressor to induce ischemia or blood restriction. The patient's images are then compared to those of a healthy heart under the same circumstances to determine the health of the patient's heart.

Functional MRI (fMRI)

Neural activity in the brain is directly correlated to the amount of blood flow in that area of the brain. fMRI looks at the blood flow in the brain to determine brain activity.

Specifically, fMRI uses blood oxygen level-dependent contrast (BOLD) to look at variations in blood flow, or hemodynamic response, in the brain. BOLD analyzes low and high oxygenated blood areas to look at how the brain acts under specific circumstances. It is a non-invasive technique that does not require injections or ingestibles.

A typical procedure for fMRI is to have the subject perform specific tasks while lying under the scanner. As the subject thinks through the task, their brain activates, and the MRI scanner picks up the corresponding activity. The scans are then compared to scans taken while the subject is at rest to determine how much brain activity occurred.

Spacial, temporal, and linear addition from multiple activations are all observed. It does have some clinical application, but it is mainly helpful for clinical research. Resting-State fMRI is a newer technique that maps brain activity while the subject is resting or in a task-negative state. Other more recent methods are also being studied that use different biomarkers than BOLD signals.

Real-time MRI

One limitation to cine imaging is that it takes 5-10 seconds to collect the needed images and requires the patient to hold their breath. Real-time MRI or real-time cine imaging can take images in less than 1 second per slice and does not require patients to hold their breath.

In conventional MRI scanning, k-space or spatial frequencies in magnetic resonance images are scanned; however, this is very time-consuming. Real-time MRI sacrifices this special resolution to increase the speed that the images can be captured.

Early iterations employed eco-planar methods to achieve this. Recently advances have been made to remove these shortcomings by using iterative reconstruction algorithms. This technique can get a temporal resolution of 20-30 milliseconds which is a marked improvement.

It also offers rapid, continuous data acquisition and does not suffer from undersampling, achieving high-quality images with 5-10% of the data needed for standard MRI reconstruction.

Determining Consciousness

The question of consciousness has remained somewhat elusive in medical terms. Though there is a hard line and requirements to determine if a patient is alive or dead are very clear, consciousness can be much more ambiguous.

Unconsciousness is roughly defined as having the inability to report subjective experience. Do patients that undergo anesthesia or more enduring states of unconsciousness have or lack markers that we can measure and observe?

A team of scientists may have uncovered this very answer. This diverse group published a paper in February of 2019 that outlines possible neural patterns that may indicate various levels of consciousness.

The team looked at 159 subjects spanning four independent research sites and recorded their fMRI data. This data was then compared against fMRI data from patients with unresponsive wakefulness syndrome and those in minimally conscious states. Their fMRI BOLD signals were taken across 42 brain regions and revealed four distinct patterns of brain activity.

Two patterns, in particular, were shared equally across all three groups and may indicate it as a transitional state. They concluded that these patterns show great promise of being markers for conscious and unconscious brain states and should be looked at further to determine if they are.

Additionally, they postulated that this knowledge might give us the ability to affect patients in either unconscious or conscious states.

Harnessing the Power of MRI Machines & Techniques

In this post, we explored the fascinating world of MRI techniques and methods, shedding light on the incredible capabilities of these imaging machines that have revolutionized the field of medical diagnostics.

From their humble beginnings to the cutting-edge technology available today, MRI machines play a crucial role in detecting, diagnosing, and monitoring various medical conditions.

We delved into the different types of MRI machines used in hospitals and other medical facilities, each with unique features and applications.

Closed, wide, and open MRI machines have provided physicians with flexible options to accommodate patients of varying needs and preferences, ensuring a comfortable and efficient scanning experience.

Additionally, we explored the advancements in MRI technology, such as functional MRI (fMRI), which enables the study of brain activity, and diffusion tensor imaging (DTI), which allows for visualizing nerve fiber bundles.

These innovations have expanded the horizons of medical research and deepened our understanding of the human body.

It is worth noting the immense impact of machine learning and artificial intelligence on MRI techniques. By developing sophisticated algorithms, these technologies aid in image reconstruction, noise reduction, and automated analysis, empowering medical professionals to make more accurate diagnoses and provide personalized treatment plans.

In years to come, we can anticipate even more breakthroughs in MRI technology, ushering in an era of faster, more precise, and patient-centric imaging. With each new discovery, we move closer to unraveling the human body’s mysteries and improving healthcare outcomes for countless individuals worldwide.