How Magnetic Resonance Imaging Works & How Leasing Benefits You

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.

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.

Perfusion

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.

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.

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.

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.