Magnetic Resonance Imaging

Introduction to Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) patient and machine

Lesson Objectives

Learn how MRI images are produced
Learn about different MRI sequences
Learn what normal tissues look like on standard MRI sequences
Learn the major strengths and weaknesses of MR imaging
Learn some common indications for MRI imaging

How does an MRI machine work?

MRI machine annotated

An MRI machine is a coil within a coil within a coil…

The largest coil is used to make the main magnetic field.
Electric current is always running through large coil
The current generates a very strong magnetic field directed through the bore of the MRI machine.
Most clinical MRI machines produce either a 1.5 Tesla or 3 Tesla magnetic field. (Tesla is a measure of magnetic field strength)

The other coils (radio frequency coil and gradient coils) are turned on and off as needed to manipulate the magnetic field while acquiring images.

How does an MRI machine make images?

How an MRI machine makes an image.

The body contains approximately 5 billion billion billion protons
Protons, when placed in a magnetic field, start to spin in a special way (this is called precession)
A radiofrequency pulse (basically radio wave) is emitted by a transmitter which rotates the protons into an orientation that makes them emit their own radiofrequency signal
A receiver coil is used to collect the signal coming from the protons
A computer is used to translate that signal into an image

What is a pulse sequence?

Inversion recovery pulse sequence

A pulse sequence is a pre-programmed set of changing magnetic gradients used to produce a specific type of MRI image (T1-weighted image, T2-weighted image, Diffusion-weighted etc.)
A pulse sequence is what we tell the MRI machine to do in order to make the type of image we want.
Pulse sequences are often written in this form. Much like a music score, they are read left to right and the symbols describe which event happens in which order. You don't need to know how to interpret these.

Common MRI sequences

T1 Weighted Imaging (T1)

T2 weighted imaging

T2 Weighted Imaging (T2)

T1 with Fat Saturation

T1 with Fat Saturation (T1 fat-sat)

T2 with Fat Saturation

T2 with Fat Saturation (T2 fat-sat)

Fluid Suppression (FLAIR)

Diffusion Weighted Imaging (DWI)

Diffusion Weighted Imaging (DWI

Susceptibility weighted imaging (SWI)

Susceptibility Weighted Imaging (SWI)

Proton Density

T1 with IV Gadolinium Contrast (T1-post)

Notice that the same tissue type (muscle, fat, abdominal organs, brain tissue, bone etc.) look different on different sequences
Each sequence is designed to highlight different tissue types or different diseases
There are over 100 different MRI sequences
The simplest and most commonly used sequences are T1 and T2

Tissue representation in T1 and T2 weighted images

Fluid represented in a T1 and T2 weighted image

T1: Dark	T2: Bright

Fat represented in a T1 and T2 weighted image

T1: Bright	T2: Dark

Muscle represented in a T1 and T2 weighted image.

T1: Isointense	T2: Isointens

White matter represented in a T1 and T2 weighted image

T1: Isointense	T2: Isointense
Signal intensity (brightness) is often described as relative signal intensity. For example: On T1 weighted images, white matter is hyperintense (brighter) compared to gray matter On T2 weighted images, white matter is hypointense (darker) compared to gray mate

Test your knowledge

Signal intensity - T1 and T2 weighted images

Signal intensity (brightness) is often described as relative signal intensity. For example:

On T1 weighted images, the liver is hyperintense (brighter) compared with the spleen
On T1 weighted images, the liver and spleen are both hyperintense (brighter) compared to cerebrospinal fluid

What is fat saturation?

Fat saturation

Fat saturation is an additional technique that can be added on to any sequence

Normally fat is bright on all sequences (see how fat is bright on the T1 and T2 weighted images)
When fat saturation is employed, the signal coming from fat is suppressed (see how fat is dark on the T1 fat sat and T2 fat sat image)

Test your knowledge

What is gadolinium?

Gadolinium examples

Gadolinium is an intravenous contrast agent used in MRI
Once injected, gadolinium makes certain tissues appear brighter on MRI and helps make certain diagnoses
In this example, the T1 weighted image doesn't show the brain tumor well
After injection of gadolinium contrast, the brain tumor is very easy to see on the T1-post contrast images

Strengths of MRI

Excellent soft tissue contrast (can distinguish between different tissues well)
Advanced MRI imaging techniques (fMRI, Cardiac MRI, DTI/Tractography, angiography etc. )
No Radiation (unlike CT and x-ray)

Weaknesses of MRI

Exams are time consuming (usually 45-60 min)
Price (Average cost ~$2600, varies by exam and institution)
Some implanted devices or retained metal from injuries can be contraindications (ie. Pacemakers or shrapnel injury)
Moving while scanning can degrade image quality (not ideal if patients are agitated, claustrophobic, in lots of pain, or unable to follow commands)

Common Indications for MRI

MRI can be used to characterize benign and malignant tumors in the abdominal organs. This is a case of multiple hemangiomas (benign vascular masses) in the liver characterized on a T1 post-contrast MRI sequence.
MRI can be used to diagnose infections in the bones. This is a case of an abscess and osteomyelitis in the proximal tibia.
MRI is very useful in the diagnosis of ischemic stroke. In acute stroke the movement of water is restricted due to cell swelling and cell membrane dysfunction. DWI is an advanced MRI technique that shows bright signal when the movement of water is restricted and helps detect acute stroke.
The difference between cerebrospinal fluid and the spinal cord on MRI helps identify cases of spinal cord compression In this case there are metastatic masses in the spinal canal compressing the thoracic spinal cord
Cardiac MRI can be used to evaluate the structure of the heart (in this case there is hypertrophic cardiomyopathy) It can also be used to evaluate the function of the heart by taking multiple images during the cardiac cycle The blood perfusion to the heart can be assessed by using special post-contrast MRI techniques
MRI is used in the diagnosis of breast cancer. MRI can be used for screening in high risk patients (for example patients with BRCA1 or 2 mutations). MRI can also be used in patients with known breast cancer to look for additional areas of cancer. This is a case of left sided breast cancer.
Fetal MRI can be performed to make the diagnosis of congenital or developmental abnormalities prior to delivery. This is a case of twins where twin-twin transfusion syndrome was suspected on prior imaging. This MRI shows normal twin pregnancy.
MRI can be used for evaluating vessels. This is a post-contrast MR angiogram showing chronic occlusion of the aorta and proximal iliac arteries.