MRI
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Introduction
MRI, or magnetic resonance imaging, uses radio waves in short bursts. Some protons absorb this energy, while in other's their magnetic effects are cancelled. When the radio pulse is turned off, the protons relax and release the energy, like an echo, that is sensed and turned into an image.
MRI can show a great deal of variations in normal or abnormal states.
It cannot detect hemorrhage in the brain.
Mechanism of MRI
T1- and T2-weighted images are based on the variation in time required to receive the "echo" from relaxed protons. Signal comes from time of relaxation following withdrawal of energy.
Evaluates vascular, inflammatory, neoplastic, metabolic conditions. Most pathologic conditions accumulate water (edema), which is dark in T1 and light in T2.
T1-weighted MRI measures the return to magnetic state of protons whose energy was cancelled. This takes more time.
T2-weighted MRI measures protons who entered a higher energy state. They lose their energy rapidly.
Uses of MRI
- head
- breast
Head MRI
MRI can be useful for assessing malignancy, inschemia, or infarct.
tissue |
T1 |
T2 |
bone |
black |
black |
air |
black |
black |
white matter |
light grey |
dark grey |
gray matter |
dark grey |
light grey |
fat |
white |
grey |
CSF |
black |
white |
pathology |
T1 |
T2 |
edema |
dark grey |
light grey/white |
enhanced tumour |
white |
rarely done |
infarct |
dark grey |
light grey/white |
ischemia |
dark grey |
light grey/white |
Contrast will accumulate wherever BBB is broken down. It is used on a T1-weighted images.
Diffusion weighted images are based on Brownian motion; picks up edema. They are great for picking up strokes within a few minutes.
Breast MRI
As breasts are small, and require increased signal. Many also have access for biopsy. Often takes 40-45 minutes, requiring patient to lie still. so general screening is contraindicated.
Uses contrast to provide enhancement, and is therefore invasive. It is quite specific, though there is a high risk of false positives: specicificity of 30-40%.
Indications for MRI
The American College of Radiology has released guidelines in 2010 (Lee et al, 2010).
screening in high-risk women: (2.5-4 fold increase in sensitivity) >20%; formal risk assessment important
- BRCA
- radiation to chest (ie for lymphoma)
- high-risk syndromes
when mammography is difficult
- dense breasts, but only with other risk factors
- prior silicone injections
breast cancer
- at time of diagnosis, for staging (though can lead to false positives
- axillary adenopathy, unkown primary
lesion characterization
problem-solving
testing breast implant integrity
It is NOT indicated in the following:
- calcifications (use mammography and biopsy)
Risks
Additional Resources
Lee et al. 2010. ACR guidelines.
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