What is magnetic resonance imaging?|
This article was prepared by Dr Kevin Lee of Mercy Radiology Group.
Magnetic Resonance Imaging (MRI) is a powerful imaging modality which produces cross sectional tomographic images similar to those produced by computed tomography (CT). Image acquisition is based on the physical principles of Nuclear Magnetic Resonance (NMR) which is an apparently safe interaction between radio waves and certain atomic nuclei in the body when they are in the presence of a strong magnetic field.
How does an MRI work?
Certain atomic nuclei that have an odd number of protons or neutrons possess a characteristic known as "spin". Since the nucleus is positively charged it generates a small magnetic field when it spins. It thus behaves like a small bar magnet and tends to align with a strong external magnetic field. If these nuclei of a magnetised object are exposed to a short burst of energy in the form of radio waves, the nuclei begin to spin or precess in phase.
For this to occur the burst of RF energy must be at the same frequency as that of the precessing nuclei ie. The resonant frequency. When the nuclei precess in phase, resonance has occurred. As energy has been added to the magnetised object, it tries to restore equilibrium and in so doing emits radio signals. These signals can be detected by extremely sensitive antennae and the signals emitted are dependent on the tissue characterisation of the object. To transform these signals into an image requires sophisticated hardware, considerable mathematical calculation and hence powerful computers.
As hydrogen is the most abundant atom in the body, clinical MR imaging involves the imaging of the hydrogen nucleus, the proton. In the basic MR examination, data acquisition is manipulated to display images that reflect three properties of tissue viz. the T1 relaxation time, T2 relaxation time and proton density. In brief terms, T1 weighted images give exquisite anatomic detail while T2 weighted images are sensitive to tissue abnormalities. Images are displayed on a grey scale format.
Why is MRI used?
Clinical MR imaging has evolved rapidly since the early 1980’s and is a complementary modality to other techniques used in radiology such as ultrasound, CT, angiography and scintiscanning.
Its advantages include:
- No ionising radiation
- Multi planar imaging
- Superior contrast resolution
- Non invasive
- Free of bone artefacts
Its disadvantages include:
- Relatively expensive
- Relatively long scan times
- Less sensitive to fine calcification
- Inferior bone detail
- The "MRI" environment
Are there reasons not to use MRI?
The magnetic field poses a potential hazard to patients with internal ferromagnetic objects in critical positions eg. ferromagnetic intracranial aneurysm clips, metallic foreign bodies in the eyes. Implanted electrical devices such as pacemakers and cochlear implants can be disrupted. The majority of MR imagers have a configuration that may induce patient claustrophobia. Electrical support devices and anaesthetic equipment has had to be specially developed for the MR environment.
What are the best uses of MRI?
The strongest applications for MRI have been in assessment of the neurologic and musculoskeletal systems. For most disorders of the brain and spine, MRI is the best screening modality. It demonstrates greater sensitivity to tumour and inflammation detection than CT. Tumours are more precisely localised and the multi planar capability allows surgeons better visualisation of lesions.