MRI Flashcards
Invasive/non-invasive?
Non-invasive
Image it produces
three dimensional detailed anatomical images
What does it use?
powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field.
What does radiofrequency current do when turned on?
When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field.
What does radiofrequency current do when turned off?
When the radiofrequency field is turned off, the MRI sensors can detect the energy released as the protons realign with the magnetic field.
What occurs to protons when current is off?
The time it takes for the protons to realign with the magnetic field, as well as the amount of energy released, changes depending on the environment and the chemical nature of the molecules. Physicians can tell the difference between various types of tissues based on these magnetic properties.
Fourier transformation
Used to convert the frequency information contained in the signal from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of gray in a matrix arrangement of pixels. By varying the sequence of RF pulses applied & collected, different types of images are created.
Repetition Time
Repetition Time (TR) is the amount of time between successive pulse sequences applied to the same slice.
Time to Echo
Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal.
How does an MRI work?
To obtain an MRI image, a patient is placed inside a large magnet and must remain very still during the imaging process in order not to blur the image. The faster the protons realign, the brighter the image.
What does MRI see?
MRI scanners are particularly well suited to image the non-bony parts or soft tissues of the body. The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT; for this reason, MRI is often used to image knee and shoulder injuries.In the brain, MRI can differentiate between white matter and grey matter and can also be used to diagnose aneurysms and tumors.
Risks
The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects; it is strong enough to fling a wheelchair across the room. Patients should notify their physicians of any form of medical or implant prior to an MR scan.
People with implants, particularly those containing iron
Nerve Stimulation
Contrast agents
Claustrophobia
T1 vs T2
Tissue can be characterized by two different relaxation times – T1 and T2. T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field. T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.
T1 vs T2 weighted
The most common MRI sequences are T1-weighted and T2-weighted scans. T1-weighted images are produced by using short TE and TR times. The contrast and brightness of the image are predominately determined by T1 properties of tissue. Conversely, T2-weighted images are produced by using longer TE and TR times. In these images, the contrast and brightness are predominately determined by the T2 properties of tissue.
Limitations of MRI
Subject to motion artifact
Inferior to CT in detecting acute hemorrhage
Inferior to CT in detection of bony injury
Requires prolonged acquisition time for many images
