Flashcards in MRI Deck (65):
What is MRI based on?
- Signal from hydrogen nuclei in water molecules. (hydrogen protons)
Describe the 6 step process of attainment of an MRI.
- Patient placed in scanner with coils that produce a very strong magnetic field
- Protons align either parallel to the magnetic field, or in the opposite direction creating a net magnetization called LONGITUDINAL MAGNETIZATION
- Pulse of RF waves applied at right angle to magnetization
- Protons realign transversely, absorbing energy
- RF pulse removed, and aided by 180 degree RF pulse, protons realign longitudinally
- Absorbed energy is released, inducing a current in the receiver coil, which acts as the basis for MRI imaging
What does realignment and decay refer to?
- Realignment from transverse magnetization, and the release of energy.
Does T1 recovery refer to protons gaining longitudinal magnetization, or losing their transverse magnetization?
- Regaining longitudinal magnetization, and losing their energy.
Does T2 decay refer to regain of longitudinal magnetization, or the loss of transverse magnetization?
- Loss of transverse magnetization toward equilibrium
How is T1 relaxation related to imaging?
The time of return to longitudinal magnetization differs for separate tissues
How is T2 decay related to imaging?
- A slow decay leads to a stronger signal
What differences in tissues create the contrast seen in MRI images?
- Proton density
What 2 parameters are most important in creating contrast in an image?
What is TE?
The time at which the signal is captured.
What is TR?
The time at which the RF pulse is repeated to again displace the protons.
Does T1 imaging have long or short TE/TR times?
Does T2 have long or short TE/TR times?
What type of tissue is captured in T1-weighted imaging?
Tissues that rapidly return to longitudinal magnetization.
What type of tissue is captured in T2-weighted imaging?
Tissues that are more reluctant to give up energy.
Which type of weighted imaging has a grainier display?
Which type of imaging provides good anatomic detail?
Which type of imaging provides detail of structures with free water?
What are the patient information markers in an MRI?
- Patient name
- Case number
- Date of exam
- Body part
What technical information markers are present on an MRI?
- Sequence used (TR/ TE time)
What is a fast SE sequence?
- 90 degree pulse
- Multiple 180 degree pulses
- Several images in same slice position without increasing imaging time
What is a proton density sequence? What does the image look like?
- Long TR (>2,000ms)
- Short TE (20 - 30 ms)
- Image based on proton density
- Similar in appearance to T1, but provides greater anatomic detail
What is the most common type of inversion recovery sequence?
Short tau inversion recovery (STIR)
What are the TR and TE lengths in STIR?
- Long TR
- Short TE, but not as short as in T1 or PD
What type of tissue is suppressed in STIR imaging?
How is the image quality affected by the use of STIR?
Poor and grainy.
What is STIR particularly adept at imaging?
Bone marrow edema.
What differentiates GRE distinctly from SE?
- RF pulse only partly flips the next magnetization vector in the transverse plane.
How quickly does GRE sequences generate images?
- Very quckly
What is the image quality of a GRE image?
- High quality
How thick are the slices of GRE images?
- Very thin
What tissues does GRE images create a high contrast between?
What are the 5 elements of an MRI scanner?
- Main magnet
- Gradient coils
- RF coils
What is the strength of an MRI magnet in terms of tesla?
0.3 - 3.
How many times greater than the strength of the earth's magnetic field is one tesla?
How many gradient coils are there?
3; one for each orthogonal plane.
What are the 2 functions of an RF field?
- Transmit RF pulses to alter proton alignment
- Receive signals from protons
What are 3 advantages of open scanners?
- Scan claustrophobic or obese patients
- Reduce scanning noise
- Can perform tests/procedures during scanning
What are 3 disadvantages of open scanners?
- Lower field strength, requiring adjustment of sequences
- Lower signal to noise ratio (worse image)
- Longer scanning times
From which direction are coronal MRI images viewed?
From the front. (facing patient)
From which direction are axial images viewed?
From which direction are sagittal images viewed?
From left to right for either side of the body.
In radiographic images, anatomy and pathology is identified by radiodensity; what is the equivalent for MRI?
What is the signal intensity of fat on T1 and T2?
What is the signal intensity of free water on T1 and T2?
T1: Very low
What is the signal intensity of tendons, ligaments, and menisci on T1 and T2?
What is the signal intensity of cortical bone on T1 and T2?
What is the signal intensity of red bone marrow on T1 and T2?
What is the signal intensity of yellow bone marrow on T1 and T2?
What is the signal intensity of muscle in T1 and T2?
What is the signal intensity of cartilage in T1 and T2?
On which weighted image is signal intensity high for inflammation?
On which weighted image is signal intensity high for hemmorage?
On which weighted image is signal intensity high for fractures?
T2. (with a dark band)
On which weighted image is signal intensity high for early and late avascular necrosis?
On which weighted image is signal intensity high for chronic hemmorage?
On which weighted image is signal intensity high for soft tissue tumors?
What is the most important intrinsic factor in image creation of MRI?
What are the 2 most important extrinsic factors in image creation of MRI?
- Magnet strength
What 5 things does MRI image best?
- Changes/ variation in bone marrow
- Soft tissue detail
- Meniscal tears
- Differential diagnosis of herniation vs other nerve root problems
- Staging neoplasms
What is the 1 contraindication, and 2 concerns for the use of MRI on patients?
- Surgical clip displacement
- Pace maker interference
- Sedating patients who can't stay still
What is the technique used to evaluate joint capsules in MRI?
What is MR Myelography used to evaluate?
- Subarachnoid space and spinal canal without the use of a contrast medium
What are 4 advantages of MRI over CT?
- Greater contrast resolution on soft tissue imaging
- Greater ability to image organs surrounded by bone
- No ionizing radiation
- Less risk of false negatives due to multiple sequences