MR Parameters and Trade-offs Flashcards
What the goals in an ideal medical imaging?
- high signal, low noise (SNR)
- high resolution
- excellent contrast (CNR)
- minimum artifacts
- good coverage
- short scan time
- reproducibility
What are the factors affecting the MR Image Quality?
- TR and TE
- Slice Thickness and Order
- Averaging
- Image Matrix
- Patient Motion
- Equipment performance
- Inter-slice Gap
- FOV
- Bandwidth
- No. of Echoes
- Motion Compensations
- Coils
- Anatomy
What are factors out of our control that determine SNR?
- magnetic field strength
- magnet shimming
- patient motion
- T1 and T2 of the tissue
- Coil “Q” Factor (sensitivity)
- Coil Filling Factor
- Anatomy
What are factors controlled by the protocol determining SNR?
- TR
- TE
- Flip-Angle
- Receiver Bandwidth
- Voxel Size
- FOV
- Matrix Size
What are the differences between surface and volume coils?
Surface: better SNR, but bad intensity distribution
Volume: good B1 districution but lower SNR
MR Signal comes from the _________.
MR noise comes from ______.
selected slice; the entire body in the coil
SNR is proportional to:
slice volume / sensitive volume of the coil
What increases the image resolution at the expense of reduction of SNR?
- Thinner slices
- increasing matrix size
- decreasing FOV
What is the equation for signal averaging & SNR?
SNR => square root of averages
=> means directly proportional to
Bandwidth determines the _.
spectrum of frequencies sampled
What is the difference between high and low bandwidths?
High bandwidth: reduce the time of sample = faster scans and shorter TEs, but decreases the SNR
Low bandwidth: improve the SNR but increases the min TE
Pixel Area Equation
Pixel area = matrix size / FOV
Voxel Equation
Voxel = pixel area x slice thickness
SNR is directly proportional to:
Voxel size & square root of averages & (1/Bandwidth)
Time of scan is directly proportional to:
SNR
Time of scan depends on:
- number of averages
- oversampling
- TR
- resolution
What is protocol optimization?
a compromise between SNR adn time of scan
Scan time equation
phase encoding x averages x TR
What is SAR?
Specific Absorption Ratio; the RF power absorbed per unit of mass of an object, measured in watts per kilogram
What is the FDA limit for SAR of the body and head?
Body: 2 W/kg over the whole body for 15 min
Head: 3.2 W/kg over the head for 10 min
SAR depends on
- Bo squared
- Type of pulse sequence
- pulse duration
- number of pulses
- number of slices
- flip angle
- TR
