MRI: QA and Safety

Question 1

What are the required qualities of a test object for QA?

Answer 1

Must test the scanner under routine conditions.

• Appropriate for TR and TEs used for clinical imaging
• Relaxation times mimicking tissue (T1, T2, T1/T2 ratio)

High-temperature stability
- Relaxation times can be temperature dependent

Minimal susceptibility effects - introduce distortion

• Differences in magnetic susceptibility to free space
• Minimal differences in magnetic susceptibility within the object

Question 2

What are the three basic regions of a QA test object and what do they measure?

Answer 2

Grid of holes - test geometry
Flood Field - Uniformity
Wedges - Slice Position/Profile

Question 3

What components of the scanner affect SNR?

Answer 3

Poor RF calibration/amplifier problem - low flip angle
RF coil performance
Pre-amp problem

Question 4

How is SNR measured?

Answer 4

Tested on uniform flood field.
Collect two images with identical parameters
Subtract the scans
Measure the Signal (S) as the mean across five ROI in the basic image.
Measure standard deviation (SD) of noise in the same ROI in the difference image.
SNR = SQRT(2)*S/SD

Question 5

What components of the scanner affect uniformity?

Answer 5

RF coil problem (channels not behaving equally)
Shim problem (poor B(0) homogeneity)

Question 6

How is uniformity measured?

Answer 6

Measured on the flood field image
Measure intensity profile across the image - should be flat
Measure SD of the signal over the largest area of object - uniform intensity produces narrow distribution.

Question 7

What components of the scanner affect ghosting/stability?

Answer 7

RF amplitude
RF phase
Gradient strength
Noise on room temperature shims

Question 8

What components of the scanner affect the image geometry?

Answer 8

Gradient amplifier problem (axes not equally calibrated)
Pre-emphasis performance
Shim problem (poor B(0) homogeneity)

Question 9

What components of the scanner affect the slice profile/thickness/offset?

Answer 9

RF waveform controller.

Question 10

What hazards arise from an MRI scanner?

Answer 10

Static magnetic field
Time-varying magnetic fields (from Magnetic gradients and RF)
Cryogens
Contrast Agents

Question 11

What hazards arise from the static magnetic field?

Answer 11

Permanent hazard
Mechanical effects on metal objects giving rise to particular forces on the object (indirect effects)
- Transational force,rotational force, eddy current damping.
Biological effect (direct effects)

Question 12

What hazards arise from the time varying magnetic field

Answer 12

Peripheral nerve stimulation
Acoustic noise
Heating
RF burns

Question 13

What RF exposure standards exist?

Answer 13

Specific Absorption Rate of 2 (normal) or 4W/kg (first-level controlled) averaged over 6min

Question 14

At what point should hearing protection be given to the patient?

Answer 14

If the scan is likely to exceed 99dB.

Question 15

What exposure standard exist for the static field?

Answer 15

2T for normal operating mode
2-4T for first-level controlled operation
Above 4T for second-level controlled operation

Question 16

What should be included in MRI local rules?

Answer 16

Procedures to follow in an emergency
Contact details for:
- MR responsible person
- MR safety advisor
- MR authorised persons
Controlled access areas

Question 17

What magnetic field strength magnetic field defines an MR Controlled area?

Answer 17

0.5mT

Question 18

What are the three categories of medical devices classified by the ASTM standard?

Answer 18

MR Safe - poses no known hazards to all MR environments
MR Conditional - an item that has been shown to pose no known in a specified MR environment with specific conditions of use.
MR Unsafe - an item that is known to pose hazards in all MR environments