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
MR Conditional
MR Unsafe

Question 19

Name three hazards that may be present when a subject is imaged with MRI, not specific to individual patients.

Answer 19

Static magnetic field;
Time varying magnetic fields from either MRI gradients or RF irradiation;
Cryogens;
Contrast agents

Question 20

Name 2 possible dangers associated with a cryogen quench?

Answer 20

The gas should be carried away from the examination room in the quench pipe – however failure would release asphyxiant gas to the room. Additionally, the sudden absorption of latent heat will lower the temperature of the interior apparatus, though the patient tunnel should be sufficiently insulated.

Question 21

What creates the acoustic noise heard during an MRI? How can it be reduced? What are the consequences of reducing it?

Answer 21

This is due to the Lorenz force induced in the gradient coils when current is pulsed in the presence of a static magnetic field. It can be reduced by reducing the slew rate of the gradients: potentially this may lead to longer TR or TE in some sequences.

Question 22

Joe is chatting to the radiographer before his MRI scan. He recently celebrated giving up smoking by getting a new tattoo. What hazards might scanning Joe present, and how should they be managed?

Answer 22

Specific hazards to Joe (aside from the usual level of risk), would be that (1) he may be wearing a nicotine patch, which often contain metallic mesh, and pose a risk of burns. They should be removed before the examination. (2) His tattoo might pose a potential burn risk depending on its size and colour. He should be advised to press the panic button and let the radiographers know if he feels any discomfort or heating from the area of his tattoo

Question 23

What magnitude of fringe magnetic field must be contained with a controlled area?

Answer 23

0.5 mT or 5G

Question 24

In what areas are magnetic fields in excess of the controlled area permitted?

Answer 24

Radiographers control room

Equipment room

Question 25

Name three different types of regions that an MRI test object would contain and name their purpose?

Answer 25

There is usually

(i) a regular grid of holes in the signal (regular geometric signal void) which allow the measurement of geometric distortion;
(ii) a flood field to measure the uniformity of the signal; and
(iii) wedges or inclined planes, which allow the examination of the accuracy of slice position and slice profile.

Question 26

Describe how the measurement of intensity uniformity might be assured.

Answer 26

A flood field phantom should be used (or the flood field range of a multi-use phantom). The uniformity of the signal intensity is assessed by measuring the standard deviation of the signal over the largest possible area of object

Question 27

What practical problems are there in quality assuring the radiofrequency chain? What can we assure using our approaches?

Answer 27

The problem is that the QA methods really only apply directly to the use of birdcage head coils or the in-built body coil. Most of the other coils in use in present day MRI are multi-channel receiver coils and their spatial coil sensitivity is highly non-uniform. Therefore uniformity and SNR are not readily definable with these coils. Many reconstruction algorithms for these coils (including parallel imaging) tend to manipulate the sensitivity profile and filter noise outside the object. We can however assure the performance of the gradient and radiofrequency transmit elements of the scanner.

Question 28

Where does the noise in the image come from?

Answer 28

The noise in the image is ideally only derived from electrical (or Johnson noise) and is generated within the receiver electronics.