MRI: QA and Safety Flashcards
What are the required qualities of a test object for QA?
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
What are the three basic regions of a QA test object and what do they measure?
Grid of holes - test geometry
Flood Field - Uniformity
Wedges - Slice Position/Profile
What components of the scanner affect SNR?
Poor RF calibration/amplifier problem - low flip angle
RF coil performance
Pre-amp problem
How is SNR measured?
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
What components of the scanner affect uniformity?
RF coil problem (channels not behaving equally) Shim problem (poor B(0) homogeneity
How is uniformity measured?
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.
What components of the scanner affect ghosting/stability?
RF amplitude
RF phase
Gradient strength
Noise on room temperature shims
What components of the scanner affect the image geometry?
Gradient amplifier problem (axes not equally calibrated)
Pre-emphasis performance
Shim problem (poor B(0) homogeneity)
What components of the scanner affect the slice profile/thickness/offset?
RF waveform controller.
What hazards arise from an MRI scanner?
Static magnetic field
Time-varying magnetic fields (from Magnetic gradients and RF)
Cryogens
Contrast Agents
What hazards arise from the static magnetic field?
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)
What hazards arise from the time varying magnetic field
Peripheral nerve stimulation
Acoustic noise
Heating
RF burns
What RF exposure standards exist?
Specific Absorption Rate of 2 (normal) or 4W/kg (first-level controlled) averaged over 6min
At what point should hearing protection be given to the patient?
If the scan is likely to exceed 99dB.
What exposure standard exist for the static field?
2T for normal operating mode
2-4T for first-level controlled operation
Above 4T for second-level controlled operation
What should be included in MRI local rules?
Procedures to follow in an emergency. Contact details for: - MR responsible person - MR safety advisor - MR authorised persons Controlled access areas
What magnetic field strength magnetic field defines an MR Controlled area?
0.5mT
What are the three categories of medical devices classified by the ASTM standard?
MR SAFE
MR Conditional
MR Unsafe
Name three hazards that may be present when a subject is imaged with MRI, not specific to individual patients.
Static magnetic field;
Time varying magnetic fields from either MRI gradients or RF irradiation;
Cryogens;
Contrast agents
Name 2 possible dangers associated with a cryogen quench?
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.
What creates the acoustic noise heard during an MRI? How can it be reduced? What are the consequences of reducing it?
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.
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?
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
What magnitude of fringe magnetic field must be contained with a controlled area?
0.5 mT or 5G
In what areas are magnetic fields in excess of the controlled area permitted?
Radiographers control room
Equipment room
Name three different types of regions that an MRI test object would contain and name their purpose?
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.
Describe how the measurement of intensity uniformity might be assured.
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
What practical problems are there in quality assuring the radiofrequency chain? What can we assure using our approaches?
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.
Where does the noise in the image come from?
The noise in the image is ideally only derived from electrical (or Johnson noise) and is generated within the receiver electronics.
