Free MRSO Radiofrequency (RF) Field Bioeffects and Specific Absorption Rate (SAR) Questions and Answers — Questions and Answers
Question 1: Specific Absorption Rate (SAR) is a measure of the:
- Total RF power transmitted by the coil.
- Rate of RF energy absorption per unit mass of tissue. (Correct answer)
- Peak B1 field strength in microtesla.
- Change in tissue temperature per minute.
Correct answer: Rate of RF energy absorption per unit mass of tissue.
SAR is the primary metric used to quantify the patient's exposure to RF energy. It is defined as the rate at which RF energy is absorbed by the body per unit of mass and is measured in watts per kilogram (W/kg).
Question 2: Under the FDA's guidelines for Normal Operating Mode, what is the whole-body average SAR limit averaged over 6 minutes?
- 1.5 W/kg
- 4.0 W/kg
- 2.0 W/kg (Correct answer)
- 8.0 W/kg
Correct answer: 2.0 W/kg
For Normal Operating Mode, which requires no special monitoring, the FDA sets the whole-body average SAR limit at 2.0 W/kg. This limit is designed to ensure that the RF energy deposited does not raise the body's core temperature by more than a safe amount.
Question 3: How does SAR deposition change when the static magnetic field strength (B0) is doubled (e.g., from 1.5T to 3.0T), assuming all other sequence parameters are kept the same?
- It is halved.
- It is doubled.
- It is quadrupled. (Correct answer)
- It remains the same.
Correct answer: It is quadrupled.
SAR is proportional to the square of the Larmor frequency, and the Larmor frequency is directly proportional to the B0 field strength. Therefore, doubling the B0 field strength (e.g., from 1.5T to 3.0T) results in a four-fold (quadrupled) increase in SAR, as SAR is proportional to B0 squared.
Question 4: Scanning in the First Level Controlled Operating Mode requires which of the following?
- The patient must be sedated.
- Approval from the hospital's ethics committee.
- Continuous monitoring and medical supervision. (Correct answer)
- Use of a scanner with a field strength of 1.5T or less.
Correct answer: Continuous monitoring and medical supervision.
First Level Controlled Operating Mode allows for SAR levels that may produce significant physiological stress. Therefore, it requires explicit confirmation from the MRSO or a designated delegate, continuous verbal and visual contact with the patient, and monitoring of vital signs as deemed appropriate by medical staff.
Question 5: What is the most common cause of serious RF-related thermal injuries (burns) to patients in the MRI environment?
- Exceeding the whole-body SAR limit.
- Ferromagnetic materials in the patient's clothing.
- Formation of a conductive loop by skin contact or cables. (Correct answer)
- Malfunction of the RF body coil.
Correct answer: Formation of a conductive loop by skin contact or cables.
The most frequent cause of RF burns is the formation of a conductive loop. This can occur when a patient's skin touches the bore of the magnet (skin-to-bore), when two skin surfaces touch (e.g., thigh-to-thigh), or when ECG cables or other conductive wires form a loop on the patient, allowing RF energy to concentrate and cause severe heating.
Question 6: The term B1+rms refers to the root-mean-square of the transmitted RF magnetic field. What is its primary role in MR safety?
- It measures the strength of the static magnetic field.
- It is a prospective estimate used to control SAR. (Correct answer)
- It quantifies the level of acoustic noise.
- It determines the likelihood of peripheral nerve stimulation.
Correct answer: It is a prospective estimate used to control SAR.
B1+rms is a prospective measure used by the MRI system to estimate and control SAR before the scan begins. It is directly related to the amount of RF power that will be deposited, allowing the scanner to predict SAR levels and ensure they remain within safe regulatory limits for the chosen operating mode.
Free MRSO Radiofrequency (RF) Field Bioeffects and Specific Absorption Rate (SAR) Questions and Answers
Specific Absorption Rate (SAR) is a measure of the: