MRI Specific Absorption Rate

MR Specific Absorption Rate (SAR) is a web based training that will provide the Technologists with information regarding the definition and the safety aspects of Specific Absorption Rate and the Food and Drug administration and International Electrotechnical commission guidelines and limits. The dependencies of SAR and the ability to identify the SAR operating modes, SAR limits, as well as parameters and sequences that reduce SAR will also be discussed.          You should now be able to: Define SAR Understand Safety Aspects of SAR FDA/IEC guidelines and limits Look-Ahead Monitor and SAR-Man Dependencies of SAR Identify Operating Modes of SAR Identify SAR Limits Exceeded Identify Parameters that reduce SAR Identify Sequences that reduce SAR      Thank you! Proceed to the assesment page to complete the training. Specific Absorption Rate (SAR) Definition SAR is defined as the RF power absorbed per unit of mass of an object when exposed to a radio frequency (RF) electromagnetic field: Specific to the Body Part and tissue Absorption of the RF into the body and tissue Rate of RF exposure and absorption                                         Unit of measure = Watts per kilogram (W/kg) RF Exposure Application of time varying electromagnetic fields with frequencies in MHz range generating heat within tissue being exposed Heating is a major effect of RF exposure Safety considerations focus on temperature increases Safety Aspects of SAR Safety aspects must be considered: Capacity of patient's thermo regulation to dissipate heat is respective of global RF load If not limited to tolerable values, life threatening episodes could be a consequence Localized high SAR values in excess could lead to local burns Guidelines are issued which set limits for SAR in order to limit RF load to tolerable values Upon successful completion of this course, technologist will be able to: Define SAR   Understand Safety Aspects of SAR   FDA/IEC guidelines and limits   Look-Ahead Monitor and SAR-Man   Dependencies of SAR   Identify Operating Modes of SAR   Identify SAR Limits Exceeded   Identify  Parameters that reduce SAR   Identify Sequences that reduce SAR                                                       FDA/IEC Guidelines Different guidelines issuing limits respective of RF exposure during MR examination United States Food & Drug Administration (FDA) International Electro-technical Commission (IEC) Accepted in most countries (including US)   SAR scales with square of main magnetic field as well as with square of flip angle assuming constant pulse duration Interest mainly on dependency of B1 field, since B1 rules flip angle Doubling main magnetic field strength & pulse flip angle quadruples SAR 1.5 T 3T  SAR increases by factor of 4 FDA SAR Guidelines RF fields lead to warming of the body tissue. In this context, an important value per body weight is the specific absorption rate or SAR Specific Absorption Rate (SAR) Site Dose Time (min) equal to or greater than: SAR (W/kg) Whole body averaged over 15 4 Head averaged over 10 3 Head or Torso per gram of tissue 5 8 Extremeties per gram of tissue 5 12   2. IEC 60601-2-33 ICE 60601-2-33 is the international standard for the safety of magnetic resonsnace equipment intended for medical diagnosis. CDRH representatives participated in the development of this standard. It was released in July 1995 and addresses most of the important safety issues associated with MR, such as instructions for use and safe operating levels. The NEMA standards for measuring acoustic noise, dB/dr and SAR have been incorporated into the IEC standard.  However, the IEC standard does not address performance issues, such as SNR, image uniformity, geometric distortion and slice thickness.       FDA – Food and Drug Administration FDA provides GUIDELINES FDA does not impose strict and explicit SAR limits What is Guidance? Good Guidance Practice (GGP) documents are documents prepared fro CDRH staff, regulated industry and the public that relate to: Processing, content, and evaluation of regulatory submissions Design, production, manufacturing, and testing of regulated products Inspection and enforcement procedures Guidance documents do not create or confer any rights for or any person and do not operate to bind FDA or the public.An alternative approach may be used if such approach satisfies the requirements of the applicable statue, regulations, or both.        Parameters that affect SAR   Parameters that MAY affect SAR TR TR  ­SAR ) Number of Slices Resolution/Phase encoding Steps Additional RF Pulses (Spectral FS, Inversion, MT) Pulse sequence type IPAT Pre-Saturation Pulses Allowed delay RF Pulse Type (Fast/Normal/Low) VERSE Hyperecho Flip Angle        Turbo Factor (Echo Train Length) Concatenations     Patient Registration - Estimates body part volume and mass  Determines position of body parts relative to transmission field volume based on table landmark Calculates RF power of scan to be executed Estimates power absorbed by each body part (SAR) Compares these estimates to defined SAR limits   SAR and stimulation monitoring can be operated in two modes which are set during patient registration Normal Mode SAR and dB/dt limits are set to low values Warming of patient is kept to a minimum and stimulation can be excluded First Level Mode SAR and dB/dt limits are set to higher values Increased SAR Values may cause patient to experience notable heat increases Increased dB/dt values may cause patient to experience slight peripheral nerve stimulation Safety Aspects of SAR Protocols will not load if advanced calculation exceeds SAR limits SAR Limits Exceeded dialog window appears Current Area of dialog window Shows current parameters selected RF Pulses and VERSE Available at 3T & Aera 1.5T Reduce SAR by changing shape of excitation and refocusing RF Pulses Reduce SAR by up to 45% VERSE pulses are implemented RF Pulse Type Fast RF Pulse Type Low SAR   Pulse Type VERSE Fast RF ON Normal RF OFF Low SAR ON (*x**) * Uses a shorter RF pulse with less than perfect slice profile ** Replaced with standard RF pulse if IR is selected The normal operating mode requires only routine monitoring of the patient. The first level controlled mode requires medical suspension. Medical supervision requires a positive assessment by a qualified medical practitioner of the risk versus benefit for the scan, or a decision by a qualified surrogate of the practitioner that the patient satisfies a set of objective criteria formulated by the medical practitioner. Equipment that is capable of operating in the first level controlled mode must display the mode defined by dB/dt and the predicted value of the SAR. When an operating parameter reaches the lower limit of the first level controlled mode, an indication must appear on the console and the operator must take a deliberate action to proceed with the scan. Also, the dB/dt and SAR values must be recorded with the image data.     Spatially localized temperature limits Operating Mode Rise of body core temperature (°C) Head (°C) Torso (°C) Extremities (°C) NORMAL 0.5 38 39 40 FIRST LEVEL CONTROLLED 1 38 39 40 SECOND LEVEL CONTROLLED >1 >38 >39 >40 Differentiates two groups of SAR limits strong>Global SAR Limits Whole body, exposed part of body, head strong>Local SAR Limits Head, Trunk, Extremities Guidelines Specifies rules when to observe which limits depending on type of transmit coil, volume or local RF transmit (Tx) coil Volume Tx coils Global SAR limits only Local Tx coils Local SAR limits + whole body SAR limit IEC – SAR Limits Whole body SAR SAR averaged over patient’s total body mass over specified time Partial body SAR (Exposed Part) SAR averaged over patient’s body mass exposed by volume of RF transmit coil over specified time Head SAR SAR averaged over mass of patient's head over specified time Local SAR SAR averaged over any 10 grams of tissue of patient's body over specified time Patient Registration input – Estimate body part volume and mass     • Up to 18 years • Dimensions of cylinders are derived from   statistical growth data • Adults (>18 yrs) • Values are extrapolated assuming that   body measurements are constant • Based on NORMAL MEASUREMENTS   DURING GROWTH • Statistical data with dependency on age • World statistics taken from   NCHS/CDC –National Center for    Health Statistics/Center for   Disease Control   Head cylinder volume Head circumference Calculates head cylinder radius based on age and sex – entered Patient Registration Head cylinder length – calculated from determined radius and mass       Calf radius Reasonable value for overall leg cylinder radius from world statistics Length of leg cylinder “Sitting Height” ratio – function of age and sex – used with overall height entered in Patient Registration from world statistics Calf circumference Function of age and sex – determines leg cylinder radius  Leg cylinder volumes       Torso cylinder volume   Torso cylinder length Subtract head and leg cylinder length from patient’s height Torso cylinder mass Determined by difference in total patient mass and of head and leg cylinder masses Torso cylinder radius Determined by both length and mass                           Thin and Thick Patients Thin or thick patients taken into account only for torso cylinder Assumes head and leg circumference variance is smaller compared to torso Radius of torso cylinder varies based on total patient weight Head radius not affected by weight Total weight – function of age and sex Checks validity of weight entered in Patient Registration          Patient 300 pounds 7 feet tall Patient 300 pounds 5 feet tall       Determines position of body parts relative to transmission field volume based on table landmark       Cylinders Defines mass of specific body parts used to calculate specific SAR for each body part Defines location of each body part on table   Safety Aspects of SAR Proposals Area of dialog window   Shows suggested changes to examination parameters which will allow examination to continue   . Within a range: Frequency bandwidth = constant Slice gradient strength = variable Across a range: Frequency bandwidth = variable Slice gradient strength = constant Sat band Thickness range (mm) Peak Pulse Voltage (relative to TRAref) Relative SAR 3 - 10 1.0115 1.000 11 - 110 2.362 5.453 111 - 150 2.560 6.405 NOTE - for Avanto 1.5T VB15; may vary for other systems and software configurations   Maximum Delay time after the end of the measurement Exception: Studies using contrast agents and dynamic imaging where timing is critical, allowed delay should not be used Note: 3T systems not recommended to use allowed delay’s >60 seconds   Normal RF Pulse with good slice profile with optimal SAR Low SAR Extended RF Pulse duration Reduced SAR Fast Short RF Pulse duration Higher SAR Aera 1.5T & 3T uses VERSE for Fast RF          Rotation of magnetization from a direction (e.g., longitudinal, transverse, etc) at end of RF Pulse Flip angle affects amount of contrast and signal in image  Spin Echo Flip angle refers to initial RF pulse (i.e. Flip 90˚) Turbo Spin Echo Flip Angle refers to refocusing pulses (i.e.  Flip 180˚) Recommended refocusing flip angle: ≥160˚ SAR is reduced by ~20%, signal reduction is ~ 3%     Change in SAR and SNR dependent on T1 and TR specified Patient Registration input– Estimate body part volume and mass Height (required) Weight (required) Sex (required) Age (required   HEATING (power in, Pin) COOLING (power out, Pout) HEATING > COOLING (Pin > Pout) Core Temperature Rises Power in - Power out = Power absorbed = SAR HEATING (power in, Pin) COOLING (power out, Pout) Head cylinder volume Torso cylinder volume Leg cylinder volumes RF Power squared when field strength doubled B1 (RF) ~ B0 (physics contraints) RF power ~ B12~ B02 (engineering constraint) SAR ~ RF power ~ B02(physiological constraint) B0 M B1 3T 1.5T B0 M B1 B1 (RF) ~ B0 (physics constraint) RF power ~ B12 ~ B02 (engineering constraint) SAR ~ RF power ~ B02 (physiological constraint) SAR RF Power (kW) B0 (Tesla) 0 1 2 3 12 10 8 6 4 2 0 Empirically determined RF Power for a 180° angle versus field strength for whole body applications Sum of pulse energies divided by time   90 180 SE TSE (low SAR) (high SAR) 90 180 180 180 180 180 Integrated Parallel Acquisition Techniques (iPAT) There are two types of iPAT techniques: GRAPA mSENSE Reduces # of phase lines   Scan time reduction Decrease SNR FFT FFT GRAPPA Recon SENSE Recon coli 1 coli 2 mSENSE and GRAPPA Standard RF Pulse VERSE Pulse - Reduces SAR by changing shape of excitation and refocusing RF pulses Gs RF time VERSE RF Pulse ⇒Fast and Low Power Slow/Low Power Fast/High Power time time RF RF Gs Gs SAR Reduction Techniques - Hyperecho Turbo Spin Echo Hypercho - 3T 90° 90° 120° 150° 180° 150° 120° 180° 180° 180° 180° 180°