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Tips and Tricks for SAR and Stimulation Reduction Job Aid

This Job Aid discusses MR techniques to reduce SAR and Stimulation.

Tips and Tricks for SAR and Stimulation Reduction Job Aid MAGNETOM Systems Our MAGNETOM systems provide a limitation of SAR and stimulation values to the issued limits for the NORMAL and FIRST LEVEL CONTROLLED OPERATING MODE according to FDA requirements and requirements from the international MR safety standard IEC 60601-2-33. However, there are patients where measurements with reduced SAR and / or stimulation levels might be advisable. These could be patients which have already indicated the sense of heat and / or peripheral nerve stimulation in previous measurements, or which are especially sensitive (e.g. children). This information provides tips and tricks how to reduce the SAR and stimulation burden on protocol level by changing respective protocol parameters. Please note that the online monitoring only stops scans which exceed the normal and first operating mode level. For more information please refer to Scanning Operator Manual of your system. Please also note that the prediction of SAR and stimulation values uses certain assumptions which might cause slight differences between predicted and real measurement values. There might be situations, e.g. triggered protocols where the timing changes during the measurement, where the actual SAR or stimulation values differ from the predicted values significantly. If additional limitation of SAR is needed: The real measurement values need to be constantly monitored and the measurement needs to be stopped manually. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 SAR in general The Specific Absorption Rate (SAR) is the power deposited within the human body by the radio frequency (RF) pulses used for the excitation of spins. The quantity is measured in the unit Watt per kg (W/kg). MAGNETOM systems provide a SAR estimation before each scan (prediction) and monitor SAR values during each scan (current monitor). SAR Information SAR Monitor Operating Mode Current Measurement Normal Next Measurement . Normal Mode First Level Prediction Status Patient Protocol Current Displayed values belong to the current patient! Name Predicted Value Unit Whole Body 7.7 % Exposed Body 5.7 % Theda 13.0 % Head Local 0.0 % The actual B1 rms value can be much higher than the predicted B1+ rms value displayed here. Do not scan patients with implants based on this value. For more information see operator manual. Whole Body 0.00 0.07 0.91 W/lb Calculation Time 11:08:28 Close It is important to note that SAR also depends on the patient anatomy and the patient position within the bore – hence it will differ from patient to patient, even if identical protocols are used. For a precise estimation of the SAR level MAGNETOM systems need to perform the adjustment of the radio frequency transmit chain. This is typically done after protocol editing just before the real measurement. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Strategies to Reduce SAR There are a couple of strategies to reduce SAR levels. Each strategy can be realized by changing the corresponding protocol parameter within the sequence parameter cards: A) Low SAR RF pulses: Many sequences support low SAR pulses. They usually make use of slightly longer pulse durations which reduce the power of the pulses while maintaining the flip angle. Low SAR pulses might require slightly longer TE and TR times. Dot @ 4:50 min Auto @ None @ 0.7x0.7x5.0 mm3 1.00 Routine Contrast Resolution Geometry System Physio Inline Sequence Part 1 Part 2 Assistant Sequence Name qtir Bandwidth 191 - Hz/Px Dimension 2D Echo Spacing 12.0 ¥ ms RF Pulse Type Low SAR Define Turbo Factor Fast Normal Turbo Factor 15 + Gradient Mode Low SAR Echo Trains per Slice 15 + Flow Compensation None B) Longer TR times: In most protocols increasing TR will spread out the RF pulses over a longer duration so that the average power deposited in the patient is lowered. C) Reduced flip angles: The flip angle of the protocol controls the amplitude of the RF pulses. Reducing the flip angle also reduces the amplitude of the pulses so that the SAR level decreases. In some cases, even a slight reduction in flip angle can reduce the SAR level significantly as SAR scales with the second power of the flip angle. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 D) Reducing number of slices: The number of slices is directly linked to the number of RF pulses used during the measurement. Reducing this value while keeping TR constant will reduce SAR. E) For protocols based upon the TSE sequence: The Turbo Factor defines how many refocusing pulses are used after an excitation pulse. Reducing this value while keeping TR constant will reduce SAR and increase scan time. F) Fat saturation methods: All fat saturation methods introduce additional RF pulses into the measurements. Depending on the chosen fat saturation methods the amount of additional pulses and their contribution to the SAR level are quite different. In general, methods which use either inversion pulses (e.g. STIR) or adiabatic pulses (e.g. SPAIR) add a high amount of SAR. DIXON based fat suppression also increases SAR because two echoes are acquired with additional pulses. G) Saturation bands: Saturation bands add RF pulses and hence increase the SAR level. H) Number of concatenations: The number of concatenations in combination with the chosen TR can influence the SAR level. In measurements without concatenations the scanner will acquire a part of each slice before it returns to the first slice to acquire the second part of the slice. This interval – the duration between two consecutive excitations within one slice – is defined as TR. When using concatenations, the stack of slices will be split into parts and the scanner will only acquire slices herein before returning to a slice for the next part of the slice. Hence, setting two concatenations will either reduce TR by a factor of two (at the same scan time) or keep TR by doubling the total scan duration. The interplay of these two parameters defines the number of pulses per time and therefore influences the SAR level. I) Patient positioning: SAR supervision is performed for several aspects: Head SAR, Whole Body SAR, and exposed Body SAR. For all these aspects the scanner needs to know the patient position relative to the scanner. For patients with their heads within the head coil, the position can be determined with high precision. In contrast, for patients which are not positioned within the head coil the position usually is quite unknown (e.g. feet first exams). In these cases, the scanner applies a worst-case model for the SAR calculation which might overestimate the SAR significantly. Hence, if possible, position the patient within the head coil and register Head First Supine. J) Scan Assistant (Sequence > Assistant): Scan protocols offer a SAR assistant where the user is able to define how the system can automatically adjust parameters. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Dot 8 4:50 min Auto @ None @ 0.7x0.7x5.0 mm3 1.00 Routine Contrast Resolution Geometry System Physio Inline Sequence Part 1 Part 2 Assistant SAR Assistant Off Off TR Flip Angle Flip Angle, TR Allowed Delay TR, Flip Angle General remark: Spin-echo based sequences like SE, TSE, BLADE, SPACE and HASTE usually have higher SAR levels than gradient-echo based sequences like FLASH, GRE, and VIBE. For further information to the different parameters, please refer to the relevant operator manuals. Checking of SAR prediction The estimation of the SAR levels requires to perform the adjustment of the radio frequency transmit chain. This adjustment scan is usually performed when the user presses the “Go” button just before the real measurement. The behavior assumes that there is no user interaction between adjustment scans and the real measurement. Optimizing SAR however requires exactly this step – checking the SAR prediction after the adjustment scans and editing protocol parameters before the real measurement. Following setup improves the user workflow: A) Set up protocol with the option Wait for User to Start. See Figure 1. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Step 1 - 11_se_sag Step 3 - t1_f12d_sag General General Start measurement without further preparation Protocol Parameters Start measurement without further preparation Protocol Parameters Wait for user to start Voice Commands Wait for User to Start Voice Commands Execution . Single measurement Image Management O Single Measurement Execution Auto Load Repeated Measurement Image Management Repeated measurement Copy References Auto Load Auto start Auto Start Preview Copy References Prio recon Prio Recon Auto Open Inline Display Preview Auto open inline display Auto Close Inline Display Auto close inline display Auto Open Contrast Injector Dialog Figure 1: Protocol with option Wait for User to Start (left N/4, right N/X) B) Edit protocol. C) Press the Go button. This triggers only the adjustment scans; the real measurement waits for the user to press the Continue button. D) Check SAR prediction after completion of the adjustments. E) If SAR levels are higher than expected, press Skip. The protocol will not be scanned but aborted. SARLimit(s) Exceeded ? x Step 23 - cine_seg_lax BORE Temperature 76.1 ºF Operating Mode Current Measurement Normal Mode Next Measurement . Normal Mode First Level Protocol Parameters Current Recommended Flip Angle 90 deg 65 deg TR 600 ms 800 ms OK Open Protocol Skip F) Right-click on aborted protocol and choose Repeat and Open for parameter editing. G) Repeat steps C) to F) until expected SAR level is predicted. H) Press Continue. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Please note that this workflow allows to optimize the predicted SAR level but does not influence the supervision. The online SAR supervision still monitors the normal / first level SAR limits. Stimulation in general Peripheral nerve stimulation (PNS) is the effect that some nerves of the peripheral nerve systems get activated by switching gradients. This can cause discomfort, the sense of pain / hot skin and uncontrolled muscle contractions. In general, PNS is not considered to be dangerous because the stimulation of vital organs such as the heart have a much higher stimulation limit than the peripheral nerves. The model used for the prediction of PNS does not include patient specific information such as the position so that stimulation limits of a certain protocol are identical for all patients. However, different patients might have different thresholds for stimulation effects so that it might be advised to reduce stimulation. Stimulation is mainly dominated by how fast and how long gradients are switched. The physiological answer of a nerve to a switching magnetic field decays with a certain time constant so that switching of multiple gradients shortly after another may build up a stimulation effect. SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Strategies to reduce stimulation There are a couple of strategies to reduce stimulation levels proposed by Stimulation Limit Exceeded Window which will appear (A-C): Recommendations ? X Step 23 - cine_seg_lax Protocol Parameters Current Recommended FOV Read 340 mm 360 mm Min. Gradient Rise Time 5.88 [us/(mT/m) 8.43 us/(mT/m) Slice Thickness 6.0 mm 7.0 mm Additional Changes OK Open Protocol Skip A) Using a different gradient mode/change rise time: All scanners offer different gradient modes: Whisper, Normal, Fast, Ultrafast1. Lowering the gradient mode usually reduces how fast and how much the gradients are switched so that the stimulation effect decreases. This change might come at the expense of slightly longer TE and TR times. Please use change rise time with care as it might have influence on image contrast and acquisition time TA. B) Increasing the FOV: A larger FOV (at identical base resolution) increases the size of voxels and causes the scanner to use lower gradients. When keeping the timing identical this leads to lower stimulation values. C) Increasing slice thickness: A higher slice thickness causes the scanner to use lower gradients which lead to lower stimulation values. D) Lowering the base resolution: A lower base resolution (at identical FOV size) increases the size of voxels and causes the scanner to use lower gradients. When keeping the timing identical this leads to lower stimulation values. The strategies presented above serve as tool kit for reducing SAR and stimulation in protocols. None of the strategies will fit to all protocols, but in most cases at least one of the strategies will work. Often the combination of more than one strategy can reduce the impact on image quality and scan time. 1 not available on all systems SIEMENS Unrestricted Published by Siemens Healthcare GmbH © Siemens Healthcare GmbH, 2020 Master Template Effective Date: 3/5/2020 | HOOD05162003074494 Healthineers Job Aid Effective Date: 2-Mar-21 I HOOD05162003151913 US Job Aid Effective Date: 3.18.2021 | HOOD05162003179744 Job Aid I Tips and Tricks for SAR and Stimulation Reduction Effective Date: 2-Mar-21 I HOOD05162003151913 US Effective Date: 3.18.2021 I HOOD05162003151913 Please note that the learning material is for training purposes only! For the proper use of the software or hardware, please always use the Operator Manual or Instructions for Use (hereinafter collectively “Operator Manual”) issued by Siemens Healthineers. This material is to be used as training material only and shall by no means substitute the Operator Manual. Any material used in this training will not be updated on a regular basis and does not necessarily reflect the latest version of the software and hardware available at the time of the training. The Operator's Manual shall be used as your main reference, in particular for relevant safety information like warnings and cautions. Note: Some functions shown in this material are optional and might not be part of your system. Certain products, product related claims or functionalities (hereinafter collectively “Functionality”) may not (yet) be commercially available in your country. Due to regulatory requirements, the future availability of said Functionalities in any specific country is not guaranteed. Please contact your local Siemens Healthineers sales representative for the most current information. The reproduction, transmission or distribution of this training or its contents is not permitted without express written authority. Offenders will be liable for damages. 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