Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI) refers back to the measure of radiofrequency (RF) power absorbed by the physique throughout the scan. It’s expressed in watts per kilogram (W/kg). Extreme RF power absorption can result in tissue heating, doubtlessly inflicting hurt to the affected person. Decreasing this absorbed power is essential for affected person security and regulatory compliance. For instance, modifying pulse sequence parameters or using parallel transmission strategies are strategies employed to realize this discount.
Minimizing the potential for thermal harm is paramount in MRI procedures. Decrease SAR values translate on to a safer examination for the affected person, notably weak populations similar to pregnant girls and youngsters. Traditionally, limitations on sequence parameters have been the first technique of management; nonetheless, advances in coil know-how and pulse sequence design supply extra subtle approaches. Efficient management enhances the affected person expertise, reduces the danger of antagonistic occasions, and ensures adherence to established security tips set by regulatory our bodies.
A number of elements affect SAR, and numerous methods could be carried out to mitigate its ranges. These embody optimizing pulse sequence parameters, choosing applicable RF coils, using parallel imaging strategies, and contemplating changes to affected person positioning. Understanding these numerous aspects is important for sustaining affected person security whereas maximizing picture high quality and scan effectivity. The following sections will delve into these points, offering an in depth overview of approaches to scale back RF power deposition throughout MRI examinations.
1. Pulse Sequence Optimization
Pulse sequence optimization represents a essential element in mitigating Particular Absorption Price (SAR) throughout Magnetic Resonance Imaging (MRI). Choosing and configuring pulse sequences judiciously considerably impacts the quantity of radiofrequency (RF) power deposited into the affected person’s tissues. Understanding the interaction between sequence parameters and SAR is important for sustaining affected person security with out compromising diagnostic picture high quality.
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Quick Spin Echo (FSE) / Turbo Spin Echo (TSE) Parameter Changes
The echo prepare size (ETL) in FSE/TSE sequences instantly influences SAR. Longer ETLs result in elevated RF power deposition. Decreasing the ETL, whereas doubtlessly rising scan time or reducing signal-to-noise ratio (SNR), could be a precious technique for SAR discount. As an example, in a backbone imaging protocol, reducing the ETL from 16 to eight may considerably decrease SAR, notably in sufferers with metallic implants. Cautious consideration of the trade-off between ETL, scan time, and SNR is paramount.
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Repetition Time (TR) Optimization
Growing the repetition time (TR) typically decreases SAR. Whereas an extended TR reduces the variety of RF pulses utilized per unit time, it might additionally prolong the general scan period. For instance, in cardiac imaging, rising the TR from 500ms to 700ms could yield a noticeable SAR discount. Nevertheless, the potential affect on picture distinction and scan effectivity should be completely evaluated. Consideration of the precise scientific indication informs the optimum TR choice.
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RF Pulse Form Modulation
The form of the RF pulses utilized in a pulse sequence considerably impacts the bandwidth of power deposited. Using pulses with decrease peak amplitudes and broader period can obtain the identical excitation profile with much less SAR. As an example, using a sinc pulse with applicable windowing reduces the utmost RF energy in comparison with an oblong pulse. This technique requires cautious design to keep away from compromising the slice profile and picture high quality. Specialised software program instruments are sometimes employed to design and consider RF pulse shapes.
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Gradient Optimization
Whereas gradients don’t instantly contribute to SAR via RF power deposition, their interplay with the RF pulses can not directly affect SAR. Optimizing gradient waveforms can decrease eddy currents and ringing artifacts, doubtlessly permitting for shorter inter-pulse delays. Shorter delays can lead to a extra environment friendly sequence with decrease whole SAR. Cautious consideration of gradient rise occasions and amplitudes is important for environment friendly and SAR-conscious pulse sequence design.
In abstract, optimizing pulse sequences for SAR discount requires a radical understanding of the connection between sequence parameters, RF power deposition, and picture high quality. Even handed changes to ETL, TR, RF pulse shapes, and gradient waveforms, contemplating the scientific indication and affected person traits, can considerably cut back SAR whereas sustaining diagnostic efficacy. A multi-faceted strategy, incorporating these methods, is important for selling affected person security in MRI.
2. RF Coil Choice
Radiofrequency (RF) coil choice performs a pivotal position in Particular Absorption Price (SAR) administration inside Magnetic Resonance Imaging (MRI). The selection of coil instantly influences the effectivity of RF power supply and, consequently, the quantity of power absorbed by the affected person. Choosing applicable coils, alongside optimized sequence parameters, is paramount in mitigating potential thermal dangers.
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Coil Sort and Geometry
The kind and geometry of the RF coil considerably affect SAR. Physique coils, designed for whole-body imaging, usually exhibit larger SAR in comparison with localized floor coils. This distinction arises as a result of physique coils generate a extra homogeneous RF subject encompassing a bigger quantity of tissue. Conversely, floor coils, positioned nearer to the area of curiosity, focus the RF subject, doubtlessly lowering the general SAR. As an example, using a devoted knee coil as an alternative of a physique coil for knee imaging can considerably lower whole-body SAR publicity. Coil geometry should be rigorously thought-about along side the precise anatomical area being imaged.
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Parallel Imaging Capabilities
RF coils geared up with parallel imaging capabilities supply a way to scale back SAR not directly. Parallel imaging strategies leverage a number of coil components to amass knowledge sooner, thereby lowering the scan time and, consequently, the overall RF power delivered to the affected person. The acceleration issue achievable with parallel imaging relies on the variety of coil components and the coil geometry. Increased acceleration elements translate to shorter scan occasions and decrease SAR. Implementing parallel imaging methods with appropriate coils is an efficient strategy to minimizing RF power deposition.
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Transmit Effectivity
The transmit effectivity of an RF coil refers to its skill to ship RF power successfully to the goal tissue. Coils with larger transmit effectivity require much less energy to realize the specified excitation, leading to decrease SAR values. Components influencing transmit effectivity embody coil design, impedance matching, and loading results. Superior coil designs, incorporating options similar to energetic detuning and optimized ingredient placement, can improve transmit effectivity. Choosing coils with superior transmit traits contributes to lowered RF power absorption.
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Coil Cooling Programs
Whereas in a roundabout way influencing the preliminary RF power deposition, coil cooling techniques play a job in managing the warmth generated throughout extended MRI scans. Environment friendly cooling techniques assist dissipate warmth from the coil components, stopping extreme temperature will increase that would not directly have an effect on SAR measurements or affected person consolation. Moreover, secure coil temperatures contribute to extra constant picture high quality. Integrating coils with efficient cooling mechanisms gives a further layer of security and efficiency throughout MRI examinations.
In abstract, RF coil choice is an integral side of SAR administration in MRI. The suitable selection of coil, contemplating its kind, geometry, parallel imaging capabilities, transmit effectivity, and cooling system, considerably impacts the quantity of RF power absorbed by the affected person. A complete strategy, integrating coil choice with pulse sequence optimization and different SAR discount methods, is important for making certain affected person security and maximizing diagnostic worth.
3. Parallel Imaging
Parallel imaging is a key method utilized to scale back Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI). The basic precept behind parallel imaging entails the simultaneous acquisition of knowledge from a number of receiver coils. This multi-channel acquisition permits for a discount within the variety of phase-encoding steps required to reconstruct a picture, instantly resulting in a lower in scan time. Since SAR is instantly proportional to the period of radiofrequency (RF) power deposition, shortening the scan time via parallel imaging inherently lowers the general SAR skilled by the affected person. As an example, a routine mind MRI sequence may take 5 minutes with out parallel imaging. Implementation of parallel imaging with a discount issue of two might cut back the scan time to 2.5 minutes, successfully halving the RF power deposited and, consequently, reducing SAR.
The effectiveness of parallel imaging in lowering SAR relies on a number of elements, together with the coil geometry, the variety of coil components, and the reconstruction algorithm employed. Increased acceleration elements, achieved with superior coil arrays, allow larger reductions in scan time and SAR. Nevertheless, rising the acceleration issue may introduce artifacts and cut back signal-to-noise ratio (SNR). Subsequently, cautious optimization of parallel imaging parameters is important to steadiness SAR discount with picture high quality. Scientific protocols should be tailored to particular affected person wants and scanner capabilities to maximise the advantages of parallel imaging. For instance, pediatric imaging typically prioritizes SAR discount, necessitating the usage of larger acceleration elements, even on the expense of barely lowered picture high quality, which could be compensated for via different sequence changes.
In conclusion, parallel imaging presents a precious device for mitigating SAR in MRI. By lowering scan time, this system instantly lowers the quantity of RF power absorbed by the affected person. Whereas cautious consideration should be given to potential trade-offs between SAR discount and picture high quality, parallel imaging stays an integral part of a complete SAR administration technique. Challenges stay in optimizing parallel imaging for particular functions and affected person populations, however ongoing developments in coil know-how and reconstruction algorithms proceed to boost its efficacy in selling affected person security in MRI environments.
4. Obligation Cycle Discount
Obligation cycle discount instantly influences Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI). The obligation cycle represents the fraction of time throughout which radiofrequency (RF) power is actively transmitted. Consequently, a lower within the obligation cycle interprets to a proportional discount within the RF power deposited into the affected person’s tissues, thereby decreasing SAR. For instance, if a pulse sequence has an obligation cycle of fifty%, halving that obligation cycle to 25% would theoretically halve the SAR, assuming all different parameters stay fixed. This direct relationship underscores the significance of obligation cycle optimization as a core element of minimizing SAR publicity.
Methods to scale back the obligation cycle typically contain modifying pulse sequence parameters to reduce the period of time the RF transmitter is actively engaged. This may be achieved via strategies similar to shortening RF pulse durations, rising inter-pulse delays, or optimizing gradient waveforms to reduce the period of every echo prepare. The effectiveness of those strategies hinges on sustaining acceptable picture high quality; due to this fact, cautious consideration should be given to potential trade-offs between SAR discount and diagnostic efficiency. In backbone imaging, for example, implementing a sparse sampling method alongside optimized gradient waveforms can considerably cut back the obligation cycle, leading to substantial SAR discount with out considerably impacting picture decision.
Efficient obligation cycle discount necessitates a complete understanding of pulse sequence design and its affect on RF power deposition. Challenges come up in complicated imaging situations the place excessive signal-to-noise ratio or speedy acquisition occasions are required. Balancing these calls for with the necessity for SAR mitigation requires cautious protocol optimization and a radical understanding of the interaction between numerous sequence parameters. Finally, obligation cycle discount represents a essential lever within the broader effort to reduce SAR and guarantee affected person security in MRI, notably in sequences with inherently excessive RF power calls for.
5. Flip Angle Adjustment
Flip angle adjustment represents a big technique for reducing Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI). The flip angle instantly influences the quantity of radiofrequency (RF) power deposited into the affected person. The next flip angle calls for a stronger RF pulse, resulting in larger power absorption and, consequently, elevated SAR. Conversely, lowering the flip angle lowers the power requirement, thereby mitigating SAR. The connection is direct: a linear discount in flip angle corresponds to a quadratic lower in energy deposition. For instance, reducing the flip angle from 90 levels to 60 levels ends in a considerable discount in RF energy, considerably decreasing SAR. The significance of flip angle adjustment is magnified in sequences with inherently excessive RF power calls for, similar to quick spin echo or turbo spin echo sequences.
Sensible utility of flip angle adjustment requires cautious consideration of its affect on picture distinction and signal-to-noise ratio (SNR). Decreasing the flip angle reduces sign depth, which may compromise picture high quality. Subsequently, changes should be made judiciously, balancing SAR discount with diagnostic necessities. Methods like variable flip angle schemes or pushed equilibrium sequences can partially compensate for the sign loss related to lowered flip angles. In scientific observe, for example, when imaging pediatric sufferers or people with metallic implants, protocols typically prioritize SAR discount by using decrease flip angles, coupled with elevated scan averaging or different strategies to take care of acceptable SNR.
Flip angle adjustment stays a cornerstone of SAR administration in MRI. The problem lies in optimizing flip angles to realize a steadiness between affected person security and diagnostic efficacy. Ongoing analysis focuses on creating superior pulse sequence designs that decrease SAR whereas preserving picture high quality. Regardless of the potential trade-offs, a radical understanding of the connection between flip angle, RF power deposition, and picture traits is essential for making certain affected person security and maximizing the diagnostic worth of MRI examinations. This parameter manipulation gives a readily accessible technique of controlling RF power publicity throughout imaging procedures.
6. Affected person Positioning
Affected person positioning considerably influences Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI) via its impact on radiofrequency (RF) subject distribution. The human physique interacts with the RF subject, and its orientation throughout the scanner bore impacts the quantity of power absorbed in numerous tissues. Particular positioning can both focus or disperse the RF power, instantly impacting SAR values. For instance, if a affected person’s limbs are in shut proximity to the scanner partitions, localized SAR could improve in these areas. Subsequently, constant and standardized positioning protocols are important for minimizing peak SAR and making certain affected person security.
Sensible implementation of SAR-conscious affected person positioning entails cautious consideration of anatomical landmarks and coil placement. Centering the area of curiosity throughout the bore, utilizing padding to take care of constant spacing between the affected person and the coil, and avoiding direct contact with the scanner partitions are essential steps. Standardized positioning protocols, built-in into the MRI technologist’s workflow, decrease variability and cut back the potential for inadvertently rising SAR. As an example, throughout cardiac MRI, making certain the affected person is aligned centrally within the bore and using dielectric pads to create uniform spacing can considerably enhance RF subject homogeneity and decrease total SAR.
In abstract, affected person positioning is an integral element of SAR administration in MRI. By optimizing the affected person’s orientation throughout the RF subject, peak SAR values could be minimized, thereby contributing to a safer imaging surroundings. Whereas affected person positioning alone can’t eradicate SAR issues, it serves as an vital adjunct to pulse sequence optimization, coil choice, and different SAR discount methods. Adherence to standardized protocols and cautious consideration to element are important for maximizing the advantages of correct positioning and making certain affected person well-being throughout MRI examinations.
7. Dielectric Padding
Dielectric padding represents a passive but efficient strategy to mitigating Particular Absorption Price (SAR) in Magnetic Resonance Imaging (MRI). It entails the strategic placement of supplies with excessive permittivity between the affected person and the radiofrequency (RF) coil. These supplies alter the RF subject distribution, influencing the quantity of power absorbed by the affected person’s tissues. Correct utility of dielectric padding contributes to a extra homogeneous RF subject, lowering localized areas of excessive SAR.
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Mechanism of Motion
Dielectric pads operate by rising the native dielectric fixed, resulting in a redistribution of the electrical subject. The upper permittivity materials acts as a “lens,” bending the electrical subject strains and reducing their focus in areas instantly adjoining to the RF coil. This impact ends in a extra uniform RF subject and a discount in peak SAR values. A typical instance contains utilizing pads stuffed with distilled water or different high-permittivity fluids. The effectiveness of this technique depends upon the fabric’s dielectric properties, its thickness, and its placement relative to the coil and the affected person.
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Influence on RF Area Homogeneity
The first advantage of dielectric padding lies in its skill to enhance RF subject homogeneity. Within the absence of padding, variations in tissue conductivity and geometry can result in areas of concentrated RF power deposition. By introducing a dielectric materials, these inhomogeneities are mitigated, resulting in a extra even distribution of RF power all through the imaging quantity. That is notably vital when imaging areas with complicated anatomy or in sufferers with implanted metallic gadgets, the place RF subject distortions can considerably elevate SAR domestically.
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Scientific Purposes and Concerns
Dielectric padding finds functions throughout a variety of scientific MRI examinations. It’s notably helpful in physique imaging, the place the comparatively massive dimension of the affected person and the complicated tissue interfaces contribute to RF subject inhomogeneity. It’s also typically employed in pediatric imaging, the place heightened sensitivity to RF power deposition necessitates cautious SAR administration. Necessary concerns embody making certain the pad materials is MRI-compatible, non-toxic, and doesn’t introduce artifacts into the photographs. Moreover, the pads needs to be positioned constantly throughout sufferers to take care of predictable SAR discount.
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Limitations and Synergistic Results
Dielectric padding, whereas useful, just isn’t a standalone answer for SAR discount. Its effectiveness is proscribed by the dielectric properties of the obtainable supplies and the geometry of the imaging situation. It’s best when used along side different SAR discount methods, similar to pulse sequence optimization and parallel imaging strategies. When utilized synergistically, these strategies contribute to a complete strategy to SAR administration, enhancing affected person security whereas sustaining diagnostic picture high quality.
In conclusion, dielectric padding serves as a precious device within the arsenal of strategies designed to lower SAR in MRI. Its skill to enhance RF subject homogeneity, when carried out strategically, contributes to a safer imaging surroundings. Whereas dielectric padding’s particular person affect is comparatively modest, it’s best employed along side different methods to realize complete SAR discount.
Steadily Requested Questions
The next questions handle frequent issues and misconceptions concerning managing Particular Absorption Price (SAR) throughout Magnetic Resonance Imaging (MRI) examinations. The intent is to offer clear, factual info to boost understanding of this essential side of affected person security.
Query 1: What constitutes a suitable SAR worth in MRI?
Acceptable SAR ranges are outlined by regulatory our bodies, such because the FDA in america and equal companies globally. These ranges fluctuate relying on the area of the physique being imaged and the mode of operation (e.g., Regular, First Degree Managed). Exceeding these limits poses a threat of tissue heating and potential hurt to the affected person. The MRI system’s software program screens and limits SAR to make sure compliance with these laws.
Query 2: How does pulse sequence choice have an effect on SAR?
Completely different pulse sequences deposit various quantities of radiofrequency (RF) power. Quick Spin Echo (FSE) or Turbo Spin Echo (TSE) sequences, which make use of lengthy echo trains, typically deposit extra RF power than gradient echo sequences. Understanding the RF traits of various sequences is essential for choosing protocols that decrease SAR whereas sustaining diagnostic picture high quality.
Query 3: Can parallel imaging really cut back SAR, or does it merely masks the issue?
Parallel imaging legitimately reduces SAR by shortening scan time. By buying knowledge from a number of receiver coils concurrently, fewer phase-encoding steps are required, resulting in a shorter RF publicity. This can be a direct discount in power deposition, not a masking impact. Nevertheless, the effectiveness depends upon coil design and reconstruction algorithms, and its utility should be rigorously balanced with picture high quality concerns.
Query 4: Is there a common SAR discount technique relevant to all MRI examinations?
No single technique is universally relevant. SAR discount requires a multifaceted strategy tailor-made to the precise affected person, imaging area, scientific indication, and MRI system. A mix of pulse sequence optimization, coil choice, affected person positioning, and different strategies, similar to dielectric padding, is usually essential to successfully handle SAR.
Query 5: Does SAR discount at all times compromise picture high quality?
SAR discount can, in some situations, require trade-offs with picture high quality. Nevertheless, cautious protocol optimization and the usage of superior strategies can decrease this affect. The purpose is to realize a steadiness between affected person security and diagnostic efficacy. In sure conditions, prioritizing affected person security by lowering SAR could necessitate barely lowered picture decision or signal-to-noise ratio, which may generally be compensated for via different strategies.
Query 6: What position does the MRI technologist play in SAR administration?
MRI technologists play an important position in SAR administration. They’re chargeable for choosing applicable protocols, making certain appropriate affected person positioning, and monitoring SAR ranges throughout the scan. They have to even be educated about SAR discount strategies and be capable to adapt protocols as wanted to make sure affected person security whereas sustaining diagnostic picture high quality. Their adherence to established protocols and a focus to element are essential for efficient SAR management.
Efficient SAR administration depends on a complete understanding of RF power deposition and the varied elements that affect it. A proactive strategy, combining data of pulse sequences, coil traits, affected person positioning, and different strategies, is important for making certain affected person security throughout MRI examinations.
The following sections will talk about superior strategies for SAR modeling and simulation, offering additional insights into optimizing MRI protocols for affected person security.
Ideas for Efficient SAR Administration in MRI
Efficient discount of Particular Absorption Price (SAR) throughout Magnetic Resonance Imaging (MRI) requires diligent utility of greatest practices throughout all phases of the imaging course of. Constant adherence to those tips minimizes potential dangers related to RF power deposition, making certain affected person security with out compromising diagnostic picture high quality.
Tip 1: Prioritize Pulse Sequence Choice. The selection of pulse sequence considerably impacts SAR. Every time clinically possible, choose sequences with decrease RF power calls for, similar to gradient echo strategies, over sequences with lengthy echo trains, similar to quick spin echo (FSE). When FSE sequences are essential, optimize the echo prepare size to the minimal acceptable for diagnostic necessities.
Tip 2: Maximize Parallel Imaging Acceleration. Make the most of parallel imaging strategies to scale back scan time. Increased acceleration elements translate on to decrease RF power deposition. Nevertheless, be conscious of the potential affect on signal-to-noise ratio (SNR) and artifact technology, and alter different parameters accordingly to take care of acceptable picture high quality.
Tip 3: Implement Obligation Cycle Optimization Methods. Scale back the share of time the RF transmitter is energetic by shortening RF pulse durations, rising inter-pulse delays, or optimizing gradient waveforms. Nevertheless, the general picture acquisition time needs to be additionally famous for sufferers well being situation.
Tip 4: Calibrate Flip Angle Changes Judiciously. Scale back flip angles the place diagnostically acceptable, understanding that decrease flip angles lower RF power deposition. Variable flip angle schemes or pushed equilibrium sequences can partially compensate for sign loss related to lowered flip angles, sustaining picture distinction.
Tip 5: Reinforce Standardized Affected person Positioning Protocols. Guarantee constant affected person positioning throughout all MRI examinations. Middle the area of curiosity throughout the bore, use padding to take care of constant spacing between the affected person and the coil, and keep away from direct contact with the scanner partitions. Standardized protocols decrease variability and the potential for localized SAR will increase.
Tip 6: Implement Dielectric Padding Strategically. Make the most of dielectric pads with excessive permittivity to redistribute the RF subject, lowering localized areas of excessive SAR. Make sure the pad materials is MRI-compatible, non-toxic, and positioned constantly throughout sufferers to take care of predictable SAR discount. Particularly with the sufferers with identified implants.
Tip 7: Guarantee Common Coaching and Competency Assessments for MRI Technologists. Present ongoing coaching to MRI technologists on SAR administration ideas and greatest practices. Conduct common competency assessments to make sure adherence to protocols and the efficient utility of SAR discount strategies. By conserving everybody up to date with new medical know-how, SAR can lower.
Persistently using these tips contributes considerably to minimizing SAR publicity, enhancing affected person security, and maximizing the advantages of MRI. Proactive consideration to those methods is paramount for fostering a tradition of security in MRI environments.
The following part summarizes the important thing factors mentioned on this article, reinforcing the significance of a multi-faceted strategy to SAR administration.
Conclusion
This text has explored lower SAR of MRI by inspecting numerous methods, starting from pulse sequence optimization and RF coil choice to affected person positioning and the usage of dielectric padding. A recurring theme is the need of a multifaceted strategy, combining strategies to realize important SAR discount with out unduly compromising picture high quality. Every technique presents each advantages and limitations, requiring cautious consideration of trade-offs and the precise scientific context.
The continuing pursuit of strategies for lower SAR of MRI is paramount to affected person security. As MRI know-how evolves, continued analysis and improvement are important to refining current strategies and discovering novel approaches for minimizing RF power deposition. The collective dedication of researchers, producers, and scientific practitioners stays important to making sure that MRI continues to offer invaluable diagnostic info with the bottom attainable threat to sufferers.
