Determination of gradient magnetic field-induced acoustic noise associated with the use of echo planar and three-dimensional,fast spin echo techniques

The purpose of this study was to assess gradient magnetic-field-induced acoustic noise levels associated with the use of echo planar imaging (EPI) and three-dimensional fast spin echo (3D-FSE) pulse sequences. Acoustic noise measurements were obtained from two different high field-strength MR systems (1.5 T, Siemens and General Electric Co.) under ambient noise conditions and the use of EPI and 3D-FSE pulse sequences. Parameters were selected to produce “worst case” acoustic noise levels. Acoustic noise recordings were made at the entrance, the center, and at the exit of the magnet bores with a specially designed microphone that was unperturbed by electromagnetic fields. The highest ambient noise levels (A-weighted scale) were 67 dB (Siemens: the same values were recorded at the center and at the exit) and 78 dB (General Electric Co.; recorded at the exit). The highest acoustic noise levels recorded during activation of the gradient magnetic fields were 114 dB (Siemens) and 115 dB (General Electric Co.) and those occurred at the centers of the MR systems with the use of the EPI technique. Gradient magnetic fields associated with the use of EPI and 3D-FSE techniques produced acoustic noise levels that were within permissible levels recommended by federal guidelines.     

Interventional MR imaging at 1.5 T: quantification of sound exposure

Sound pressure levels (SPLs) during interventional magnetic resonance (MR) imaging may create an occupational hazard for the interventional radiologist (ie, the potential risk of hearing impairment). Therefore, A-weighted and linear continuous-equivalent SPLs were measured at the entrance of a 1.5-T MR imager during cardiovascular and real-time pulse sequences. The SPLs ranged from 81.5 to 99.3 dB (A-weighted scale), and frequencies were from 1 to 3 kHz. SPLs for the interventional radiologist exceeded a safe SPL of 80 dB (A-weighted scale) for all sequences; therefore, hearing protection is recommended.     

Efficacy of passive acoustic screening: implications for the design of imager and MR-suite

PURPOSE: To investigate the efficacy of passive acoustic screening in the magnetic resonance (MR) environment by reducing direct and indirect MR-related acoustic noise, both from the patient's and health worker's perspective. MATERIALS AND METHODS: Direct acoustic noise refers to sound originating from the inner and outer shrouds of the MR imager, and indirect noise to acoustic reflections from the walls of the MR suite. Sound measurements were obtained inside the magnet bore (patient position) and at the entrance of the MR imager (health worker position). Inner and outer shrouds and walls were lined with thick layers of sound insulation to eliminate the direct and indirect acoustic pathways. Sound pressure levels (SPLs) and octave band frequencies were acquired during various MR imaging sequences at 1.5 T. RESULTS: Inside the magnet bore, direct acoustic noise radiating from the inner shroud was most relevant, with substantial reductions of up to 18.8 dB when using passive screening of the magnetic bore. At the magnet bore entrance, blocking acoustic noise from the outer shroud and reflections showed significant reductions of 4.5 and 2.8 dB, respectively, and 9.4 dB when simultaneously applied. Inner shroud coverage contributed minimally to the overall SPL reduction. CONCLUSION: Maximum noise reduction by passive acoustic screening can be achieved by reducing direct sound conduction through the inner and outer shrouds. Additional measures to optimize the acoustic properties of the MR suite have only little effect.     

Anatomic MR images obtained with silent sequences

The authors evaluated silent magnetic resonance (MR) imaging sequences for their suitability in providing high-spatial-resolution anatomic images that are of sufficient quality to be useful in a clinical setting. The authors compared the images obtained with a silent rapid acquisition with relaxation enhancement (RARE) sequence to its standard counterpart with respect to signal-to-noise ratio, distribution of gray level, and spatial resolution. No real differences were observed between the standard and the silent RARE MR images. Anatomic images were also acquired with a silent spin-echo sequence. Acoustic noise levels with the silent sequences were at least 22 dB (A-weighted scale) lower than those with standard sequences, without loss of image quality.     

Verbal communication in MR environments: effect of MR system acoustic noise on speech understanding

PURPOSE: To assess the masking effect of magnetic resonance (MR)-related acoustic noise and the effect of passive hearing protection on speech understanding. MATERIALS AND METHODS: Acoustic recordings were made at 1.5 T at patient and operator (interventionalist in the MR suite) locations for relevant pulse sequences. In an audiologic laboratory, speech-to-noise ratios (STNRs) were determined, defined as the difference between the absolute sound pressure levels of MR noise and speech. The recorded noise of the MR sequences was played simultaneously with the recorded sentences at various intensities, and 15 healthy volunteers (seven women, eight men; median age, 27 years) repeated these sentences as accurately as possible. The STNR that corresponded with a 50% correct repetition was used as the measure for speech intelligibility. In addition, the effect of passive hearing protection on speech intelligibility was tested by using an earplug model. RESULTS: Overall, speech understanding was reduced more at operator than at patient location. Most problematic were fast gradient-recalled-echo train and spiral k-space sequences. As the absolute sound pressure level of these sequences was approximately 100 dB at patient location, the vocal effort needed to attain 50% intelligibility was shouting (>77 dB). At operator location, less effort was required because of the lower sound pressure levels of the MR noise. Fast spoiled gradient-recalled-echo and echo-planar imaging sequences showed relatively favorable results with raised voice at operator location and loud speaking at patient location. The use of hearing protection slightly improved STNR. CONCLUSION: At 1.5 T, the level of MR noise requires that large vocal effort is used, at the operator and especially at the patient location. Depending on the specific MR sequence used, loud speaking or shouting is needed to achieve adequate bidirectional communication with the patient. The wearing of earplugs improves speech intelligibility.     

Acoustic noise reduction in MRI using Silent Scan: an initial experience

PURPOSE

Acoustic noise during magnetic resonance imaging (MRI) is the main source for patient discomfort and leads to verbal communication problems, difficulties in sedation, and hearing impairment. Silent Scan technology uses less changes in gradient excitation levels, which is directly related to noise levels. Here, we report our preliminary experience with this technique in neuroimaging with regard to subjective and objective noise levels and image quality.

MATERIALS AND METHODS

Ten patients underwent routine brain MRI with 3 Tesla MR750w system and 12-channel head coil. T1-weighted gradient echo (BRAVO) and Silenz pulse sequence (TE=0, 3D radial center-out k-space filling and data sampling with relatively small gradient steps) were performed. Patients rated subjective sound impression for both sequences on a 6-point scale. Objective sound level measurements were performed with a dedicated device in gantry at different operation modes. Image quality was subjectively assessed in consensus by two radiologists on a 3-point scale.

RESULTS

Readers rated image quality as fully diagnostic in all patients. Measured mean noise was reduced significantly with Silenz sequence (68.8 dB vs. 104.65 dB with BRAVO, P = 0.024) corresponding to 34.3% reduction in sound intensity and 99.97% reduction in sound pressure. No significant difference was observed between Silenz sound levels and ambient sounds (i.e., background noise in the scanner room, 68.8 dB vs. 68.73 dB, P = 0.5). The patients’ subjective sound level score was lower for Silenz compared with conventional sequence (1.1 vs. 2.3, P = 0.003).

CONCLUSION

T1-weighted Silent Scan is a promising technique for acoustic noise reduction and improved patient comfort.Acoustic noise during magnetic resonance imaging (MRI) is one of the main sources of patient complaints and discomfort. Evidence from the scientific literature suggests that neonates may have an increased response to acoustic noise, elderly and pediatric patients may be confused due to MRI acoustic noise, sedated patients may experience additional discomfort due to high noise levels, and certain drugs may increase hearing sensitivity (1). Acoustic noise produced by the magnetic resonance (MR) scanner itself may have an influence on speech understanding during the exam, making communication with the patient difficult (2). Studies show that the use of headphones in patients under general anesthesia during MRI reduces spontaneous arm and leg movements significantly (3). Temporary shifts in hearing thresholds were reported in patients scanned without ear protection (4). Thus, reduction of the acoustic noise can increase patient comfort during MRI, and acceptance of the procedure may be increased. As involuntary patient movements and temporary hearing problems would be reduced, image quality is expected to improve when MR acoustic noise is reduced. Recently a new technology called Silent Scan, using a new prototype Silenz pulse sequence, has been introduced for brain imaging. Our aim is to report our initial experiences with this technology, focusing on acoustic noise levels and image quality.     

MRI acoustic noise: Sound pressure and frequency analysis

The large gradient colls used in MRI generate, simultaneously with the pulsed radiofrequency (RF) wave, acoustic noise of high intensity that has raised concern regarding hearing safety. The sound pressure levels (SPLs) and power spectra of MRI acoustic noise were measured at the position of the human head in the isocenter of five MRI systems and with 10 different pulse sequences used in clinical MR scanning. Each protocol, including magnetization-prepared rapid gradient echo (MP-RAGE; 113 dB SPL linear), fast gradient echo turbo (114 dB SPL linear), and spin echo Tl/2 mm (117 dB SPL linear), was found to have the high SPLs, rapid pulse rates, amplitude-modulated pulse envelopes, and multi-peaked spectra. Slice thickness and SPL were inversely related, and Tl-weighted images generated more intense acoustic noise than the proton-dense T2-weighted measures. The unflltered linear peak values provided more accurate measurements of the SPL and spectral content of the MRI acoustic noise than the commonly used dB A-weighted scale, which filters out the predominant low frequency components. Fourier analysis revealed predominantly low frequency energy peaks ranging from .05 to approximately.     

Potential hearing loss resulting from MR imaging

To determine if the loud noise generated by magnetic resonance (MR) imaging equipment is capable of inducing hearing loss, the hearing of 24 patients was tested before and after MR imaging. Fourteen patients were imaged without ear protection, and six (43%) suffered a temporary, mild loss of hearing (less than or equal to 15 dB at at least one frequency). Ten patients were imaged with ear protection, and only one experienced any hearing loss. Therefore, the noise generated by MR imagers may cause temporary hearing loss, and earplugs can prevent this loss. All threshold changes had returned to within 10 dB of baseline by 15 minutes after completion of the second audiometric test.     

Noise correction for the exact determination of apparent diffusion coefficients at low SNR.

Noise in MR image data increases the mean signal intensity of image regions due to the usually performed magnitude reconstruction. Diffusion-weighted imaging (DWI) is especially affected by high noise levels for several reasons, and a decreasing SNR at increasing diffusion weighting causes systematic errors when calculating apparent diffusion coefficients (ADCs). Two different methods are presented to correct biased signal intensities due to the presence of complex noise: 1) with Gaussian intensity distribution, and 2) with arbitrary intensity distribution. The performance of the correction schemes is demonstrated by numerical simulations and DWI measurements on two different MR systems with different noise characteristics. These experiments show that noise significantly influences the determination of ADCs. Applying the proposed correction schemes reduced the bias of the determined ADC to less than 10% of the bias without correction. Magn Reson Med 45:448-453, 2001.     

Investigation of acoustic noise on 15 MRI scanners from 0.2 T to 3 T

Acoustic noise levels for fast MRI pulse sequences were surveyed on 14 systems with field strengths ranging from 0.2 T to 3 T. A microphone insensitive to the magnetic environment was placed close to the magnet isocenter and connected via an extension cable to a sound level meter outside the scan room. Measured noise levels varied from 82.5 +/- 0.1 dB(A) for a 0.23 T system to 118.4 +/- 1.3 dB(A) for a 3 T system. Further measurements on four of the closed-bore systems surveyed showed that: 1) pulse sequence parameters (particularly FOV and TR) were more influential in determining noise level than field strength, 2) the noise level was found to vary along the z-direction with a maximum near the bore entrance, and 3) in one of two systems tested there was a significant increase in noise with a volunteer present instead of a test object. The results underline the importance of hearing protection for patients and for staff spending extended periods in the scan room.     

Quiet PROPELLER MRI Techniques Match the Quality of Conventional PROPELLER Brain Imaging Techniques

BACKGROUND AND PURPOSE:Switching of magnetic field gradients is the primary source of acoustic noise in MR imaging. Sound pressure levels can run as high as 120 dB, capable of producing physical discomfort and at least temporary hearing loss, mandating hearing protection. New technology has made quieter techniques feasible, which range from as low as 80 dB to nearly silent. The purpose of this study was to evaluate the image quality of new commercially available quiet T2 and quiet FLAIR fast spin-echo PROPELLER acquisitions in comparison with equivalent conventional PROPELLER techniques in current day-to-day practice in imaging of the brain.MATERIALS AND METHODS:Thirty-four consecutive patients were prospectively scanned with quiet T2 and quiet T2 FLAIR PROPELLER, in addition to spatial resolution–matched conventional T2 and T2 FLAIR PROPELLER imaging sequences on a clinical 1.5T MR imaging scanner. Measurement of sound pressure levels and qualitative evaluation of relative image quality was performed.RESULTS:Quiet T2 and quiet T2 FLAIR were comparable in image quality with conventional acquisitions, with sound levels of approximately 75 dB, a reduction in average sound pressure levels of up to 28.5 dB, with no significant trade-offs aside from longer scan times.CONCLUSIONS:Quiet FSE provides equivalent image quality at comfortable sound pressure levels at the cost of slightly longer scan times. The significant reduction in potentially injurious noise is particularly important in vulnerable populations such as children, the elderly, and the debilitated. Quiet techniques should be considered in these special situations for routine use in clinical practice.

Acoustic noise generated during MR imaging contributes to patient discomfort. Problems associated with high levels of acoustic noise include annoyance, anxiety, and verbal communication difficulties between the patient and operator.^1,2 In addition, the very high noise pressure levels can cause hearing loss. Temporary shifts in hearing thresholds have been reported in 43% of the patients scanned without ear protection and with improperly fitted earplugs.³ In extreme cases, permanent hearing impairment can occur.^3–5 Noise is of particular concern in populations vulnerable to hearing loss such as the very young and elderly and those who may not be able to manage the effectiveness of earplug placement such as patients with psychiatric disorders or reduced levels of consciousness.⁵ Fetal noise exposure is also a concern.⁶The primary source of acoustic noise in MR imaging procedures is the pulsed currents generated in gradient coils for spatial encoding of the MR signal.⁷ These currents, in the presence of the strong static magnetic field of the MR imaging system, induce significant (Lorentz) forces that cause vibrations in the gradient coils, which, in turn, generate a compression wave in the air perceived as the scanner noise.^8–10 Previous methods used to ameliorate the high acoustic noise levels of clinical MR imaging include acoustic insulation of the scanner bore, resulting in reduced bore diameter and gradient waveform shaping/filtering^11,12; bandwidth limiting¹³; and restricting gradient performance—each trading image quality and acquisition speed for only modest noise reduction. More recent studies have demonstrated that innovative pulse-sequence modifications can be applied to achieve substantial reductions of acoustic noise while maintaining image quality. Novel, almost silent sequences have recently become available.^14–17 Before these new techniques can be widely adopted, validation against traditional techniques must be performed.In this study, we evaluated the image quality of new commercially available quiet T2 PROPELLER (Q-T2) and quiet T2 FLAIR PROPELLER (Q-FLAIR) sequences in comparison with our standard of care conventional T2 PROPELLER (C-T2) and T2 FLAIR PROPELLER (C-FLAIR) techniques. To our knowledge, this is the first study to assess the performance of quiet T2 and quiet T2 FLAIR MR imaging applications in day-to-day clinical practice.     

Sound-level measurements and calculations of safe noise dosage during EPI at 3 T

This paper describes systematic methods for measuring and controlling sound levels within a magnetic resonance scanner. The methods are illustrated by application to the acoustic noise generated by a 3 T scanner during echoplanar imaging (EPI). Across five measurement sessions, sound pressure levels at the center of the head gradient coil ranged from 122 to 131 dB SPL [123 to 132 dB(A)]. For protection against damaging noise exposure, UK and US industrial guidelines stipulate that the maximum permitted daily noise dosage is equivalent to 90 dB(A) for 8 hours, where noise dosage is a function of the level of an acoustic signal and the length of exposure to it. Without hearing protection, this equivalent level would be exceeded by less than 5 seconds of exposure to the measured levels of scanner acoustic noise. These findings highlight the importance of noise reduction and hearing protection for those exposed to the acoustic noise generated during EPI.     

Highly accelerated cardiovascular MR imaging using many channel technology: concepts and clinical applications

Cardiovascular magnetic resonance imaging (CVMRI) is of proven clinical value in the non-invasive imaging of cardiovascular diseases. CVMRI requires rapid image acquisition, but acquisition speed is fundamentally limited in conventional MRI. Parallel imaging provides a means for increasing acquisition speed and efficiency. However, signal-to-noise (SNR) limitations and the limited number of receiver channels available on most MR systems have in the past imposed practical constraints, which dictated the use of moderate accelerations in CVMRI. High levels of acceleration, which were unattainable previously, have become possible with many-receiver MR systems and many-element, cardiac-optimized RF-coil arrays. The resulting imaging speed improvements can be exploited in a number of ways, ranging from enhancement of spatial and temporal resolution to efficient whole heart coverage to streamlining of CVMRI work flow. In this review, examples of these strategies are provided, following an outline of the fundamentals of the highly accelerated imaging approaches employed in CVMRI. Topics discussed include basic principles of parallel imaging; key requirements for MR systems and RF-coil design; practical considerations of SNR management, supported by multi-dimensional accelerations, 3D noise averaging and high field imaging; highly accelerated clinical state-of-the art cardiovascular imaging applications spanning the range from SNR-rich to SNR-limited; and current trends and future directions.     

Comparison of accuracy and interreader agreement in side-by-side versus independent evaluations of MR imaging of the medial collateral ligament of the elbow

RATIONALE AND OBJECTIVES. The authors compared independent and side-by-side evaluation of magnetic resonance (MR) images of the medial collateral ligament (MCL) of the elbow, with regard to sensitivity, specificity, and interreader agreement MATERIALS AND METHODS. Six MR imaging sequences were used to image the MCLs in 28 cadaveric specimens, eight with surgically created lesions. Two reading methods were used. For independent evaluation, the images were first evaluated independently and rated on a five-point scale by two musculoskeletal radiologists experienced in interpreting MR images and blinded to the MCL integrity. The images were then reevaluated on the same scale by both readers after at least 2 weeks, with images from all six sequences shown side by side. For each MR sequence and reading method, the sensitivity and specificity were estimated nonparametrically, and differences were tested with the McNemar test. Interreader agreement was assessed with a K statistic, and differences were tested with Z and chi2 tests after adjustment for the dependence structure between correlated K statistics. RESULTS: For all sequences, side-by-side evaluation generally yielded higher specificity than independent evaluation, as well as better agreement between readers. CONCLUSION: Observer performance is superior when multiple MR imaging pulse sequences are reviewed simultaneously rather than independently and separately. Side-by-side review of different MR pulse sequences enabled higher accuracy and lower interreader variability for evaluation of the elbow MCL. These findings have implications for the design of studies to optimize MR imaging protocols by using multiple pulse sequences and multiple readers.     

Assessment of motion gating strategies for mouse magnetic resonance at high magnetic fields

PURPOSE: To assess the performance of motion gating strategies for mouse cardiac magnetic resonance (MR) at high magnetic fields by quantifying the levels of motion artifact observed in images and spectra in vivo. MATERIALS AND METHODS: MR imaging (MRI) of the heart, diaphragm, and liver; MR angiography of the aortic arch; and slice-selective 1H-spectroscopy of the heart were performed on anesthetized C57Bl/6 mice at 11.75 T. Gating signals were derived using a custom-built physiological motion gating device, and the gating strategies considered were no gating, cardiac gating, conventional gating (i.e., blanking during respiration), automatic gating, and user-defined gating. Both automatic and user-defined modes used cardiac and respiratory gating with steady-state maintenance during respiration. Gating performance was assessed by quantifying the levels of motion artifact observed in images and the degree of amplitude and phase stability in spectra. RESULTS: User-defined gating with steady-state maintenance during respiration gave the best performance for mouse cardiac imaging, angiography, and spectroscopy, with a threefold increase in signal intensity and a sixfold reduction in noise intensity compared to cardiac gating only. CONCLUSION: Physiological gating with steady-state maintenance during respiration is essential for mouse cardiac MR at high magnetic fields.     

Partial flip angle MR imaging

Theoretical analysis predicts that performing magnetic resonance (MR) imaging with partial (less than 90 degrees) flip angles can reduce imaging times two- to fourfold when lesions with elevated T1 values are being examined. This time savings occurs because repetition time (TR) is reduced when imaging is performed with partial flips. Partial flip MR imaging can also improve signal-to-noise ratio (S/N) in fast body imaging. For this study, analytical tools were used to predict image contrast and S/N for short TR, partial flip sequences. Experimental implementation of the short TR, partial flip sequences that analytical work had predicted would be optimal supported the analytical predictions and demonstrated their validity. Partial flip MR imaging is applicable to reducing imaging time only when the ratio of signal differences to noise exceeds threshold values in conventional MR images. Partial flip sequences can be used to advantage in MR imaging of both the head and the body, and the observed effects are predictable through theoretical analysis.     

A comparative study of MR imaging profile of titanium pedicle screws

Objective: We compared the MR imaging profile of three different types of titanium pedicle screw implants in common usage in a human cadaveric model. We additionally compared the change in temperature during imaging among three constructs.Material and Methods: Titanium-based lumbar pedicle screw/rod constructs from three manufacturers were implanted sequentially in a human cadaveric spine. MR imaging was then performed using both conventional spin-echo sequences and advanced imaging pulse sequences. Changes in tissue temperature were also measured during imaging to assess differences among the various implants. MR images were compared in a blinded fashion by two neuroradiologists.Results: No significant differences in imaging profile were noted between the three types of titanium implants with regards to their MR artifact profile. Fast spin-echo sequences led to a decrease in perceptible MR artifacts. Moreover, there were no significant differences in temperature increase among the three manufacturers (mean increase 0.5°C) during imaging.Conclusion: Slight differences in the percentage of titanium among the three pedicle screw systems does not appear to result in artifact differences during MR imaging. Therefore, with regard to imaging profile considerations, the three systems studied should be considered interchangeable.     

Magnetic resonance noise measurements and signal‐quantization effects at very low noise levels

The well‐known noise distributions of magnetic resonance imaging (MRI) data (Rayleigh, Rician, or non‐central chi‐distribution) describe the probability density of real‐valued (i.e., floating‐point) signal intensities. MR image data, however, is typically quantized to integers before visualization or archiving. Depending on the scaling factors applied before the quantization and the signal‐to‐noise ratio (SNR), very low noise levels with substantial artifacts due to the quantization process can occur. The purpose of this study was to analyze the consequences of the signal quantization, to determine the theoretical absolute lower limit for noise measurements in discrete data, and to evaluate an improved method for noise and SNR measurements in the presence of very low noise levels. Image data were simulated with original noise levels of between 0.02 and 2.00. Noise measurements were performed based on the properties of background and foreground data using the conventional approach, which exploits the standard deviation or mean value of the signal, and a maximum‐likelihood approach based on the relative frequencies of the observed discrete signal intensities. Substantial deviations were found for the conventionally determined noise levels, while noise levels comparable to or lower than the quantization error can be accurately estimated with the proposed maximum‐likelihood approach. Magn Reson Med 60:1477–1487, 2008. © 2008 Wiley‐Liss, Inc.     

MR measurement of pulsatile pressure gradients

A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01–0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.