A new technique for MR elastography of the supraspinatus muscle: A gradient-echo type multi-echo sequence

Development of a suitable vibration pad for renal MR elastography

The aim of this study was to develop a vibration pad suitable for renal MR elastography (MRE). Chronic kidney disease (CKD) is a progressive condition affecting >800 million people worldwide. Renal fibrosis is a common pathological feature of CKD that causes fibrotic regions to be much stiffer than those in normal renal tissues. Therefore, MRE can be used to diagnose CKD because it can image organ stiffness. In MRE, the shear modulus is obtained from the wavelength of the shear waves. Therefore, it is highly important to propagate shear waves with sufficient vibration strength in the tissue. By using a three-dimensional (3D) printer, we created a "Flexible Pad" suitable for renal MRE. The Flexible Pad was placed under the back of the participant in the supine position and deformed in response to the participant's weight, adhering closely to the body surface. Six healthy volunteers participated in this study. Our Flexible Pad allowed for coherent shear waves (clear waves with little scattering and interference) to be efficiently transmitted to the kidney deep-lying tissues in the abdomen. The shear moduli of the kidney (n = 6) were 8.95 ± 0.84 kPa in the right kidney and 9.70 ± 0.99 kPa in the left kidney. Our results indicate that using our Flexible Pad for renal MRE can provide a more reliable measurement of renal shear modulus.

Intersession Repeatability of Diffusion‐Tensor Imaging in the Supraspinatus and the Infraspinatus Muscles of Volunteers

BACKGROUND

Quantifying the rotator cuff (RC) muscles' viscoelasticity could provide outcome relevant information in patients with RC tears. MR-elastography requires robust diffusion-tensor imaging (DTI) to account for tissue anisotropy in muscles stiffness computation.

PURPOSE

To assess the repeatability of DTI parameters in the supraspinatus and infraspinatus muscles and to explore DTI tractography conformity with the muscles' anatomy.

STUDY TYPE

Prospective.

SUBJECTS

Six healthy volunteers underwent three consecutive shoulder MRI sessions about 10 minutes apart.

FIELD STRENGTH/SEQUENCE

3T/T1-vibe Dixon and Spin echo EPI DTI (12 gradient encoding directions, b-values 500 and 800 sec/mm² ).

ASSESSMENT

Supraspinatus and infraspinatus muscles were segmented on the T1-vibe Dixon sequence. DTI image quality was assessed using a quantitative threshold based on the signal-to-noise ratio (SNR). The eigenvalues ( λ 1 , λ 2 , λ 3 ), fractional anisotropy (FA) and mean diffusivity were calculated. DTI tractography was visually assessed.

STATISTICAL TESTS

DTI parameters within-subject intersession repeatability was assessed with Bland-Altman analysis and the coefficient of variation (CV). Repeatability was considered good for CV < 10%.

RESULTS

The SNR between diffusion-weighted and non-diffusion-weighted images was greater than 3, which aligns with standards for estimating DTI parameters. The FA showed the lowest mean bias (-0.007; 95% confidence interval [CI] -0.031 to 0.018) whereas the λ₁ had the highest mean bias (0.146 × 10^-3  mm² /sec; CI -0.034 to 0.326 × 10^-3  mm² /sec). CVs of the DTI parameters varied between 3.5% (FA) and 8.4% (λ₃ ) for the supraspinatus and between 3.2% (λ₁ ) and 6.8% (λ₃ ) for the infraspinatus. Tractography provided muscle fiber representations in three-dimensional space concordant with RC anatomy.

DATA CONCLUSION

DTI of the supraspinatus and infraspinatus muscles achieved an adequate SNR, allowing the measurement of the DTI metrics with good repeatability, and thus can be used for optimizing stiffness estimation in these anisotropic tissues.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Fast acquisition of propagating waves in humans with low-field MRI: Toward accessible MR elastography

Most commonly used at clinical magnetic fields (1.5 to 3 T), magnetic resonance elastography (MRE) captures mechanical wave propagation to reconstruct the mechanical properties of soft tissue with MRI. However, in terms of noninvasively assessing disease progression in a broad range of organs (e.g., liver, breast, skeletal muscle, and brain), its accessibility is limited and its robustness is challenged when magnetic susceptibility differences are encountered. Low-field MRE offers an opportunity to overcome these issues, and yet it has never been demonstrated in vivo in humans with magnetic fields <1.5 T mainly because of the long acquisition times required to achieve a sufficient signal-to-noise ratio. Here, we describe a method to accelerate 3D motion-sensitized MR scans at 0.1 T using only 10% k-space sampling combined with a high-performance detector and an efficient encoding acquisition strategy. Its application is demonstrated in vivo in the human forearm for a single motion-encoding direction in less than 1 min.

Quantification of the Elastic Property of Normal Thigh Muscles Using MR Elastography: Our Initial Experience

Purpose

This study aimed to apply MR elastography (MRE) to achieve in vivo evaluation of the elastic properties of thigh muscles and validate the feasibility of quantifying the elasticity of normal thigh muscles using MRE.

Materials and Methods

This prospective study included 10 volunteer subjects [mean age, 32.5 years, (range, 23-45 years)] who reported normal activities of daily living and underwent both T2-weighted axial images and MRE of thigh muscles on the same day. A sequence with a motion-encoding gradient was used in the MRE to map the propagating shear waves in the muscle. Elastic properties were quantified as the shear modulus of the following four thigh muscles at rest; the vastus medialis, vastus lateralis, adductor magnus, and biceps femoris.

Results

The mean shear modulus was 0.98 ± 0.32 kPa and 1.00 ± 0.33 kPa for the vastus medialis, 1.10 ± 0.46 kPa and 1.07 ± 0.43 kPa for the vastus lateralis, 0.91 ± 0.41 kPa and 0.93 ± 0.47 kPa for the adductor magnus, and 0.99 ± 0.37 kPa and 0.94 ± 0.32 kPa for the biceps femoris, with reader 1 and 2, respectively. No significant difference was observed in the shear modulus based on sex (p < 0.05). Aging consistently showed a statistically significant negative correlation (p < 0.05) with the shear modulus of the thigh muscles, except for the vastus medialis (p = 0.194 for reader 1 and p = 0.355 for reader 2).

Conclusion

MRE is a quantitative technique used to measure the elastic properties of individual muscles with excellent inter-observer agreement. Age was consistently significantly negatively correlated with the shear stiffness of muscles, except for the vastus medialis.

A novel measurement to evaluate supraspinatus tendinopathies with strain elastography

BACKGROUND

In addition to the traditional strain ratio (STR), a novel measurement termed hard percent of the whole tendon, provided by the device's software and resembling shear-wave elastography, was included in the study.

PURPOSE

To evaluate the correlation between magnetic resonance imaging (MRI) and strain elastography findings in cases with rotator cuff tendinopathy, including tears.

MATERIAL AND METHODS

As a prospective study, cases suggestive of rotator cuff tendinopathy were included. After MRI evaluation by two observers, grading of the elastography examination was performed by a third radiologist. For the first measurement, the region of interest (ROI) was chosen from the gray-scale evaluation corresponding to the area showing a MRI pathological signal. Stiffness was assessed with the STR. For the second measurement, the whole visible tendon was measured as the ROI. The hard percent (unaffected areas as a percent) was measured. Correlation coefficients between MRI grade and the two strain value measurements were calculated.

RESULTS

There were 93 cases (31 men, 62 women) with 112 involved tendons. The correlation coefficient between MRI grade and age was calculated as 0.56, between MRI grade and STR it was -0.51, and with the novel measurement the correlation coefficient was -0.53.

CONCLUSION

This study showed a good association of findings of tendon stiffness with those of tendinopathy on MRI. Using the hard ratio as a novel measurement, its correlation with MRI grade was as reliable as the STR. We also experienced that the benefit of elastography is a challenging issue for defining small ruptures.

Biomedical Applications of Electromagnetic Detection: A Brief Review

This paper presents a review on the biomedical applications of electromagnetic detection in recent years. First of all, the thermal, non-thermal, and cumulative thermal effects of electromagnetic field on organism and their biological mechanisms are introduced. According to the electromagnetic biological theory, the main parameters affecting electromagnetic biological effects are frequency and intensity. This review subsequently makes a brief review about the related biomedical application of electromagnetic detection and biosensors using frequency as a clue, such as health monitoring, food preservation, and disease treatment. In addition, electromagnetic detection in combination with machine learning (ML) technology has been used in clinical diagnosis because of its powerful feature extraction capabilities. Therefore, the relevant research involving the application of ML technology to electromagnetic medical images are summarized. Finally, the future development to electromagnetic detection for biomedical applications are presented.

Stiffness change of the supraspinatus muscle can be detected by magnetic resonance elastography

Magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE) are imaging techniques to measure stiffness of the soft tissue using magnetic resonance imaging (MRI) and ultrasound images, respectively. The purpose of this study was to explore the feasibility of the MRE measurement to evaluate the change in supraspinatus (SSP) muscle stiffness before and after rotator cuff tear, and to compare the result with those of SWE. Six swine shoulders were used. The skin and subcutaneous fat were removed, and the stiffness value of the SSP muscle was measured by MRE and SWE. The MRE measurement was performed with 0.3 T open MRI and the vibration from a pneumatic driver system with active driver to a passive driver to create the shear wave in the tissue. The passive driver was placed on the center of the SSP muscle. The stiffness was estimated from the wave images using local frequency estimation methods. In the SWE measurement, the probe of the ultrasound was placed on the center of the SSP muscle. The shear wave propagation speed was measured at a depth of 1 cm from the surface, and the stiffness was calculated. After those measurements, the rotator cuff tendon was detached from the greater tuberosity, and MRE and SWE measurements were then performed in the same manner again. The differences in the stiffness values were compared between before and after the rotator cuff tendon tear on both the MRE and SWE measurements. The results indicated that stiffness values on MRE and SWE were 9.3 ± 1.8 and 10.0 ± 1.2 kPa respectively before the rotator cuff tear, and 7.3 ± 1.3 and 8.0 ± 0.8 kPa respectively after the tendon detachment. Stiffness values were significantly lower after the tendon detachment on both the MRE and SWE measurements (p < 0.05). Our results demonstrated that stiffness values of the SSP muscle on MRE and SWE were lower after rotator cuff detachment. From this result, MRE may be a feasible method for quantification of the change in rotator cuff muscle stiffness.

Magnetic resonance elastography of the supraspinatus muscle: A preliminary study on test-retest repeatability and wave quality with different frequencies and image filtering

The purpose of this study was to determine an optimal condition (vibration frequency and image filtering) for stiffness estimation with high accuracy and stiffness measurement with high repeatability in magnetic resonance elastography (MRE) of the supraspinatus muscle. Nine healthy volunteers underwent two MRE exams separated by at least a 30 min break, on the same day. MRE acquisitions were performed with a gradient-echo type multi-echo MR sequence at 75, 100, and 125 Hz pneumatic vibration. Wave images were processed by a bandpass filter or filter combining bandpass and directional filters (bandpass-directional filter). An observer specified the region of interest (ROI) on clear wave propagation in the supraspinatus muscle, within which the observer measured the stiffness. This study assessed wave image quality according to two indices, as a substitute for the assessment of the accuracy of the stiffness estimation. One is the size of the clear wave propagation area (ROI size used to measure the stiffness) and the other is the qualitative stiffness resolution score in that area. These measurements made by the observer were repeated twice at least one month apart after each MRE exam. This study assessed the intra-examiner and observer repeatability of the stiffness value, ROI size and resolution score in each combination of vibration frequency and image filter. Repeatability of the data was analyzed using the intraclass correlation coefficient (ICC) and 95% limits-of-agreement (LOA) in Bland-Altman analysis. The analyses on intra-examiner and observer repeatability of stiffness indicated that the ICC and 95% LOA were not varied greatly depending on vibration frequency and image filter (intra-examiner repeatability, ICC range, 0.79 to 0.88; 95% LOA range, ±23.95 to ±32.42%, intra-observer repeatability, ICC range, 0.98 to 1.00; 95% LOA range, ±5.10 to ±10.99%). In the analyses on intra-examiner repeatability of ROI size, ICCs were rather low (ranging from: 0.03 to 0.69) while 95% LOA was large in all the combinations of vibration frequency and image filter (ranging from: ±62.66 to ±83.33%). In the analyses on intra-observer repeatability of ROI size, ICCs were sufficiently high in the total combination of vibration frequency and image filter (ranging from 0.80 to 0.87) while the 95% LOAs were better (lower) in the bandpass-directional filter than the bandpass filter (bandpass directional filter vs. bandpass filter, ±28.81 vs. ±54.83% at 75 Hz; ±25.63 vs. ±37.83% at 100 Hz; ±34.51 vs. ±43.36% at 125 Hz). In the analyses on intra-examiner and observer repeatability of resolution score, the mean difference (bias) between the two exams (or observations) was significantly low and there was almost no difference across all the combinations of vibration frequency and image filter (range of bias: -0.11-0.11 and -0.17-0.00, respectively). Additionally, effects of vibration frequency and image filter on wave image quality (ROI size and resolution score) were assessed separately in each exam. Both mean ROI size and resolution score in the bandpass-directional filter were larger than those in the bandpass filter. Among the data in the bandpass-directional filter, mean ROI size was larger at 75 and 100 Hz, and mean resolution score was larger at 100 and 125 Hz. Taking into consideration with the results of repeatability and wave image quality, the present results suggest that optimal vibration frequency and image filter for MRE of the supraspinatus muscles is 100 Hz and bandpass-directional filter, respectively.

A new technique for motion encoding gradient-less MR elastography of the psoas major muscle: A gradient-echo type multi-echo sequence

The present study aimed to develop vibration techniques for magnetic resonance (MR) elastography (MRE) of the psoas major muscle (PM). Seven healthy volunteers were included. MRE was performed with motion-encoding gradient (MEG)-less multi-echo MRE sequence, which allows clinicians to perform MRE using conventional MR imaging. In order to transmit mechanical vibration of the pneumatic type to the PM, a long narrow vibration pad was designed using a 3D printer, and the optimum vibration techniques were verified. The vibration pad was placed under the lower back, with the volunteers in the supine position. The results indicated that the PM vibrated well through the transmitted vibration from the lumbar spine, which suggests that the placement of a narrow vibration pad under the supine body, along the lumbar spine, allows the vibration of the PM. The shear modulus of the PM (n = 7) was 1.23 ± 0.09 kPa (mean ± SEM) on the right side and 1.22 ± 0.15 kPa on the left side, with no significant difference (t-test, P > 0.05). Increased stiffness of the muscle due to continuous local contraction may be an important cause of non-specific low back pain (LBP). The present vibration techniques for MRE of the PM provide a quantitative diagnostic tool for changes in muscle stiffness associated with non-specific LBP.

Musculoskeletal application and validation of speckle-tracking ultrasonography

Background

Diseased, injured, or dysfunctional skeletal muscles may demonstrate abnormal function and contractility. Currently, only few in vivo imaging techniques are able to characterize the contractile properties of muscle tissue. This study aimed to test the hypothesis that muscle strain can be tracked in two upper extremity skeletal muscles by speckle-tracking ultrasonography (STU) and correlates with isometric muscle contractions.

Methods

A convenience sample of 10 healthy, adult volunteers with normal shoulder function were tested. The 5 women and 5 men had a mean age of 45 years (range: 39–59 years) and BMI < 30. STU was applied to the supraspinatus (SS) and biceps brachii (BB) muscles using a M11 L-MHz linear transducer (frequency 8–15 MHz) hooked to a Vivid E 9TM ultrasound machine. Strain validation was performed by correlating peak strain against standardized sub-maximal, isometric load conditions of the two muscles (20–80% of maximal voluntary contraction) using a custom-built muscle dynamometer based on strain-gauge technique. Data were analyzed offline using the EchoPac speckle-tracking software and were blinded to the examiner.

Results

Intramuscular strain measured by STU in the SS and BB muscles showed moderate to strong correlations with external muscle load (SS: r = − 0.76, p < 0.0001 and BB: r = − 0.60, p < 0.0001). We found strain to vary from approximately 10–20% during increasing submaximal, isometric conditions.

Conclusions

We demonstrate that STU can be applied on healthy skeletal musculature (SS and BB muscles). The observed correlations between strain and isometric contractions suggest a valid technique. However, the concept of measuring muscle strain non-invasively needs further investigation for validity, accuracy, responsiveness, and reliability before its therapeutic and research potential can be realized.

[Influence of Vibration Waveform on MR Elastography]

The purpose of this study was to investigate an influence of vibration waveform on magnetic resonance elastography (MRE). MRE is an innovative imaging technique for the non-invasive quantification of the elasticity of soft tissues through the direct visualization of propagating shear waves in vivo using a special phase-contrast magnetic resonance imaging sequence. Since the elasticity of soft tissue calculates from the wavelength of propagating shear waves, it is necessary to propagate sine-wave-shape shear wave at the target soft tissue. However, due to the various factors; i.e. overload of vibration generator, poor contact between imaging object and vibration pad, etc.; it may be difficult to generate a simple sine wave. This work was focused on change vibration waveforms; i.e. square wave, triangle wave, saw-tooth wave; which is induced by the various factors. Phantom experimental results demonstrated that when square and saw-tooth waveforms of 25 Hz vibration frequency, into the phantom, the waveform of propagating wave was not similar to sine waveform. It may influence on the MRE that in case of the waveforms has low frequency and square or saw-tooth like waveforms.

Simultaneous acquisition of magnetic resonance elastography of the supraspinatus and the trapezius muscles

We developed a Magnetic Resonance elastography (MRE) technique using a conventional magnetic resonance imaging (MRI), which allows a simultaneous elastography of the supraspinatus and trapezius muscles, by designing a new wave transducer (vibration pad) and optimizing the mechanical vibration frequency. Five healthy volunteers underwent an MRE. In order to transmit the mechanical vibration (pneumatic vibration) to the supraspinatus and trapezius muscles, a new vibration pad was designed using a three-dimensional (3D) printer. The vibration pad was placed on the skin 2 cm medial and 2 cm cephalad the deltoid tubercle. MRE acquisition was performed with a multi-slice gradient-echo type multi-echo MR sequence, which allows MREs even in a conventional MRI; two oblique axial images of the supraspinatus and trapezius muscles were obtained simultaneously. Vibration frequencies were set at 50-150 Hz, with a 25 Hz step. Wave image quality in each frequency was analyzed using a phase-to-noise ratio (PNR) and clarity of propagating wave that was assessed by two readers qualitatively. In the supraspinatus muscle, the wave images were of good quality especially at frequencies >75 Hz. In the trapezius muscle, the wave images were of better quality at low frequencies (50 and 75 Hz) compared with high frequencies (100-150 Hz). The PNR of both muscles were higher at low frequencies. The mean stiffness in the trapezius muscle (7.26 ± 2.13 kPa at 75 Hz) was larger than those in the supraspinatus muscle (4.16 ± 0.50 kPa at 75 Hz). The results demonstrated that our MRE technique allows simultaneous assessment of the stiffness in the supraspinatus and trapezius muscles using a conventional MRI, and that optimal vibration frequency for simultaneous MRE of these muscles is 75 Hz. This technique provides a new means for early detection of abnormality in the shoulder.

Development of a robust diffusion-MR elastography (dMRE) technique to mitigate intravoxel phase dispersion

Diffusion-magnetic resonance elastography (dMRE) is an emerging practical technique that can acquire diffusion magnetic resonance imaging and MRE simultaneously. However, a signal loss attributable to intravoxel phase dispersion (IVPD) interferes with the calculation of the apparent diffusion coefficient (ADC). This study presents an approach to dMRE that reduces the influence of IVPD by introducing a new pulse sequence. The existing and proposed techniques were performed using a phantom comprising five rods with different elasticities at 60 Hz vibration to investigate the accuracy of previous and proposed dMRE techniques. The measures of ADC and stiffness, obtained by using both dMRE techniques, were compared with conventional spin-echo (SE) diffusion and SE-MRE. Then, we evaluated those differences by using the mean of absolute differences (MAD) in each rod within the phantom. The results of the MAD of the stiffness from both dMRE techniques showed almost no difference. In contrast, the value of the ADC MAD (MAD ≒ 0.16 × 10^-3 mm²/s), obtained in the soft region within the phantom with the previous dMRE technique, was large. This value was about 2.7 times that of the value produced by the proposed dMRE technique. This difference must reflect the degree of influence of IVPD in both techniques. These results demonstrate that our dMRE technique is a robust method for addressing the signal loss attributable to IVPD.

Shear Wave Elastography Can Predict Passive Stiffness of Supraspinatus Musculotendinous Unit During Arthroscopic Rotator Cuff Repair for Presurgical Planning

PURPOSE

To determine the feasibility of shear wave elastography (SWE) with B-mode ultrasound in predicting the stiffness of the rotator cuff muscle before arthroscopic rotator cuff repair to evaluate the difficulty of the surgical procedure, as well as to compare SWE with the Goutallier stage on magnetic resonance imaging (MRI).

METHODS

Thirty-eight patients with a full-thickness supraspinatus tear requiring arthroscopic rotator cuff repair participated. The Goutallier stage of fatty infiltration on MRI was measured before surgery, as was the SWE modulus of the anterior superficial, anterior deep, posterior superficial, and posterior deep (PD) regions of the supraspinatus muscle. To measure the stiffness of the supraspinatus musculotendinous unit during surgery, the supraspinatus tendon was axially stretched until the anatomic insertion site was reached, and force per deformation was recorded. The correlation between stiffness of the supraspinatus and SWE value in each region of the supraspinatus muscle or Goutallier stage was determined. In addition, patients were divided into 2 groups: (1) In the complete footprint coverage group, greater than 50% of the footprint was covered during the stiffness measurement, and (2) in the incomplete footprint coverage group, less than 50% of the footprint was covered during the stiffness measurement. Differences in SWE value and Goutallier stage were measured between the 2 groups.

RESULTS

The best correlation of stiffness with the SWE modulus of the PD muscle of the supraspinatus was R = 0.69, and the correlation of stiffness with the Goutallier stage on MRI was R = 0.48. The SWE value of the PD region was greater in the incomplete footprint coverage group than in the complete footprint coverage group, although the Goutallier stage was not significantly different.

CONCLUSIONS

The highest correlation with stiffness of the supraspinatus musculotendinous unit was with the SWE modulus of the PD muscle, as compared with SWE evaluation of the other regions or the Goutallier stage on MRI. Ultrasound SWE can predict the stiffness of the supraspinatus musculotendinous unit best.

CLINICAL RELEVANCE

Rotator cuff retraction adds difficulty to arthroscopic rotator cuff repair. Ultrasound SWE may be used for presurgical planning.

[The Development of Vibration System for Applying Magnetic Resonance Elastography (MRE) to the Supraspinatus Muscle]

Palpation is a standard clinical tool to diagnose abnormal stiffness changes in soft tissues. However, it is difficult to palpate the supraspinatus muscle because it locates under the trapezius muscle. The magnetic resonance elastography (MRE) uses harmonic mechanical excitation to quantitatively measure the stiffness (shear modulus) of both the superficial and deep tissues. The purpose of this study was to build a vibration system for applying the MRE to the supraspinatus muscle. In this study, a power amplifier and a pneumatic pressure generator were used to supply vibrations to a vibration pad. Six healthy volunteers underwent MRE. We investigated the effects of position (the head of the humerus and the trapezius muscle) of the vibration pad on the patterns of wave propagation (wave image). When the vibration pad was placed in the trapezius muscle, the wave images represented clear wave propagation. On the other hand, when the vibration pad was placed in the head of the humerus, the wave images represented unclear wave propagation. This result might be caused by wave interferences resulting from the vibrations from bones and an intramuscular tendon of the supraspinatus muscle. The mean shear modulus also was 8.12 ± 1.83 (mean ± SD) kPa, when the vibration pad was placed in the trapezius muscle. Our results demonstrated that the vibration pad should be placed in the trapezius muscle in the MRE of the supraspinatus muscle.