Novel materials in magnetic resonance imaging: high permittivity ceramics, metamaterials, metasurfaces and artificial dielectrics

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple “dielectric pads” for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.

Diffusion-weighted imaging in prostate cancer

Diffusion-weighted imaging (DWI), a key component in multiparametric MRI (mpMRI), is useful for tumor detection and localization in clinically significant prostate cancer (csPCa). The Prostate Imaging Reporting and Data System versions 2 and 2.1 (PI-RADS v2 and PI-RADS v2.1) emphasize the role of DWI in determining PIRADS Assessment Category in each of the transition and peripheral zones. In addition, several recent studies have demonstrated comparable performance of abbreviated biparametric MRI (bpMRI), which incorporates only T2-weighted imaging and DWI, compared with mpMRI with dynamic contrast-enhanced MRI. Therefore, further optimization of DWI is essential to achieve clinical application of bpMRI for efficient detection of csPC in patients with elevated PSA levels. Although DWI acquisition is routinely performed using single-shot echo-planar imaging, this method suffers from such as susceptibility artifact and anatomic distortion, which remain to be solved. In this review article, we will outline existing problems in standard DWI using the single-shot echo-planar imaging sequence; discuss solutions that employ newly developed imaging techniques, state-of-the-art technologies, and sequences in DWI; and evaluate the current status of quantitative DWI for assessment of tumor aggressiveness in PC.

A half-century of innovation in technology-preparing MRI for the 21st century

MRI developed during the last half-century from a very basic concept to an indispensable non-ionising medical imaging technique that has found broad application in diagnostics, therapy control and far beyond. Due to its excellent soft-tissue contrast and the huge variety of accessible tissue- and physiological-parameters, MRI is often preferred to other existing modalities. In the course of its development, MRI underwent many substantial transformations. From the beginning, starting as a proof of concept, much effort was expended to develop the appropriate basic scanning technology and methodology, and to establish the many clinical contrasts (e.g., T₁, T₂, flow, diffusion, water/fat, etc.) that MRI is famous for today. Beyond that, additional prominent innovations to the field have been parallel imaging and compressed sensing, leading to significant scanning time reductions, and the move towards higher static magnetic field strengths, which led to increased sensitivity and improved image quality. Improvements in workflow and the use of artificial intelligence are among many current trends seen in this field, paving the way for a broad use of MRI. The 125th anniversary of the BJR is a good point to reflect on all these changes and developments and to offer some slightly speculative ideas as to what the future may bring.

Reduced field-of-view diffusion-weighted imaging (DWI) in patients with gastric cancer: Comparison with conventional DWI techniques at 3.0T: A preliminary study

To evaluate the qualitative image quality and quantitative apparent diffusion coefficient (ADC) value of reduced field-of view (rFOV) and full field-of-view (fFOV) diffusion-weighted imaging (DWI) sequences at 3.0 T in patients with gastric cancer.Fifty-three patients (37 males, 16 females; mean age, 63.3 ± 10.3 years) with 60 lesions with gastric cancer who underwent magnetic resonance (MR) scans, including both rFOV-DWI and fFOV-DWI, were retrospectively analyzed. Two observers subjectively evaluated image quality for both the fFOV-DWI and rFOV-DWI sequences regarding the anatomic details, distortion, lesion conspicuity, artifacts, and overall image quality. The mean ADC values of gastric cancer were calculated. The Wilcoxon test and paired samples t test were used. Interobserver agreement was assessed using kappa statistics.The mean scores based on the 2 observers demonstrated significant differences in image quality in terms of anatomic details, distortion, lesion conspicuity, artifacts and overall image quality at both b values between rFOV-DWI and fFOV-DWI (P < .05) in the whole gastric area. rFOV-DWI yielded significantly better scores in image quality at b = 800 seconds/mm (P < .05) in patients with esophagogastric junction cancers, but there were no significant differences in the gastric corpus and gastric antrum region. The mean tumor ADC values of rFOV-DWI were significantly lower than those of fFOV-DWI (1.237 ± 0.228 × 10-3 mm/second vs 1.683 ± 0.322 × 10-3 mm/second, P < .001).rFOV-DWI yielded significantly better image quality (anatomic details, distortion, lesion conspicuity, artifacts, overall image quality) and more accurate ADC measurements than fFOV-DWI did.

Effect of Dual-Source Radiofrequency Transmission on Left Ventricular Measurements and Measurement Reproducibility at 3.0 T Cardiac MR Imaging: Comparison with Conventional Single-Source Transmission Reference

RATIONALE AND OBJECTIVES

To prospectively assess effect of dual-source radiofrequency (RF) transmission on left ventricular (LV) measurements and measurements reproducibility at 3.0 T MR using balanced steady-state free precession (b-SSFP) cine imaging, compared to the conventional single-source RF transmission reference approach.

MATERIALS AND METHODS

Cardiac b-SSFP cine imaging was performed in 19 subjects at 3.0 T MR equipped with dual-source RF transmission. All images were analyzed to obtain LV end-diastolic volume, end-systolic volume, stroke volume, ejection fraction, mass, LV end-diastolic inferior wall thickness, and interventricular septal thickness. The difference of all LV measurements between the two imaging techniques was tested with the paired t test and the intertechnique agreement was tested through linear regression and Bland-Altman analyses. Additionally, repeated LV measurements were performed to determine intra and interobserver variability with the Bland-Altman method, the 95% limits of agreement, the coefficient of variation (CV) and the intraclass correlation coefficient.

RESULTS

Compared to conventional single-source, dual-source slightly overestimated end-diastolic volume, end-systolic volume, and stroke volume (mean differences, 3.9 mL ± 9.7, 1.1 mL ± 2.6, and 2.8 mL ± 9.1, respectively; p > 0.05), resulting in a small but significant positive bias in ejection fraction (1.5% ± 2.6; p = 0.021). Mass was significantly smaller with dual-source than with single-source (-4.0 g ± 6.5, p = 0.001). Dual-source slightly underestimated interventricular septal thickness (-0.29 mm ± 0.6, p = 0.067) and significantly underestimated LV end-diastolic inferior wall thickness (-0.55 mm ± 0.4, p < 0.0001). The two techniques in measurements correlated highly (r² = 0.81 to 0.96, p < 0.0001). Intra and interobserver variability in dual-source measurements was much lower than that in single-source, and variability values were <14.0%.

CONCLUSION

Improved image quality of b-SSFP cine imaging at 3.0 T MR with dual-source RF transmission may provide more reproducible LV measurements compared to conventional single-source approach. Dual-source RF transmission also provides a reasonable estimate of the LV measurements.

1.5 versus 3 versus 7 Tesla in abdominal MRI: A comparative study

Objectives

The aim of this study was to investigate and compare the feasibility as well as potential impact of altered magnetic field properties on image quality and potential artifacts of 1.5 Tesla, 3 Tesla and 7 Tesla non-enhanced abdominal MRI.

Materials and methods

Magnetic Resonance (MR) imaging of the upper abdomen was performed in 10 healthy volunteers on a 1.5 Tesla, a 3 Tesla and a 7 Tesla MR system. The study protocol comprised a (1) T1-weighted fat-saturated spoiled gradient-echo sequence (2D FLASH), (2) T1-weighted fat-saturated volumetric interpolated breath hold examination sequence (3D VIBE), (3) T1-weighted 2D in and opposed phase sequence, (4) True fast imaging with steady-state precession sequence (TrueFISP) and (5) T2-weighted turbo spin-echo (TSE) sequence. For comparison reasons field of view and acquisition times were kept comparable for each correlating sequence at all three field strengths, while trying to achieve the highest possible spatial resolution. Qualitative and quantitative analyses were tested for significant differences.

Results

While 1.5 and 3 Tesla MRI revealed comparable results in all assessed features and sequences, 7 Tesla MRI yielded considerable differences in T1 and T2 weighted imaging. Benefits of 7 Tesla MRI encompassed an increased higher spatial resolution and a non-enhanced hyperintense vessel signal at 7 Tesla, potentially offering a more accurate diagnosis of abdominal parenchymatous and vasculature disease. 7 Tesla MRI was also shown to be more impaired by artifacts, including residual B₁ inhomogeneities, susceptibility and chemical shift artifacts, resulting in reduced overall image quality and overall image impairment ratings. While 1.5 and 3 Tesla T2w imaging showed equivalently high image quality, 7 Tesla revealed strong impairments in its diagnostic value.

Conclusions

Our results demonstrate the feasibility and overall comparable imaging ability of T1-weighted 7 Tesla abdominal MRI towards 3 Tesla and 1.5 Tesla MRI, yielding a promising diagnostic potential for non-enhanced Magnetic Resonance Angiography (MRA). 1.5 Tesla and 3 Tesla offer comparably high-quality T2w imaging, showing superior diagnostic quality over 7 Tesla MRI.

The impact of dual-source parallel radiofrequency transmission with patient-adaptive shimming on the cardiac magnetic resonance in children at 3.0 T

The cardiac magnetic resonance (CMR) of children at 3.0 T presents a unique set of technical challenges because of their small cardiac anatomical structures, fast heart rates, and the limited ability to keep motionless and hold breathe, which could cause problems associated with field inhomogeneity and degrade the image quality. The aim of our study was to evaluate the effect of dual-source parallel radiofrequency (RF) transmission on the B1 homogeneity and image quality in children with CMR at 3.0 T. The study was approved by the institutional ethics committee and written informed consent was obtained. A total of 30 free-breathing children and 30 breath-hold children performed CMR examinations with dual-source and single-source RF transmission. The B1 homogeneity, contrast ratio (CR) of cine images, and off-resonance artifacts in cine images between dual-source and single-source RF transmission were assessed in free-breathing and breath-hold groups, respectively. In both free-breathing and breath-hold groups, higher mean percentage of flip angle (free-breathing group: 104.2 ± 4.6 vs 95.5 ± 6.3, P < .001; breath-hold group: 101.5 ± 5.1 vs 92.5 ± 6.3, P < .001) and lower coefficient of variation (free-breathing group: 0.06 ± 0.02 vs 0.09 ± 0.03, P < .001; breath-hold group: 0.07 ± 0.03 vs 0.10 ± 0.04, P = .005) were found with dual-source than with single-source RF transmission. Both the CRs in the horizontal long axis (HLA) and short axis of cine images with dual-source RF transmission was improved (P < .05 for all). The scores of off-resonance artifacts in the HLA with dual-source RF transmission were higher in both free-breathing and breath-hold groups (P < .05 for all), with substantial interreader agreement (kappa values from 0.68 to 0.74). Compared with conventional single-source, dual-source parallel RF transmission could significantly improve the B1 homogeneity and image quality for CMR in children at 3.0 T. This technology could be taken into account in CMR for children with cardiac diseases.

Cardiac MOLLI T1 mapping at 3.0 T: comparison of patient-adaptive dual-source RF and conventional RF transmission

To prospectively compare image quality and myocardial T₁ relaxation times of modified Look-Locker inversion recovery (MOLLI) imaging at 3.0 T (T) acquired with patient-adaptive dual-source (DS) and conventional single-source (SS) radiofrequency (RF) transmission. Pre- and post-contrast MOLLI T₁ mapping using SS and DS was acquired in 27 patients. Patient wise and segment wise analysis of T₁ times was performed. The correlation of DS MOLLI measurements with a reference spin echo sequence was analysed in phantom experiments. DS MOLLI imaging reduced T₁ standard deviation in 14 out of 16 myocardial segments (87.5%). Significant reduction of T₁ variance could be obtained in 7 segments (43.8%). DS significantly reduced myocardial T₁ variance in 16 out of 25 patients (64.0%). With conventional RF transmission, dielectric shading artefacts occurred in six patients causing diagnostic uncertainty. No according artefacts were found on DS images. DS image findings were in accordance with conventional T₁ mapping and late gadolinium enhancement (LGE) imaging. Phantom experiments demonstrated good correlation of myocardial T₁ time between DS MOLLI and spin echo imaging. Dual-source RF transmission enhances myocardial T₁ homogeneity in MOLLI imaging at 3.0 T. The reduction of signal inhomogeneities and artefacts due to dielectric shading is likely to enhance diagnostic confidence.

B1 homogeneity of breast MRI using RF shimming with individual specific values in volunteers simulating patients after mastectomy

Background Magnetic resonance imaging (MRI) using a 3-T MRI scanner is now widely used for clinical examinations. However, B₁ inhomogeneity becomes larger with MRI scanners using 3-T and higher. It especially becomes a problem in the breast. To improve B₁ homogeneity, a RF shimming technique has been developed. Purpose To evaluate the B₁ homogeneity of breast MRI using RF shimming with individual specific values for subjects after mastectomy. Material and Methods The subjects are healthy female volunteers who underwent normal breast imaging, followed by imaging of one breast while the other breast was bound tightly to the chest by bleached cotton cloths (simulating volunteers after mastectomy). B₁ mappings were performed with RF shimming using two techniques: (i) optimized fixed value; and (ii) individual specific values using a 3-T MRI scanner. The means and standard deviations of the B₁ maps for all slices in the breast were measured and compared between the fixed value and the individual specific value cases. Results For normal volunteers, the breast B₁ variation was not statistically significantly different between the RF shimming techniques. For volunteers after simulated surgery, the breast B₁ variation was (1.02 ± 0.29) with the fixed value and (0.98 ± 0.22) with the individual specific value ( P < 0.01). With the individual specific optimization, B₁ variation for all slices in the breast was improved for volunteers after simulated surgery. Conclusion RF shimming with individual specific values has the potential to improve the B₁ homogeneity of breast MRI in patients after mastectomy.

Left Ventricular Function Evaluation on a 3T MR Scanner with Parallel RF Transmission Technique: Prospective Comparison of Cine Sequences Acquired before and after Gadolinium Injection

Objectives

To compare cine MR b-TFE sequences acquired before and after gadolinium injection, on a 3T scanner with a parallel RF transmission technique in order to potentially improve scanning time efficiency when evaluating LV function.

Methods

25 consecutive patients scheduled for a cardiac MRI were prospectively included and had their b-TFE cine sequences acquired before and right after gadobutrol injection. Images were assessed qualitatively (overall image quality, LV edge sharpness, artifacts and LV wall motion) and quantitatively with measurement of LVEF, LV mass, and telediastolic volume and contrast-to-noise ratio (CNR) between the myocardium and the cardiac chamber. Statistical analysis was conducted using a Bayesian paradigm.

Results

No difference was found before or after injection for the LVEF, LV mass and telediastolic volume evaluations. Overall image quality and CNR were significantly lower after injection (estimated coefficient cine after > cine before gadolinium: -1.75 CI = [-3.78;-0.0305], prob(coef>0) = 0% and -0.23 CI = [-0.49;0.04], prob(coef>0) = 4%) respectively), but this decrease did not affect the visual assessment of LV wall motion (cine after > cine before gadolinium: -1.46 CI = [-4.72;1.13], prob(coef>0) = 15%).

Conclusions

In 3T cardiac MRI acquired with parallel RF transmission technique, qualitative and quantitative assessment of LV function can reliably be performed with cine sequences acquired after gadolinium injection, despite a significant decrease in the CNR and the overall image quality.

Practical improvements in the design of high permittivity pads for dielectric shimming in neuroimaging at 7T

Improvements are proposed for practical design and use of high permittivity materials in high field neuroimaging in three different areas: (i) a simple formula to predict the permittivity of tri-component aqueous-based perovskite suspensions with relative permittivities between 110 and 300, (ii) characterization of addition of a hydroxyethyl-cellulose gelling agent to improve the long-term stability and material properties of "dielectric pads", and (iii) investigation of the integration of, for example, headphones into the dielectric pads to increase patient comfort within tightly-fitting receive coil arrays.

Parallel transmission for ultrahigh-field imaging

The development of MRI systems operating at or above 7 T has provided researchers with a new window into the human body, yielding improved imaging speed, resolution and signal-to-noise ratio. In order to fully realise the potential of ultrahigh-field MRI, a range of technical hurdles must be overcome. The non-uniformity of the transmit field is one of such issues, as it leads to non-uniform images with spatially varying contrast. Parallel transmission (i.e. the use of multiple independent transmission channels) provides previously unavailable degrees of freedom that allow full spatial and temporal control of the radiofrequency (RF) fields. This review discusses the many ways in which these degrees of freedom can be used, ranging from making more uniform transmit fields to the design of subject-tailored RF pulses for both uniform excitation and spatial selection, and also the control of the specific absorption rate. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Multiparametric imaging with heterogeneous radiofrequency fields

Magnetic resonance imaging (MRI) has become an unrivalled medical diagnostic technique able to map tissue anatomy and physiology non-invasively. MRI measurements are meticulously engineered to control experimental conditions across the sample. However, residual radiofrequency (RF) field inhomogeneities are often unavoidable, leading to artefacts that degrade the diagnostic and scientific value of the images. Here we show that, paradoxically, these artefacts can be eliminated by deliberately interweaving freely varying heterogeneous RF fields into a magnetic resonance fingerprinting data-acquisition process. Observations made based on simulations are experimentally confirmed at 7 Tesla (T), and the clinical implications of this new paradigm are illustrated with in vivo measurements near an orthopaedic implant at 3T. These results show that it is possible to perform quantitative multiparametric imaging with heterogeneous RF fields, and to liberate MRI from the traditional struggle for control over the RF field uniformity.

Magnetic resonance fingerprinting (MRF) requires a uniform B₁⁺ radiofrequency field. Here the authors present plug-n-play MRF, a technique that enables multiparametric imaging with heterogeneous B₁⁺ fields, and demonstrate its utility in the presence of RF distortion caused by a metallic orthopaedic implant.

Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1 + uniformity

PURPOSE

To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B1 + uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response.

METHODS

RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation.

RESULTS

Two-port RF shimming, compared to circular polarization mode, can increase the B1 + uniformity on average by +32%. Worst-case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively.

CONCLUSION

Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. © 2015 Wiley Periodicals, Inc.

Late gadolinium enhancement cardiac imaging on a 3T scanner with parallel RF transmission technique: prospective comparison of 3D-PSIR and 3D-IR

OBJECTIVE

To qualitatively and quantitatively compare different late gadolinium enhancement (LGE) sequences acquired at 3T with a parallel RF transmission technique.

METHODS

One hundred and sixty participants prospectively enrolled underwent a 3T cardiac MRI with 3 different LGE sequences: 3D Phase-Sensitive Inversion-Recovery (3D-PSIR) acquired 5 minutes after injection, 3D Inversion-Recovery (3D-IR) at 9 minutes and 3D-PSIR at 13 minutes. All LGE-positive patients were qualitatively evaluated both independently and blindly by two radiologists using a 4-level scale, and quantitatively assessed with measurement of contrast-to-noise ratio and LGE maximal surface. Statistical analyses were calculated under a Bayesian paradigm using MCMC methods.

RESULTS

Fifty patients (70 % men, 56yo ± 19) exhibited LGE (62 % were post-ischemic, 30 % related to cardiomyopathy and 8 % post-myocarditis). Early and late 3D-PSIR were superior to 3D-IR sequences (global quality, estimated coefficient IR > early-PSIR : -2.37 CI = [-3.46 ; -1.38], prob(coef > 0) = 0 % and late-PSIR > IR : 3.12 CI = [0.62 ; 4.41], prob(coef > 0) = 100 %), LGE surface estimated coefficient IR > early-PSIR: -0.09 CI = [-1.11; -0.74], prob(coef > 0) = 0 % and late-PSIR > IR : 0.96 CI = [0.77; 1.15], prob(coef > 0) = 100 %). Probabilities for late PSIR being superior to early PSIR concerning global quality and CNR were over 90 %, regardless of the aetiological subgroup.

CONCLUSIONS

In 3T cardiac MRI acquired with parallel RF transmission technique, 3D-PSIR is qualitatively and quantitatively superior to 3D-IR.

KEY POINTS

• Late gadolinium enhancement is an essential part of a cardiac MRI examination • PSIR and IR sequences are the two possible options for LGE imaging • At 3T with parallel RF transmission, PSIR sequences are significantly better • One LGE sequence is sufficient, allowing an optimization of the acquisition time.

Assessment of Liver Perfusion by IntraVoxel Incoherent Motion (IVIM) Magnetic Resonance-Diffusion-Weighted Imaging: Correlation With Phase-Contrast Portal Venous Flow Measurements

OBJECTIVES

To prospectively verify, in vivo, Le Bihan's model of signal decay in magnetic resonance/diffusion-weighted imaging (intravoxel incoherent motion) in healthy liver parenchyma.

METHODS

Informed consent and institutional board approval were obtained. To measure both underfasting and postprandial conditions, apparent, slow, and fast diffusion (D*) coefficients and perfusion fraction of liver parenchyma, 40 healthy volunteers (19 women and 21 men) underwent a 3.0-T magnetic resonance imaging examination, including portal venous flow measurements by a 2-dimensional phase-contrast sequence, and multi-b diffusion-weighted imaging acquired before and 30 minutes after a 600-Kcal meal. Parameters were measured by fitting procedure with regions of interest drawn on the right liver lobe. Paired-sample t test was performed to search for any statistically significant difference between preprandial and postprandial values of each parameter and of portal flow. Pearson correlation coefficients were calculated to evaluate the relationship between portal flow increase and diffusion-weighted imaging parameter changes in postprandial conditions. Interobserver agreement for measurement of the intravoxel incoherent motion parameters was determined, both for preprandial and postprandial values.

RESULTS

Mean increase in postprandial portal flow was 98% (P < 0.0009). The t test did not show any statistically significant difference between the preprandial and postprandial values for apparent, slow diffusion coefficients and perfusion fraction (P ≥ 0.05), whereas a statistically significant postprandial increase (P < 0.01) of D* was detected. Correlation with portal venous flow increase at Pearson test was statistically significant for D* (P = 0.04) and nonsignificant for the other parameters. All the parameters showed wide variability, with a higher percent coefficient of variation for D*. Interobserver agreement was always greater than 0.70.

CONCLUSIONS

This study verifies Le Bihan's theory, confirming that in the liver, D* is influenced by perfusional changes related to portal venous flow.

Clinical applications of dual-channel transmit MRI: A review

This article reviews the principle of dual-channel transmit MRI and highlights current clinical applications which are performed primarily at 3 Tesla. The main benefits of dual-channel transmit compared with single-transmit systems are the increased image contrast homogeneity and the decreased scanning time due to the more accurate local specific absorption ratio estimation, meaning that less conservative safety limits are needed. The dual-transmit approach has been particularly beneficial in body imaging applications, and is also promising in terms of cardiac, spine, and fetal imaging. Future advances in transmit SENSE, the combination of dual-channel transmit with high permittivity pads, as well as the potential increase in the number of transmit channels are also discussed.

High-dose dobutamine stress steady-state free precession (SSFP) cine MRI at 3T with patient adaptive local radiofrequency (RF) shimming using dual-source RF transmission

PURPOSE

To prospectively assess the feasibility, image quality, and diagnostic accuracy of high-dose dobutamine stress magnetic resonance imaging (DSMR) using steady-state free precession (SSFP) cine imaging at 3T applying a dual-source radiofrequency (RF) excitation magnetic resonance imaging (MRI) system with parallel transmission and patient adaptive local RF shimming.

MATERIALS AND METHODS

DSMR using SSFP cine imaging was performed in 44 patients at 3T scheduled for a clinically indicated coronary angiography. The effect of conventional versus dual-source RF transmission was assessed regarding homogeneity of the B1 field, contrast-to-noise ratios (CNRs) at rest, image quality, and diagnostic accuracy of DSMR using long and short axis.

RESULTS

The mean percentage of the intended flip angle within the heart increased from 88 ± 9.1% with single-source to 103 ± 5.6% (P < 0.001) dual-source RF transmission. CNR increased for dual-source particularly at the apex (63.4 ± 24.2 vs. 36.5 ± 16.5, P < 0.001) but also at the base of the left ventricle (LV) (50.1 ± 14.8 vs. 39.3 ± 15.8, P < 0.001). Image quality of dual-source was higher both at rest (2.8 ± 0.5 vs. 2.6 ± 0.7, P < 0.001) and stress (2.5 ± 0.7 vs. 2.0 ± 1.0, P < 0.001). The number of segments with severe artifacts or nondiagnostic image quality at stress was lower with dual-source RF transmission (8% vs. 27%, P < 0.001). The diagnostic accuracy of DSMR in coronary territories using dual-source RF transmission was significantly higher (77% vs. 65%, P = 0.04).

CONCLUSION

Patient adaptive local RF shimming using dual-source RF transmission provided significantly improved image quality and higher diagnostic accuracy of SSFP during DSMR at 3T compared to conventional RF transmission.

Dual-source RF transmission in cardiac SSFP imaging at 3 T: systematic spatial evaluation of image quality improvement compared to conventional RF transmission

The purpose of this investigation was to systematically evaluate the spatial distribution of image quality improvement with dual-source radiofrequency (RF) transmission in cardiac steady-state free precession sequences at 3.0 T. Imaging with and without dual-source RF transmission was performed in 30 patients. Contrast-to-noise ratio for the left ventricular myocardium was significantly higher using dual-source RF transmission, but improvement was not uniformly distributed. The posterior myocardium showed significantly less contrast-to-noise ratio gain than all other cardiac regions. Signal-to-noise ratio increase was higher in the right than in the left ventricle. Subjective image quality was significantly enhanced by parallel RF transmission.

Three-dimensional balanced steady state free precession myocardial perfusion cardiovascular magnetic resonance at 3T using dual-source parallel RF transmission: initial experience

BACKGROUND

The purpose of this study was to establish the feasibility of three-dimensional (3D) balanced steady-state-free-precession (bSSFP) myocardial perfusion cardiovascular magnetic resonance (CMR) at 3T using local RF shimming with dual-source RF transmission, and to compare it with spoiled gradient echo (TGRE) acquisition.

METHODS

Dynamic contrast-enhanced 3D bSSFP perfusion imaging was performed on a 3T MRI scanner equipped with dual-source RF transmission technology. Images were reconstructed using k-space and time broad-use linear acquisition speed-up technique (k-t BLAST) and compartment based principle component analysis (k-t PCA).

RESULTS

In phantoms and volunteers, local RF shimming with dual source RF transmission significantly improved B1 field homogeneity compared with single source transmission (P=0.01). 3D bSSFP showed improved signal-to-noise, contrast-to-noise and signal homogeneity compared with 3D TGRE (29.8 vs 26.9, P=0.045; 23.2 vs 21.6, P=0.049; 14.9% vs 12.4%, p=0.002, respectively). Image quality was similar between bSSFP and TGRE but there were more dark rim artefacts with bSSFP. k-t PCA reconstruction reduced artefacts for both sequences compared with k-t BLAST. In a subset of five patients, both methods correctly identified those with coronary artery disease.

CONCLUSION

Three-dimensional bSSFP myocardial perfusion CMR using local RF shimming with dual source parallel RF transmission at 3T is feasible and improves signal characteristics compared with TGRE. Image artefact remains an important limitation of bSSFP imaging at 3T but can be reduced with k-t PCA.

Zoomed EPI DWI of Acute Spinal Ischemia Using a Parallel Transmission System

PURPOSE

A new method for diffusion-weighted imaging (DWI) using independent parallel transmission technique resulting in zoomed DWI was applied in four patients suffering from acute spinal cord ischemia.

METHODS

Four patients with clinical symptoms of acute spinal cord ischemia were examined on a 3 T MR-system equipped with a two-channel transmit array. Scans included T2-weighted turbo spin echo, conventional DWI, and zoomed DWI. Image evaluation was performed with regard to overall image quality, anatomic delineation of the spinal cord, and the level of confidence to establish the diagnosis of spinal cord ischemia.

RESULTS

Through spatially selective excitation, zoomed DWI allows for acquisition of high-resolution images with reduced scan time due to a reduced field of view in phase-encoding direction, resulting in zoomed images. In all cases the ischemia was demonstrated in conventional DWI as well as zoomed DWI.

CONCLUSIONS

Compared to conventional DWI, zoomed DWI enables a faster image acquisition and allowed a more detailed analysis of the spinal lesion which may be critical to attribute the lesion to a particular vessel territory.

Dual-source parallel radiofrequency transmission for magnetic resonance breast imaging at 3T: any added clinical value?

PURPOSE

To investigate the influence of dual-source parallel radiofrequency (RF) excitation on clinical breast MR images.

METHODS

A 3T MR system with both dual-source and conventional single-source RF excitations was used to examine 22 patients. Axial TSE-T2WI with fat suppression, TSE-T1WI without fat suppression, THRIVE (3D field echo) and DWI (SE-EPI) were obtained by using both excitation techniques. Image homogeneity, image contrast and lesion conspicuity were measured or independently scored by two radiologists and were compared by paired-sample t test or Wilcoxon test.

RESULTS

Both excitations revealed 24 lesions. For SE sequences using dual-source mode, image homogeneity was improved (P=0.00), scan time was reduced, and ghost artifacts on DWI were significantly reduced (P=0.00). However, image contrast was not increased and lesion conspicuity had no significant difference between two modes, except DWI on which lesion conspicuity was significantly improved (P=0.00), due to less ghost artifacts. For field-echo sequence, image homogeneity, acquisition time, image contrast and lesion conspicuity had no significant difference between the two modes.

CONCLUSIONS

Dual-source parallel RF transmission has some added value for improving breast image quality. However, its value is limited in terms of improving lesion detection and characterization.

Breast magnetic resonance imaging technique at 1.5 T and 3 T: requirements for quality imaging and American College of Radiology accreditation

Although there are multiple variations in acquisition protocols for breast magnetic resonance (MR) imaging, there is agreement that components of high-quality technique include a bilateral acquisition obtained with a dedicated breast coil. Further, key pulse sequences should be included and spatial and temporal resolution should be sufficiently high to assess lesion morphology and kinetics. Artifacts must be recognized and avoided. The American College of Radiology Breast MRI Accreditation Program requirements provide minimum standards to guide facilities in technique. MR imaging at 3 T is increasingly available and offers signal-to-noise ratio advantages over 1.5 T but also some technical challenges.

Clinical and technical considerations for high quality breast MRI at 3 Tesla

The use of breast MRI at 3 tesla (T) has increased in use substantially in recent years. Potential benefits of moving to higher field strength MRI include improved morphologic and kinetic assessment of breast lesions through higher spatial and temporal resolution dynamic contrast-enhanced MR examinations. Furthermore, higher field strength holds promise for the development of superior advanced breast MRI techniques, such as diffusion weighted imaging and MR spectroscopy. To fully realize the benefits of moving to 3T, a thorough understanding of the technical and safety challenges of higher field strength imaging specific to breast MRI is paramount. Through the use of advanced coil technology, parallel imaging, dual-source parallel radiofrequency excitation, and image-based shimming techniques, many of these limiting technical factors can be overcome to achieve high quality breast MRI at 3T.

Initial experience of MR/PET in a clinical cancer center

Magentic Resonance/positron emission tomography (PET) has been introduced recently for imaging of clinical patients. This hybrid imaging technology combines the inherent strengths of MRI with its high soft-tissue contrast and biological sequences with the inherent strengths of PET, enabling imaging of metabolism with a high sensitivity. In this article, we describe the initial experience of MR/PET in a clinical cancer center along with a review of the literature. For establishing MR/PET in a clinical setting, technical challenges, such as attenuation correction and organizational challenges, such as workflow and reimbursement, have to be overcome. The most promising initial results of MR/PET have been achieved in anatomical areas where high soft-tissue and contrast resolution is of benefit. Head and neck cancer and pelvic imaging are potential applications of this hybrid imaging technology. In the pediatric population, MR/PET can decrease the lifetime radiation dose. MR/PET protocols tailored to different types of malignancies need to be developed. After the initial exploration phase, large multicenter trials are warranted to determine clinical indications for this exciting hybrid imaging technology and thereby opening new horizons in molecular imaging.

Magnetic resonance myocardial perfusion imaging at 3.0 Tesla for the identification of myocardial ischaemia: comparison with coronary catheter angiography and fractional flow reserve measurements

AIMS

To assess image quality and diagnostic performance of 3.0 Tesla (3T) cardiac magnetic resonance (CMR) myocardial perfusion imaging with a dual radiofrequency source to detect functional relevant coronary artery disease (CAD), using coronary angiography and invasive pressure-derived fractional flow reserve (FFR) as reference standard.

METHODS AND RESULTS

We included 116 patients with suspected or known CAD, who underwent 3T adenosine myocardial perfusion CMR (resolution 2.97 × 2.97 mm) and coronary angiography plus FFR measurements in intermediate lesions. Image quality of myocardial perfusion CMR was graded on a 4-point scale (1 = poor to 4 = excellent). Diagnostic accuracy was assessed by ROC analyses using a 16-myocardial segment-based summed perfusion score (0 = normal to 3 = transmural perfusion defect) and by determining sensitivity, specificity, positive and negative predictive value on the coronary vessel territory and the patient level. Diagnostic image quality was achieved for all stress myocardial perfusion CMR studies with an average quality score of 2.5, 3.1, and 3.0 for LAD, LCX, and RCA territories. The ability of the myocardial perfusion CMR perfusion score to detect significant coronary artery stenosis yielded an area under the curve of 0.93 on ROC analysis. Values for sensitivity, specificity, positive and negative predictive value on a vessel territory level and the patient level were 89, 95, 87, 96% and 85, 87, 77, 92%, respectively.

CONCLUSION

In patients with suspected or known significant CAD, 3T myocardial perfusion CMR with standard perfusion protocols provides consistently high image quality and an excellent diagnostic performance.

Impact of 3.0 T Cardiac MR Imaging Using Dual-Source Parallel Radiofrequency Transmission with Patient-Adaptive B1 Shimming

Objectives

To prospectively evaluate the impact of 3.0 T Cardiac MR imaging using dual-source parallel radiofrequency (RF) transmission with patient-adaptive B1 shimming compared with single-source RF transmission in the RF homogeneity, image contrast and image quality.

Methods

The study was approved by the local institutional review board, and all subjects provided written informed consent. Fourteen healthy volunteers were examined at 3.0 T MR, with both the conventional single-source and the new dual-source RF transmission. B1 calibrations (RF shimming) of the heart region were performed to acquire a percent of the prescribed flip angle (FA) of B1 maps, which were used for quantitative assessment of RF homogeneity. Contrast ratios (CRs) between ventricular blood pool and septum were calculated on balanced-turbo field echo (B-TFE) cine images. The off-resonance artifacts of cine images were blindly assessed by two radiologists according to a 4-point grading-scale.

Results

A significantly lower mean coefficients of variance of the achieved FA with dual-source revealed better RF homogeneity compared to single-source (P = 0.0094). Dual-source RF shimming significantly increased the CRs (P<0.05) and reduced the off-resonance artifacts of B-TFE cine images (P<0.05). Inter-observer agreement for the off-resonance artifacts of B-TFE cine images was good to excellent (k >0.65).

Conclusions

Dual-source parallel RF transmission significantly improves the RF homogeneity, increases image contrast and reduces image artifacts of cardiac B-TFE images compared to single-source mode. This may be of value in reducing the observer-dependence of cardiac MR images and enhancing diagnostic confidence for clinical practice using CMR at 3.0 T.

Qualitative and quantitative assessment of wrist MRI at 3.0T: comparison between isotropic 3D turbo spin echo and isotropic 3D fast field echo and 2D turbo spin echo

BACKGROUND

Isotropic three-dimensional (3D) magnetic resonance imaging (MRI) has been applied to various joints. However, comparison for image quality between isotropic 3D MRI and two-dimensional (2D) turbo spin echo (TSE) sequence of the wrist at a 3T MR system has not been investigated.

PURPOSE

To compare the image quality of isotropic 3D MRI including TSE intermediate-weighted (VISTA) sequence and fast field echo (FFE) sequence with 2D TSE intermediate-weighted sequence of the wrist joint at 3.0 T.

MATERIAL AND METHODS

MRI was performed in 10 wrists of 10 healthy volunteers with isotropic 3D sequences (VISTA and FFE) and 2D TSE intermediate-weighted sequences at 3.0 T. The signal-to-noise ratio (SNR) was obtained by imaging phantom and noise-only image. Contrast ratios (CRs) were calculated between fluid and cartilage, triangular fibrocartilage complex (TFCC), and the scapholunate ligament. Two radiologists independently assessed the visibility of TFCC, carpal ligaments, cartilage, tendons and nerves with a four-point grading scale. Statistical analysis to compare CRs (one way ANOVA with a Tukey test) and grades of visibility (Kruskal-Wallis test) between three sequences and those for inter-observer agreement (kappa analysis) were performed.

RESULTS

The SNR of 2D TSE (46.26) was higher than those of VISTA (23.34) and 3D FFE (19.41). CRs were superior in 2D TSE than VISTA (P = 0.02) for fluid-cartilage and in 2D TSE than 3D FFE (P < 0.01) for fluid-TFCC. The visibility was best in 2D TSE (P < 0.01) for TFCC and in VISTA (P = 0.01) for scapholunate ligament. The visibility was better in 2D TSE and 3D FFE (P = 0.04) for cartilage and in VISTA than 3D FFE (P < 0.01) for TFCC. The inter-observer agreement for the visibility of anatomic structures was moderate or substantial.

CONCLUSION

Image quality of 2D TSE was superior to isotropic 3D MR imaging for cartilage, and TFCC. 3D FFE has better visibility for cartilage than VISTA and VISTA has superior visibility for TFCC to 3D FFE and the visibility for scapholunate ligament was best on VISTA.

Breast MR imaging at 3 T with dual-source radiofrequency transmission offers superior B1 homogeneity: an intraindividual comparison with breast MR imaging at 1.5 T

PURPOSE

To evaluate and compare B1 homogeneity for breast magnetic resonance (MR) imaging performed at 3 T with dual-source radiofrequency (RF) transmission to 1.5-T MR imaging and 3-T MR imaging with quadrature transmission.

MATERIALS AND METHODS

This prospective study received institutional review board approval and patients provided informed consent. Women (n = 25; mean age, 53 years; range, 30-68 years) suspected of having breast lesions underwent breast MR imaging examinations on comparable 1.5-T and 3-T clinical systems between February and May 2012. B1 maps were obtained at 1.5 T and at 3 T with quadrature and dual-source RF transmission. Intrabreast differences and differences in mean B1 values between right and left breasts were investigated by using two-sided multivariate analysis of variance with interaction; t tests were used to compare the differences between measured whole-breast mean B1 values and requested B1 values.

RESULTS

With quadrature transmission at 1.5 T and 3 T, the mean B1 values showed a statistically significant difference: left-breast measured B1 was -8.9% of requested B1 value at 1.5 T and -13.7% at 3 T (P < .001), whereas right-breast measured B1 was +5.4% of requested B1 value at 1.5 T (P < .001) and +2.7% at 3 T (P = .01). With dual-source RF transmission at 3 T, mean B1 values across the breasts were not statistically different, nor were the measured B1 values compared with requested B1 values (left breast, -0.6%; right breast, -0.7%). At 3 T with dual-source transmission, slight intrabreast local variations in B1 were recorded.

CONCLUSION

MR imaging at 3 T with dual-source RF transmission offered an overall B1 homogeneity for breast imaging that was better than that obtained at 1.5 T and with quadrature transmission.

Field shaping arrays: a means to address shading in high field breast MRI

PURPOSE

To develop a simple correction approach to mitigate shading in 3 Tesla (T) breast MRI.

MATERIALS AND METHODS

A slightly modified breast receive (Rx) array, which we termed field shaping array (FSA), was shown to mitigate breast shading at 3T. In this FSA, one Rx element was selectively unblocked and tuned off the Larmor frequency during the transmit (Tx) phase. The current flowing in this element during Tx created a secondary transmit field; the vector addition of this field and the one created directly by the body coil resulted in a more uniform excitation profile over the entire breast area. The receive Rx element was returned to its intended tuning during the Rx phase, ensuring unperturbed signal reception.

RESULTS

Using the FSA, improved Tx field uniformity, better fat suppression, increased image homogeneity and reduced power deposition was seen in all volunteers studied.

CONCLUSION

A simple modification of a standard breast Rx array, converting it to a field shaping array, was shown to mitigate breast shading in all volunteers studied.

Qualitative and quantitative assessment of isotropic ankle magnetic resonance imaging: three-dimensional isotropic intermediate-weighted turbo spin echo versus three-dimensional isotropic fast field echo sequences

Objective

To compare the image quality of volume isotropic turbo spin echo acquisition (VISTA) imaging method with that of the three-dimensional (3D) isotropic fast field echo (FFE) imaging method applied for ankle joint imaging.

Materials and Methods

MR imaging of the ankles of 10 healthy volunteers was performed with VISTA and 3D FFE sequences by using a 3.0 T machine. Two radiologists retrospectively assessed the tissue contrast between fluid and cartilage (F-C), and fluid and the Achilles tendon (F-T) with use of a 4-point scale. For a quantitative analysis, signal-to-noise ratio (SNR) was obtained by imaging phantom, and the contrast ratios (CRs) were calculated between F-T and F-C. Statistical analyses for differences in grades of tissue contrast and CRs were performed.

Results

VISTA had significantly superior grades in tissue contrast of F-T (p = 0.001). Results of 3D FFE had superior grades in tissue contrast of F-C, but these result were not statistically significant (p = 0.157). VISTA had significantly superior CRs in F-T (p = 0.002), and 3D FFE had superior CRs in F-C (p = 0.003). The SNR of VISTA was higher than that of 3D FFE (49.24 vs. 15.94).

Conclusion

VISTA demonstrates superior tissue contrast between fluid and the Achiles tendon in terms of quantitative and qualitative analysis, while 3D FFE shows superior tissue contrast between fluid and cartilage in terms of quantitative analysis.

Parallel imaging of the cervical spine at 3T: optimized trade-off between speed and image quality

BACKGROUND AND PURPOSE

Patients with cervical spine syndrome often experience pain during the MR examination. Our aim was to compare the quality of cervical spine MR images obtained by parallel imaging with those of nonaccelerated images, with the goal of shortening the examination time while preserving adequate image quality.

MATERIALS AND METHODS

A phantom study and examinations of 10 volunteers and 26 patients were conducted on a clinical 3T scanner. Acquisitions included axial T2WI, sagittal T2WI, T1WI, and T2TIRM sequences. Nonaccelerated sequences and accelerated sequences with different numbers of averages and different accelerations, with a scanning time reduction of 67%, were performed. For quantitative analysis, the SNR was obtained from the phantom measurements, and the NU was calculated from the volunteer measurements. For qualitative analysis, 3 independent readers assessed the delineation of anatomic structures in volunteers and the visibility of degenerative disease in patients.

RESULTS

In the phantom study, as expected, the SNR of the nonaccelerated images was higher than the SNR of the same sequence with parallel imaging. In vivo, the NU was higher when applying fewer averages or parallel imaging, compared with the nonaccelerated images. The analysis of the subjective parameters in the volunteers and patients showed that a scanning time of 48% of the original protocol could be obtained by combining the following sequences: sagittal T1WI with 1 average; sagittal T2WI with acceleration factor 3; sagittal T2TIRM with acceleration factor 2; and axial T2* GRE with acceleration factor 2.

CONCLUSIONS

Parallel imaging of the cervical spine at 3T allows shortening of the examination time by 52%, preserving adequate image quality.

Ultrahigh-field magnetic resonance imaging: the clinical potential for anatomy, pathogenesis, diagnosis and treatment planning in neck and spine disease

An increase of the magnetic field strength to ultrahigh-field yields advantageous as well as disadvantageous changes in physical effects. The beneficial increase in signal/noise ratio can be leveraged into higher spatiotemporal resolution, and an exacerbation of artifacts can impede ultrahigh-field imaging. With the successful introduction of intracranial and musculoskeletal imaging at 7 T, recent advances in coil design have created opportunities for further applications of ultrahigh-field magnetic resonance (MR) imaging in other parts of the body. Initial studies in 7 T neck and spine MR imaging have revealed promising insights and new challenges, demanding further research and methodological optimization.

Dual-source radiofrequency transmission with patient-adaptive local radiofrequency shimming for 3.0-T cardiac MR imaging: initial experience

PURPOSE

To evaluate the effect of dual-source parallel radiofrequency (RF) transmission with patient-adaptive local RF shimming on image quality, image contrast, and diagnostic confidence at routine clinical cardiac magnetic resonance (MR) imaging with use of a 3.0-T dual-channel transmit whole-body MR system.

MATERIALS AND METHODS

Written informed consent was obtained from all patients, and the study protocol was approved by the local institutional review board. Cardiac MR imaging was performed in 28 patients by using a 3.0-T MR unit equipped with a dual-source RF transmission system. The effect of conventional versus dual-source RF transmission on steady-state free precession (SSFP) cine sequences and turbo spin-echo (TSE) black-blood (BB) sequences was evaluated. The homogeneity of the B1 field and contrast-to-noise ratios (CNRs) were measured and tested for statistical significance with the paired t test. Images were analyzed qualitatively for homogeneity, the presence of off-resonance artifacts, and diagnostic confidence independently by two readers. Statistical significance was assessed with the Wilcoxon signed rank test. Inter- and intraobserver agreement was assessed with κ statistics.

RESULTS

Quantitative image analysis revealed that B1 homogeneity and CNR were significantly improved for images acquired with dual-source RF transmission compared with conventional RF transmission (P = .005). The quality of SSFP and TSE BB images of the left and the right ventricles showed a significant improvement with respect to image homogeneity and diagnostic confidence as evaluated by the readers (P = .0001, κ > 0.74). As a side effect, off-resonance artifacts were significantly reduced on SSFP images (P = .0001, κ > 0.76).

CONCLUSION

Dual-source parallel RF transmission significantly improves image homogeneity, image contrast, and diagnostic confidence compared with conventional RF transmission of cardiac SSFP and TSE BB sequences.

Improved B1 homogeneity of 3 Tesla breast MRI using dual-source parallel radiofrequency excitation

PURPOSE

To compare breast MRI B(1) homogeneity at 3 Tesla (T) with and without dual-source parallel radiofrequency (RF) excitation.

MATERIALS AND METHODS

After institutional review board approval, we evaluated 14 consecutive breast MR examinations performed at 3T that included three-dimensional B(1) maps created separately with conventional single-source and dual-source parallel RF excitation techniques. We measured B(1) values (expressed as % of intended B(1) ) on each B(1) map at nipple level in multiple bilateral locations: anterior, lateral, central, medial, and posterior. Mean whole breast and location specific B(1) values were calculated and compared between right and left breasts using paired t-test.

RESULTS

Mean whole breast B(1) values differed significantly between right and left breasts with standard single-source RF excitation (difference L-R, Δ = 9.2%; P < 0.001) but not with dual-source parallel RF excitation (Δ = 2.3%; P = 0.085). Location specific B(1) values differed significantly between right and left on single-source in the lateral (P = 0.014), central (P = 0.0001), medial (P = 0.0013), and posterior (P < 0.0001) locations. Conversely, mean B(1) values differed significantly on dual-source parallel RF excitation for only the anterior (P = 0.030) and lateral (P = 0.0003) locations.

CONCLUSION

B(1) homogeneity is improved with dual-source parallel RF excitation on 3T breast MRI when compared with standard single-source RF excitation technique.

3D isotropic turbo spin-echo intermediate-weighted sequence with refocusing control in knee imaging: comparison study with 3D isotropic fast-field echo sequence

BACKGROUND

Three-dimensional (3D) turbo spin-echo (TSE) images have been used in imaging of the extremities and comparable diagnostic performance to two-dimensional (2D) TSE images has been reported in several studies. However, comparison of the 3D isotropic TSE intermediate-weighted sequence and 3D FFE sequence in terms of image quality has not been investigated.

PURPOSE

To compare the image quality of a 3D isotropic TSE intermediate-weighted sequence with refocusing control (volume isotropic turbo spin echo acquisition [VISTA]) and a 3D isotropic fast-field echo (FFE) sequence of the knee joint.

MATERIAL AND METHODS

3.0 T knee MRI with VISTA and 3D FFE sequences was performed in 10 healthy volunteers (3 men, 7 women; age range 26-30 years). Two radiologists with specialties in the musculoskeletal system assessed tissue contrast between the fluid-cruciate ligament (F-L), fluid-meniscus (F-M), and fluid-cartilage (F-C) based on a 4-point scale (1, poor; 2, fair; 3, good; and 4, excellent). Statistical analysis for inter-observer agreement and differences in grades in tissue contrast between VISTA and 3D FFE images (Wilcoxon signed-rank) were performed. For a quantitative analysis, the signal-to-noise ratio (SNR) was obtained by imaging phantom and noise-only image. Image contrast ratios (CRs) were calculated between F-L, F-M, and F-C in volunteer images of VISTA and 3D FFE and compared statistically with a paired t-test.

RESULTS

Based on qualitative analysis, VISTA had statistically superior grades of tissue contrast in F-L (P < 0.001) and F-M (P < 0.001). 3D FFE had superior but not statistically significant (P = 0.317) grades in F-C. Based on quantitative analysis, the SNR of the phantom imaging was higher in VISTA than that in 3D FFE (28.18 vs. 14.90). VISTA had superior CRs in F-L (P < 0.001) and F-M (P < 0.001). 3D FFE had superior CR in F-C (P = 0.038).

CONCLUSION

The VISTA sequence was superior in tissue contrast between F-M and F-L, and 3D FFE was superior in tissue contrast between F-C, subjectively and quantitatively.