Numerical approach to investigate MR imaging artifacts from orthopedic implants at different field strengths according to ASTM F2119.

OBJECTIVE: This study presents an extended evaluation of a numerical approach to simulate artifacts of metallic implants in an MR environment. METHODS: The numerical approach is validated by comparing the artifact shape of the simulations and measurements of two metallic orthopedic implants at three different field strengths (1.5 T, 3 T, and 7 T). Furthermore, this study presents three additional use cases of the numerical simulation. The first one shows how numerical simulations can improve the artifact size evaluation according to ASTM F2119. The second use case quantifies the influence of different imaging parameters (TE and bandwidth) on the artifact size. Finally, the third use case shows the potential of performing human model artifact simulations. RESULTS: The numerical simulation approach shows a dice similarity coefficient of 0.74 between simulated and measured artifact sizes of metallic implants. The alternative artifact size calculation method presented in this study shows that the artifact size of the ASTM-based method is up to 50% smaller for complex shaped implants compared to the numerical-based approach. CONCLUSION: In conclusion, the numerical approach could be used in the future to extend MR safety testing according to a revision of the ASTM F2119 standard and for design optimization during the development process of implants.

Development and evaluation of a numerical simulation approach to predict metal artifacts from passive implants in MRI.

OBJECTIVE: This study presents the development and evaluation of a numerical approach to simulate artifacts of metallic implants in an MR environment that can be applied to improve the testing procedure for MR image artifacts in medical implants according to ASTM F2119. METHODS: The numerical approach is validated by comparing simulations and measurements of two metallic test objects made of titanium and stainless steel at three different field strengths (1.5T, 3T and 7T). The difference in artifact size and shape between the simulated and measured artifacts were evaluated. A trend analysis of the artifact sizes in relation to the field strength was performed. RESULTS: The numerical simulation approach shows high similarity (between 75% and 84%) of simulated and measured artifact sizes of metallic implants. Simulated and measured artifact sizes in relation to the field strength resulted in a calculation guideline to determine and predict the artifact size at one field strength (e.g., 3T or 7T) based on a measurement that was obtained at another field strength only (e.g. 1.5T). CONCLUSION: This work presents a novel tool to improve the MR image artifact testing procedure of passive medical implants. With the help of this tool detailed artifact investigations can be performed, which would otherwise only be possible with substantial measurement effort on different MRI systems and field strengths.

The traveling heads 2.0: Multicenter reproducibility of quantitative imaging methods at 7 Tesla.

OBJECT: This study evaluates inter-site and intra-site reproducibility at ten different 7 T sites for quantitative brain imaging. MATERIAL AND METHODS: Two subjects - termed the "traveling heads" - were imaged at ten different 7 T sites with a harmonized quantitative brain MR imaging protocol. In conjunction with the system calibration, MP2RAGE, QSM, CEST and multi-parametric mapping/relaxometry were examined. RESULTS: Quantitative measurements with MP2RAGE showed very high reproducibility across sites and subjects, and errors were in concordance with previous results and other field strengths. QSM had high inter-site reproducibility for relevant subcortical volumes. CEST imaging revealed systematic differences between the sites, but reproducibility was comparable to results in the literature. Relaxometry had also very high agreement between sites, but due to the high sensitivity, differences caused by different applications of the B1 calibration of the two RF coil types used were observed. CONCLUSION: Our results show that quantitative brain imaging can be performed with high reproducibility at 7 T and with similar reliability as found at 3 T for multicenter studies of the supratentorial brain.

Radiofrequency induced heating around aneurysm clips using a generic birdcage head coil at 7 Tesla under consideration of the minimum distance to decouple multiple aneurysm clips.

PURPOSE: To evaluate radiofrequency (RF) induced tissue heating around aneurysm clips during a 7T head MR examination and to determine the decoupling distance between multiple implanted clips. METHODS: A total of 120 RF exposure scenarios of clinical relevance were studied using specific absorption rate and temperature simulations. Variations between scenarios included 2 clips (18.8 and 51.5 mm length), 2 MR-operating modes, 2 head models, and 3 thermoregulation models. Furthermore, a conservative approach was developed to allow for safe scans of patients with aneurysm clips even if detailed information on the implanted clip is unknown. A dedicated simulation-based approach was applied to determine the decoupling distance between multiple implanted clips. RESULTS: For all 60 clinical scenarios with the 18.8-mm-long clip, the absolute tissue temperature remained below regulatory limits. For 15 of 60 scenarios with the 51.5-mm-long clip, limits were slightly exceeded (less than 1°C). The conservative approach led to a maximum time-averaged input power of the RF coil of 3.3W. The corresponding B1+ is 1.32 µT. A decoupling distance of 35 mm allows the aneurysm clips to be treated as uncoupled from one other. CONCLUSION: Safe scanning conditions with respect to RF-induced heating can be applied for single or decoupled aneurysm clips in a 7T ultra-high field MRI setting. Multiple aneurysm clips separated by less than 35 mm need further investigations.

Abdominopelvic 1.5-T and 3.0-T MR Imaging in Healthy Volunteers: Relationship to Formation of DNA Double-Strand Breaks.

Purpose To investigate the relationship between abdominopelvic magnetic resonance (MR) imaging and formation of DNA double-strand breaks (DSBs) in peripheral blood lymphocytes among a cohort of healthy volunteers. Materials and Methods Blood samples were obtained from 40 healthy volunteers (23 women and 17 men; mean age, 27.2 years [range, 21-37 years]) directly before and 5 and 30 minutes after abdominopelvic MR imaging performed at 1.5 T (n = 20) or 3.0 T (n = 20). The number of DNA DSBs in isolated blood lymphocytes was quantified after indirect immunofluorescent staining of a generally accepted DSB marker, γ-H2AX, by means of high-throughput automated microscopy. As a positive control of DSB induction, blood lymphocytes from six volunteers were irradiated in vitro with x-rays at a dose of 1 Gy (70-90 keV). Statistical analysis was performed by using a Friedman test. Results No significant alteration in the frequency of DNA DSB induction was observed after MR imaging (before imaging: 0.22 foci per cell, interquartile range [IQR] = 0.54 foci per cell; 5 minutes after MR imaging: 0.08 foci per cell, IQR = 0.39 foci per cell; 30 minutes after MR imaging: 0.09 foci per cell, IQR = 0.63 foci per cell; P = .057). In vitro radiation of lymphocytes with 1 Gy led to a significant increase in DSBs (0.22 vs 3.43 foci per cell; P = .0312). The frequency of DSBs did not differ between imaging at 1.5 T and at 3.0 T (5 minutes after MR imaging: 0.23 vs 0.06 foci per cell, respectively [P = .57]; 30 minutes after MR imaging: 0.12 vs 0.08 foci per cell [P = .76]). Conclusion Abdominopelvic MR imaging performed at 1.5 T or 3.0 T does not affect the formation of DNA DSBs in peripheral blood lymphocytes.

In vitro and in silico assessment of RF-induced heating around intracranial aneurysm clips at 7 Tesla.

PURPOSE: To examine radiofrequency-induced tissue heating around intracranial aneurysm clips during a 7 Tesla (T) head MR examination. METHODS: Radiofrequency (RF), temperature simulations, and RF measurements were employed to investigate the effects of polarization and clip length on the electric field (E-field) and temperature. Heating in body models was studied using both a conservative approach and realistic exposure scenarios. RESULTS: Worst-case orientation was found for clips aligned parallel to the E-field polarization. Absolute tissue temperature remained below International Electrotechnical Commission regulatory limits for 44 of 50 clinical scenarios. No significant effect on heating was determined for clip lengths below 18.8 mm, and worst-case heating was found for clip length 51.5 mm. The conservative approach led to a maximum permissible E-field of 72 V/m corresponding to B1+ of 1.2 µT, and an accepted power of 4.6 W for the considered RF head coil instead of 38.5 W without clip. CONCLUSION: Safe scanning conditions with respect to RF-induced heating can be applied depending on the information about the clip gained during screening interviews. However, force and torque measurements in the MR system shall be conducted to give a final statement on the MR safety of aneurysm clips at 7T. Magn Reson Med 79:568-581, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Non-enhanced versus low-dose contrast-enhanced renal magnetic resonance angiography at 7 T: a feasibility study.

Background Considering the currently reported association between a repetitive application and cumulative dosage of Gadolinium (Gd)-based contrast agents and Gd-deposition in brain tissue as well as the risk for the advent of nephrogenic systemic fibrosis (NSF), techniques allowing for a dose reduction become an important key aspect aside from non-enhanced magnetic resonance angiography (MRA) techniques. Thus, this study was focused on the reduction and/or complete omission of contrast agent for renal MRA at 7T. Purpose To evaluate the performance of time-of-flight MRA versus low-dose contrast-enhanced (CE) renal MRA at 7T. Material and Methods Ten healthy volunteers were examined on a 7T MR system comprising a TOF MRA and three-dimensional (3D) fast low angle shot spoiled gradient-echo sequence (FLASH) MRA after administration of one-quarter of clinical dose of gadobutrol. Qualitative image analysis was performed including overall image quality, artery delineation and presence of artifacts. Contrast ratio (CR), signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of the renal arteries were calculated. Results TOF MRA and low-CE MRA achieved comparable overall ratings, with slightly superior delineation of the main renal arteries in TOF MRA (TOF = 3.10 ± 0.75, low-CE = 2.95 ± 0.75). Segmental branches outside and inside the parenchyma were delineated significantly better on TOF MRA. Quantitative analysis demonstrated the superiority of TOF MRA, yielding higher scores for CR, SNR, and CNR. Conclusion The initial results of our study demonstrate the feasibility and comparable diagnostic performance of TOF and low-dose CE renal MRA at 7T.

7T: Physics, safety, and potential clinical applications.

With more than 60 installed magnetic resonance imaging (MRI) systems worldwide operating at a magnetic field strength of 7T or higher, ultrahigh-field (UHF) MRI has been established as a platform for clinically oriented research in recent years. Profound technical and methodological developments have helped overcome the inherent physical challenges of UHF radiofrequency (RF) signal homogenization in the human body. The ongoing development of dedicated RF coil arrays was pivotal in realizing UHF body MRI, beyond mere brain imaging applications. Another precondition to clinical application of 7T MRI is the safety testing of implants and the establishment of safety concepts. Against this backdrop, 7T MRI and MR spectroscopy (MRS) recently have demonstrated capabilities and potentials for clinical diagnostics in a variety of studies. This article provides an overview of the immanent physical challenges of 7T UHF MRI and discusses recent technical solutions and safety concepts. Furthermore, recent clinically oriented studies are highlighted that span a broad application spectrum from 7T UHF brain to body MRI. LEVEL OF EVIDENCE: 4 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1573-1589.

7 Tesla quantitative hip MRI: T1, T2 and T2* mapping of hip cartilage in healthy volunteers.

OBJECTIVES: To evaluate the technical feasibility and applicability of quantitative MR techniques (delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), T2 mapping, T2* mapping) at 7 T MRI for assessing hip cartilage. METHODS: Hips of 11 healthy volunteers were examined at 7 T MRI with an 8-channel radiofrequency transmit/receive body coil using multi-echo sequences for T2 and T2* mapping and a dual flip angle gradient-echo sequence before (T10) and after intravenous contrast agent administration (T1Gd; 0.2 mmol/kg Gd-DTPA(2-) followed by 0.5 h of walking and 0.5 h of rest) for dGEMRIC. Relaxation times of cartilage were measured manually in 10 regions of interest. Pearson's correlations between R1delta = 1/T1Gd - 1/T10 and T1Gd and between T2 and T2* were calculated. Image quality and the delineation of acetabular and femoral cartilage in the relaxation time maps were evaluated using discrete rating scales. RESULTS: High correlations were found between R1delta and T1Gd and between T2 and T2* relaxation times (all p < 0.01). All techniques delivered diagnostic image quality, with best delineation of femoral and acetabular cartilage in the T2* maps (mean 3.2 out of a maximum of 4 points). CONCLUSIONS: T1, T2 and T2* mapping of hip cartilage with diagnostic image quality is feasible at 7 T. To perform dGEMRIC at 7 T, pre-contrast T1 mapping can be omitted. KEY POINTS: • dGEMRIC of hip cartilage with diagnostic image quality is feasible at 7 T. • To perform dGEMRIC at 7 T, pre-contrast T1 mapping can be omitted. • T2(*) mapping of hip cartilage with diagnostic image quality is feasible at 7 T. • T2 and T2* relaxation times of cartilage were highly correlated at 7 T. • Best delineation of femoral and acetabular cartilage was found in T2* maps.

Morphological imaging and T2 and T2* mapping of hip cartilage at 7 Tesla MRI under the influence of intravenous gadolinium.

OBJECTIVES: To investigate the influence of intravenous gadolinium on cartilage T2 and T2* relaxation times and on morphological image quality at 7-T hip MRI. METHODS: Hips of 11 healthy volunteers were examined at 7 T. Multi-echo sequences for T2 and T2* mapping, 3D T1 volumetric interpolated breath-hold examination (VIBE) and double-echo steady-state (DESS) sequences were acquired before and after intravenous application of gadolinium according to a delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) protocol. Cartilage relaxation times were measured in both scans. Morphological sequences were assessed quantitatively using contrast ratios and qualitatively using a 4-point Likert scale. Student's t-test, Pearson's correlation (ρ) and Wilcoxon sign-rank test were used for statistical comparisons. RESULTS: Pre- and post-contrast T2 and T2* values were highly correlated (T2: acetabular: ρ = 0.76, femoral: ρ = 0.77; T2*: acetabular: ρ = 0.80, femoral: ρ = 0.72). Gadolinium enhanced contrasts between cartilage and joint fluid in DESS and T1 VIBE according to the qualitative (p = 0.01) and quantitative (p < 0.001) analysis. The delineation of acetabular and femoral cartilage and the labrum predominantly improved with gadolinium. CONCLUSIONS: Gadolinium showed no relevant influence on T2 or T2* relaxation times and improved morphological image quality at 7 T. Therefore, morphological and quantitative sequences including dGEMRIC can be conducted in a one-stop-shop examination. KEY POINTS: • Hip cartilage T2 values correlate highly before and after gadolinium at 7 T • Hip cartilage T2* values correlate highly before and after enhancement at 7 T • Morphological hip cartilage imaging benefits from intravenous gadolinium at 7 T • The delineation of acetabular and femoral cartilage can be improved by gadolinium • Morphological and quantitative sequences including dGEMRIC can be combined as a one-stop-shop examination.

7 Tesla quantitative hip MRI: a comparison between TESS and CPMG for T2 mapping.

OBJECTIVES: We aimed to evaluate the feasibility of triple-echo steady state (TESS) T2 mapping as an alternative to conventional multi-echo-spin-echo (CPMG) T2 mapping for the quantitative assessment of hip joint cartilage at 7 T. MATERIALS AND METHODS: A total of eight healthy volunteers and three patients were included. Reproducibility of both techniques was evaluated in five volunteers in five scans each. T2 relaxation times were measured by manually drawing regions of interest in multiple regions of the hip joint. Data from both methods were compared using Pearson correlation coefficient, intra-class correlation coefficient, and coefficient of repeatability. The overall image quality and presence of artifacts was assessed. RESULTS: Cartilage transplant and surrounding fluid were well depicted by both methods. Compared to CPMG, TESS provided systematically reduced T2 values (43.3 ± 7.3 vs. 19.2 ± 5.5 ms for acetabular cartilage, and 41.4 ± 5.6 vs. 21.7 ± 5.2 ms for femoral cartilage), in line with previously reported values. No correlation between both methods was found. TESS yielded a slightly better reproducibility than CPMG, while CPMG showed pronounced sensitivity to B1 inhomogeneities. CONCLUSION: TESS seems to be an attractive alternative to CPMG for improvements in quantitative hip joint imaging at 7 T, allowing shortening of the total acquisition time paired with insensitivity to B1, while rendering comparable image quality with good repeatability.

Cranial fixation plates in cerebral magnetic resonance imaging: a 3 and 7 Tesla in vivo image quality study.

OBJECTIVE: This study assesses and quantifies impairment of postoperative magnetic resonance imaging (MRI) at 7 Tesla (T) after implantation of titanium cranial fixation plates (CFPs) for neurosurgical bone flap fixation. MATERIALS AND METHODS: The study group comprised five patients who were intra-individually examined with 3 and 7 T MRI preoperatively and postoperatively (within 72 h/3 months) after implantation of CFPs. Acquired sequences included T1-weighted magnetization-prepared rapid-acquisition gradient-echo (MPRAGE), T2-weighted turbo-spin-echo (TSE) imaging, and susceptibility-weighted imaging (SWI). Two experienced neurosurgeons and a neuroradiologist rated image quality and the presence of artifacts in consensus reading. RESULTS: Minor artifacts occurred around the CFPs in MPRAGE and T2 TSE at both field strengths, with no significant differences between 3 and 7 T. In SWI, artifacts were accentuated in the early postoperative scans at both field strengths due to intracranial air and hemorrhagic remnants. After resorption, the brain tissue directly adjacent to skull bone could still be assessed. Image quality after 3 months was equal to the preoperative examinations at 3 and 7 T. CONCLUSION: Image quality after CFP implantation was not significantly impaired in 7 T MRI, and artifacts were comparable to those in 3 T MRI.

The traveling heads: multicenter brain imaging at 7 Tesla.

OBJECTIVE: This study evaluates the inter-site and intra-site reproducibility of 7 Tesla brain imaging and compares it to literature values for other field strengths. MATERIALS AND METHODS: The same two subjects were imaged at eight different 7 T sites. MP2RAGE, TSE, TOF, SWI, EPI as well as B1 and B0 field maps were analyzed quantitatively to assess inter-site reproducibility. Intra-site reproducibility was measured with rescans at three sites. RESULTS: Quantitative measures of MP2RAGE scans showed high agreement. Inter-site and intra-site reproducibility errors were comparable to 1.5 and 3 T. Other sequences also showed high reproducibility between the sites, but differences were also revealed. The different RF coils used were the main source for systematic differences between the sites. CONCLUSION: Our results show for the first time that multi-center brain imaging studies of the supratentorial brain can be performed at 7 T with high reproducibility and similar reliability as at 3T. This study develops the basis for future large-scale 7 T multi-site studies.

[(18)F]FDG PET/MRI vs. PET/CT for whole-body staging in patients with recurrent malignancies of the female pelvis: initial results.

PURPOSE: To evaluate the diagnostic potential of PET/MRI with [(18)F]FDG in recurrent ovarian and cervical cancer in comparison to PET/CT. METHODS: A group of 19 patients with suspected recurrence of pelvic malignancies (ovarian cancer, 11 patients; cervical cancer, 8 patients) scheduled for an [(18)F]FDG PET/CT were subsequently enrolled for a PET/MRI. The scan protocol comprised: (1) a T1-W axial VIBE after contrast agent adminstration, (2) an axial T2-W HASTE, (3) a coronal TIRM, (4) an axial DWI, and dedicated MR sequences of the female pelvis including (5) a T1-W VIBE before contrast agent adminstration, (6) a sagittal T2-W TSE, and (7) a sagittal T1-W dynamic VIBE. The datasets (PET/CT, PET/MRI) were rated separately by two readers regarding lesion count, lesion localization, lesion conspicuity (four-point scale), lesion characterization (benign/malignant/indeterminate) and diagnostic confidence (three-point scale). All available data (histology, prior examinations, PET/CT, PET/MRI, follow-up examinations) served as standard of reference. Median values were compared using the Wilcoxon rank sum test. RESULTS: Metastatic lesions were present in 16 of the 19 patients. A total of 78 lesions (malignant, 58; benign, 20) were described. Both PET/CT and PET/MRI allowed correct identification of all malignant lesions and provided equivalent conspicuity (3.86 ± 0.35 for PET/CT, 3.91 ± 0.28 for PET/MRI; p > 0.05). Diagnostic confidence was significantly higher for PET/MRI in malignant (p < 0.01) and benign lesions (p < 0.05). CONCLUSION: Both PET/CT and PET/MRI offer an equivalently high diagnostic value for recurrent pelvic malignancies. PET/MRI offers higher diagnostic confidence in the discrimination of benign and malignant lesions. Considering the reduced radiation dose and superior lesion discrimination, PET/MRI may serve as a powerful alternative to PET/CT in the future.

MRI at 7 Tesla and above: demonstrated and potential capabilities.

With more than 40 installed MR systems worldwide operating at 7 Tesla or higher, ultra-high-field (UHF) imaging has been established as a platform for clinically oriented research in recent years. Along with technical developments that, in part, have also been successfully transferred to lower field strengths, MR imaging and spectroscopy at UHF have demonstrated capabilities and potentials for clinical diagnostics in a variety of studies. In terms of applications, this overview article focuses on already achieved advantages for in vivo imaging, i.e., in imaging the brain and joints of the musculoskeletal system, but also considers developments in body imaging, which is particularly challenging. Furthermore, new applications for clinical diagnostics such as X-nuclei imaging and spectroscopy, which only really become feasible at ultra-high magnetic fields, will be presented.

3 Tesla high-resolution and delayed gadolinium enhanced MR imaging of cartilage (dGEMRIC) after autologous chondrocyte transplantation in the hip.

BACKGROUND: To evaluate the feasibility of 3 Tesla (T) high-resolution and gadolinium enhanced MRI of cartilage (dGEMRIC) in the thin and rounded hip cartilage of patients after acetabular matrix-based autologous chondrocyte transplantation (MACT). METHODS: Under general ethics approval, 24 patients were prospectively examined 6-31 months after acetabular MACT at 3T using high-resolution proton-density weighted (PDw) images (bilateral PD SPACE, 0.8 mm isotropic; unilateral PD-TSE coronal/sagittal, 0.8 × 0.8 resp. 0.5 × 0.5 × 2.5 mm) as well as T1 mapping (3D-FLASH, 0.78 mm isotropic) in dGEMRIC technique, and clinically scored. The cartilage transplant was evaluated using an adapted MOCART score (maximum 85 points). T1 relaxation times were measured independently by two radiologists. Here, regions of interest were placed manually in automatically calculated relaxation-maps, both in the transplant and adjacent healthy cartilage regions. Interobserver reliability was estimated by means of intraclass-correlation (ICC). RESULTS: The transplant was morphologically definable in the PDw images of 23 patients with a mean MOCART score of 69 points (60-80 points, SD 6.5). T1 maps showed a clear differentiation between acetabular and femoral cartilage, but correlation with PDw images was necessary to identify the transplant. Mean T1 relaxation times of the transplant were 616.3 ms (observer 1) resp. 610.1 ms (observer 2), and of adjacent healthy acetabular cartilage 574.5 ms (observer 1) resp. 604.9 ms (observer 2). Interobserver reliability of the relaxation times in the transplant was excellent (ICC-coefficient 0.88) and in adjacent healthy regions good (0.77). CONCLUSION: High-resolution PDw imaging with adapted MOCART scoring and dGEMRIC is feasible after MACT in the thin and rounded hip cartilage.

Experience with magnetic resonance imaging of human subjects with passive implants and tattoos at 7 T: a retrospective study.

OBJECT: Over the last decade, the number of clinical MRI studies at 7 T has increased dramatically. Since only limited information about the safety of implants/tattoos is available at 7 T, many centers either conservatively exclude all subjects with implants/tattoos or have started to perform dedicated tests for selected implants. This work presents our experience in imaging volunteers with implants/tattoos at 7 T over the last seven and a half years. MATERIALS AND METHODS: 1796 questionnaires were analyzed retrospectively to identify subjects with implants/tattoos imaged at 7 T. For a total of 230 subjects, the type of local transmit/receive RF coil used for examination, imaging sequences, acquisition time, and the type of implants/tattoos and their location with respect to the field of view were documented. These subjects had undergone examination after careful consideration by an internal safety panel consisting of three experts in MR safety and physics. RESULTS: None of the subjects reported sensations of heat or force before, during, or after the examination. None expressed any discomfort related to implants/tattoos. Artifacts were reported in 52% of subjects with dental implants; all artifacts were restricted to the mouth area and did not affect image quality in the brain parenchyma. CONCLUSION: Our initial experience at 7 T indicates that a strict rejection of subjects with tattoos and/or implants is not justified. Imaging can be conditionally performed in carefully selected subjects after collection of substantial safety information and evaluation of the detailed exposure scenario (RF coil/type and position of implant). Among the assessed subjects with tattoos, no side effects from the exposure to 7 T MRI were reported.

Aspects of postoperative magnetic resonance imaging of patients with avascular necrosis of the femoral head, treated by advanced core decompression.

OBJECTIVE: To analyze remodeling processes after advanced core decompression (ACD) in patients with avascular femoral head necrosis by means of 3T MRI and to identify indicators for clinical outcome considering the defect size and characteristics of the bone graft and of the neighboring regeneration tissue. MATERIALS AND METHODS: Thirty-four hips, with preexisting preoperative MRIs in 21 cases, were examined 1-34 months (mean 12.7) postoperatively by 3T MRI. The volume of necrosis was measured manually pre- and postoperatively to calculate absolute as well as percentage necrosis reduction. The signal intensity of the bone graft was quantified using a 4-point scale. Border phenomena between the bone graft and bone were described and classified into groups. Wilcoxon sign-rank test was used to identify correlations between the analyzed items and clinical signs of femoral head collapse after a mean follow-up time of 28.6 months (10.4-46.8). RESULTS: Mean percentage reduction of necrosis was significantly higher in asymptomatic patients (59.36%) compared to patients with signs of femoral head collapse (28.78%, p = 0.008). Signal intensity of the bone graft increased in T1w and T2w TIRM sequences over time after surgery and was significantly higher in asymptomatic patients. Five border phenomena between the bone graft and healthy bone were identified. Among them, the so-called "rail sign" representing three layers of remodeling tissue correlated with the histological observations. CONCLUSION: A variety of border phenomena representing remodeling processes have been described using 3T MRI. Beneath the percentage amount of necrosis reduction, we identified the signal intensity of the bone graft as an indicator for clinical outcome.

Usefulness of metal artifact reduction with WARP technique at 1.5 and 3T MRI in imaging metal-on-metal hip resurfacings.

OBJECTIVE: To evaluate the usefulness of the metal artifact reduction technique "WARP" in the assessment of metal-on-metal hip resurfacings at 1.5 and 3T in the context of image quality and imaging speed. MATERIALS AND METHODS: Nineteen patients (25 hip resurfacings) were randomized for 1.5 and 3T MRI, both including T1 and T2 turbo spin-echo as well as turbo inversion recovery magnitude sequences with and without view angle tilting and high bandwidth. Additional 3T sequences were acquired with a reduced number of averages and using the parallel acquisition technique for accelerating imaging speed. Artifact size (diameter, area), image quality (5-point scale) and delineation of anatomical structures were compared among the techniques, sequences and field strengths using the Wilcoxon sign-rank and paired t-test with Bonferroni correction. RESULTS: At both field strengths, WARP showed significant superiority over standard sequences regarding image quality, artifact size and delineation of anatomical structures. At 3T, artifacts were larger compared to 1.5T without affecting diagnostic quality, and scanning time could be reduced by up to 64 % without quality degradation. CONCLUSION: WARP proved useful in imaging metal-on-metal hip resurfacings at 1.5T as well as 3T with better image quality surrounding the implants. At 3T imaging could be considerably accelerated without losing diagnostic quality.

Feasibility of T2 -weighted turbo spin echo imaging of the human prostate at 7 tesla.

PURPOSE: To demonstrate that high quality T2 -weighted (T2w) turbo spin-echo (TSE) imaging of the complete prostate can be achieved routinely and within safety limits at 7 T, using an external transceive body array coil only. METHODS: Nine healthy volunteers and 12 prostate cancer patients were scanned on a 7 T whole-body system. Preparation consisted of B0 and radiofrequency shimming and localized flip angle calibration. T1 and T2 relaxation times were measured and used to define the T2w-TSE protocol. T2w imaging was performed using a TSE sequence (pulse repetition time/echo time 3000-3640/71 ms) with prolonged excitation and refocusing pulses to reduce specific absorption rate. RESULTS: High quality T2w TSE imaging was performed in less than 2 min in all subjects. Tumors of patients with gold-standard tumor localization (MR-guided biopsy or prostatectomy) were well visualized on 7 T imaging (n = 3). The number of consecutive slices achievable within a 10-g averaged specific absorption rate limit of 10 W/kg was ≥28 in all subjects, sufficient for full prostate coverage with 3-mm slices in at least one direction. CONCLUSION: High quality T2w TSE prostate imaging can be performed routinely and within specific absorption rate limits at 7 T with an external transceive body array.