RF heating due to conductive wires during MRI depends on the phase distribution of the transmit field.

In many studies concerning wire heating during MR imaging, a "resonant wire length" that maximizes RF heating is determined. This may lead to the nonintuitive conclusion that adding more wire, so as to avoid this resonant length, will actually improve heating safety. Through a theoretical analysis using the method of moments, we show that this behavior depends on the phase distribution of the RF transmit field. If the RF transmit field has linear phase, with slope equal to the real part of the wavenumber in the tissue, long wires always heat more than short wires. In order to characterize the intrinsic safety of a device without reference to a specific body coil design, this maximum-tip heating phase distribution must be considered. Finally, adjusting the phase distribution of the electric field generated by an RF transmit coil may lead to an "implant-friendly" coil design.     

RF safety of wires in interventional MRI: using a safety index.

With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wire-shaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value, assuming a straight wire, a homogeneous RF coil, and a wire that does not extend out of the tissue. The local specific absorption rate (SAR) amplification from induced currents on insulated and bare wires was calculated using the method of moments. This SAR gain was combined with a semianalytic solution to the bioheat transfer equation to generate a safety index. The safety index ( degrees C/(W/kg)) is a measure of the in vivo temperature change that can occur with the wire in place, normalized to the SAR of the pulse sequence. This index can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. For the case of a straight resonant wire in a tissue with very low perfusion, only about 100 mW/kg/ degrees C spatial peak SAR may be used at 1.5 T. But for < or =10-cm wires with an insulation thickness > or =30% of the wire radius that are placed in well-perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. Further model development to include the influence of inhomogeneous RF, curved wires, and wires that extend out of the sample are required to generate safety indices that are applicable to common clinical situations. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase.     

Measuring local RF heating in MRI: Simulating perfusion in a perfusionless phantom

PURPOSE: To overcome conflicting methods of local RF heating measurements by proposing a simple technique for predicting in vivo temperature rise by using a gel phantom experiment. MATERIALS AND METHODS: In vivo temperature measurements are difficult to conduct reproducibly; fluid phantoms introduce convection, and gel phantom lacks perfusion. In the proposed method the local temperature rise is measured in a gel phantom at a timepoint that the phantom temperature would be equal to the perfused body steady-state temperature value. The idea comes from the fact that the steady-state temperature rise in a perfused body is smaller than the steady-state temperature increase in a perfusionless phantom. Therefore, when measuring the temperature on a phantom there will be the timepoint that corresponds to the perfusion time constant of the body part. RESULTS: The proposed method was tested with several phantom and in vivo experiments. Instead, an overall average of 30.8% error can be given as the amount of underestimation with the proposed method. This error is within the variability of in vivo experiments (45%). CONCLUSION: With the aid of this reliable temperature rise prediction the amount of power delivered by the scanner can be controlled, enabling safe MRI examinations of patients with implants.     

Caring for patients with spinal metastasis during an MRI examination

IntroductionMagnetic resonance imaging (MRI) is without question the best tool used for diagnosing and evaluating spinal metastasis. An MRI examination is known to be of great value for the treatment planning and survival of these patients. Radiographers have an important role in how the quality of care is experienced by the patients during an MRI examination. The purpose of the study was to describe the radiographers’ perceptions of caring for patients with spinal metastasis during an examination with MRI.MethodsPhenomenography was used to analyze the data in this study. Ten radiographers, one male and nine females were interviewed about their perception of caring for patients with spinal metastasis during an MRI examination.ResultsThe findings showed that the radiographers’ caring perspective influenced their approach towards what they consider to be essential in the care of patients with spinal metastasis. This can impact the extent of the adjustment to the care needs of the patients. Furthermore, the findings showed that there was a strong connection between the radiographers’ care approach and preparedness to personalize the care.ConclusionThis study shows that it is important to be flexible when providing care for the patients. A person-centered care is achieved when the caring perspective is based on the patient’s view and adjustments are made in agreement with the patient.     

Induced magnetic field gradients and forces in the human head in MRI

PURPOSE: To map the induced magnetic field gradients and estimate the magnetic force in the human head during magnetic resonance imaging at 4 Tesla (T). MATERIALS AND METHODS: The magnetic field distribution in the human head was measured using two gradient-echo experiments with different echo times. The phase of the complex image ratio removed the wrapping artifact, characteristic of phase images, and was used to map the magnetic field distribution and calculate the accurate maps of the magnetic field gradients in the human head. RESULTS: The time-independent gradient fields induced by air/tissue interfaces in the head can be 50 times larger than those resulting from the magnetic field inhomogeneity of the MRI magnet. However, the associated magnetic force in the brain is by far smaller than the gravitational force. CONCLUSION: The induced gradient fields increase the magnetic force on tissues. However, even for tissue components with large magnetic susceptibility such as iron-containing proteins, this force is negligible compared with the gravitational force. Therefore, this study suggests that static and uniform magnetic fields do not have a significant risk for the tissues in the head.     

Contrast visibility for indirect MR arthrography with different protein contents and agent relaxivities at different field strengths: an in vitro model

Objectives

Protein binding and relaxivity are major determinants of the relative effectiveness of an MR arthrographic contrast agent. We sought to evaluate the optimal concentrations of high and usual relaxivity agents in two different proteinous environments at variable field strength for two MR contrast agents of different relaxivities.

Materials and methods

At 1.5, 3.0 and 7.0 T, gadobenate dimeglumine (Multihance) with high-relaxivity in proteinous environment and gadoteridol (Prohance) with more typical behavior were studied at 1.25, 2.5, 5, and 10 mmol in 1.7 g/dL and 3 g/dL albumin (mimicking protein content of normal and inflammatory synovial fluids, respectively) vs. pure normal saline, as a control. Analysis of image signal intensity (SI) and relaxivity values was done.

Results

In our study a change in concentration had no significant effect on T1 SI. In contrast, nearly every change in concentration led to a significant change in T2 SI. In 1.25 mmol concentration, there was no effect on T1 SI of either protein concentrations while higher concentrations showed significant decreased SI in either protein carrier compared to saline. The SI of Gadoteridol was significantly higher (p < 0.0001) than that of gadobenate at each of 3 T and 7 T, but was significantly lower (p < 0.001) at 1.5 T in saline solution while this was not significant for either protein carrier.Both protein carriers had significant effect on T1 (p = 0.0124) and T2 (p = 0.0118) relaxivities. Also solution concentration significantly (p < 0.01) affected both T1 and T2 relaxivities. Field strength did not affect T1 relaxivity (p = 0.02511) while it significantly affected T2 relaxivity (p < 0.001). This was significant (p = 0.035) in case of gadoteridol at 3 T.

Conclusion

1.25 mmol concentration of both gadoteridol and gadobenate solutions yields the best diagnostic T1 SI specially in higher fields (3 T and 7 T) and avoid the deleterious effect of increasing concentration on T2 SI. Gadoteridol is suggested on 3 T field indirect MR arthrograms. Protein had no positive effect on either SI or relaxivities in any joint model.     

Pacemaker lead tip heating in abandoned and pacemaker-attached leads at 1.5 Tesla MRI

Purpose

To assess the risk of RF‐induced heating in pacemaker‐attached and abandoned leads using in vitro temperature measurements at 1.5 Tesla as a function of lead length.

Materials and Methods

Five custom lead lengths, 20–60 cm, were exposed to a uniform magnitude and phase radiofrequency electric field to examine the effect of lead length on pacemaker lead tip heating for pacemaker‐attached and abandoned pacemaker leads.

Results

Abandoned and pacemaker‐attached leads show resonant heating behavior and maximum heating occurs at different lead lengths due to the differences in termination conditions. For clinical lead lengths (40–60 cm) abandoned leads exhibited greater lead tip heating compared with pacemaker‐attached leads.

Conclusion

Current recommendations for MRI pacemaker safety should highlight the possible increased risk for patients with abandoned leads as compared to pacemaker‐attached leads. J. Magn. Reson. Imaging 2011;33:426–431. © 2011 Wiley‐Liss, Inc.     

两种不同内固定材料和植骨材料治疗胸腰椎椎体骨折的疗效分析

目的：研究经后路复位内固定并行伤椎植骨对胸、腰椎骨折的治疗效果。方法：对椎体压缩性骨折和爆裂性骨折使用AF或钉棒系统复位固定后,经伤椎椎弓根向伤椎植入自固化磷酸钙人工骨和同种异体骨的治疗结果进行分析。结果：治疗432例468椎,手术后椎体高度即刻恢复100%的319椎;95%以上80椎;90%以上60椎;80%以上的9椎。398例获得随访,随访10月～8年,伤椎均获得骨性愈合,脊柱稳定性及运动功能恢复良好。结论：胸、腰椎椎体骨折经AF或钉棒系统复位固定后,伤椎植入自固化磷酸钙人工骨和同种异体骨的疗效确切,效果优良。     

Correlation between spinal cord MRI and clinical features in patients with demyelinating disease

Localisation of spinal cord lesions by MRI was correlated with neurological symptoms and signs in 16 patients with clinical and laboratory evidence of multiple sclerosis. There was good correspondence between spinal cord lesions and motor tract signs. On the other hand, superficial or deep sensory disturbances correlated with spinal cord lesions in only about a quarter of the patients. MRI of the spinal cord appeared to explain the myelopathy in 11 patients, while in 3 there was strong clinical evidence of more extensive demyelinating lesions. In 7 of the 16 patients MRI of the brain was normal.     

Comparative study of fluorodeoxyglucose positron emission tomography and magnetic resonance imaging for the detection of spinal bone marrow infiltration in untreated patients with multiple myeloma

Background: The presence and extent of osteolytic bone lesions in untreated patients with multiple myeloma are important factors in the staging of the disease, and the extent of bone lesions in multiple myeloma cases significantly influences decisions regarding therapy. Recently, fluorodeoxyglucose positron emission tomography (FDG-PET) and magnetic resonance imaging (MRI) have been used to detect bone marrow involvement in patients with multiple myeloma.

Purpose: To compare the efficacy of FDG-PET and MRI for the detection of bone marrow infiltration into the spine in untreated patients with multiple myeloma.

Material and Methods: Twenty-two patients with multiple myeloma underwent both FDG-PET and spine MRI. The examined spinal regions by MRI included 21 thoracic and lumbar spines, one lumbar spine, and 12 cervical spines. The following imaging sequences were performed: T1-weighted spin-echo MRI with and without fat suppression, and T2-weighted spin-echo MRI in the sagittal plane. In the patients with bone marrow abnormalities, an additional contrast-enhanced T1-weighted spin-echo MR image and a fat-suppressed T1-weighted spin-echo MR image were obtained. Patients were divided into three groups on the basis of the criteria defined by Durie and Salmon: stage I (n=9), stage II (n=3), and stage III (n=10). The number and location of lesions detected in both FGD-PET and MRI were recorded, and the lesions were compared using the McNemar test. Bone marrow biopsy results, the patient's clinical examinations, and other imaging findings (MRI, FDG-PET, etc.) were used as references.

Results: In stages I and II (37 lesions in 12 patients), FDG-PET and MRI detected lesions in 78% (29 of 37 lesions) and 86% (32 of 37 lesions), respectively. However, the difference between the abilities of FDG-PET and MRI to detect lesions was not statistically significant (P=0.317). In stage III (101 lesions in 10 patients), FDG-PET and MRI detected lesions in 80% (81 of 101 lesions) and 92% (93 of 101 lesions), respectively. The difference between the abilities of FDG-PET and MRI to detect lesions was statistically significant (P=0.038).

Conclusion: MRI is superior to FDG-PET in detecting bone marrow involvement in the spine of patients with advanced multiple myeloma.     

Intra- and inter-observer reliability of MRI examination of intervertebral disc abnormalities in patients with cervical myelopathy

PURPOSE: Intervertebral cervical disc herniation (CDH) is a relatively common disorder that can coexist with degenerative changes to worsen cervicogenic myelopathy. Despite the frequent disc abnormalities found in asymptomatic populations, magnetic resonance imaging (MRI) is considered excellent at detecting cervical spine myelopathy (CSM) associated with disc abnormality. The objective of this study was to investigate the intra- and inter-observer reliability of MRI detection of CSM in subjects who also had co-existing intervertebral disc abnormalities. MATERIALS AND METHODS: Seven experienced radiologists reviewed twice the MRI of 10 patients with clinically and/or imaging determined myelopathy. MRI assessment was performed individually, with and without operational guidelines. A Fleiss Kappa statistic was used to evaluate the intra- and inter-observer agreement. RESULTS: The study found high intra-observer percent agreement but relatively low Kappa values on selected variables. Inter-observer reliability was also low and neither observation was improved with operational guidelines. We believe that those low values may be associated with the base rate problem of Kappa. CONCLUSION: In conclusion, this study demonstrated high intra-observer percent agreement in MR examination for intervertebral disc abnormalities in patients with underlying cervical myelopathy, but differing levels of intra- and inter-observer Kappa agreement among seven radiologists.     

Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field.

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.