BOLD-fMRI response vs. transcranial magnetic stimulation (TMS) pulse-train length: testing for linearity

PURPOSE: To measure motor and auditory cortex blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) response to impulse-like transcranial magnetic stimulation (TMS) pulses as a function of train length. MATERIALS AND METHODS: Interleaved with fMRI at 1.5 T, TMS pulses 0.3-msec long were applied at 1 Hz to the motor cortex area for thumb. Six subjects were studied in a TR = 1 second session administering trains of 1, 2, 4, 8, and 16 pulses, and a TR = 3 seconds session administering trains of 1, 2, 4, 8, 16, and 24 pulses. A simple hemodynamic model with finite recovery and saturation was used to quantitatively characterize the BOLD-fMRI response as a function of train length. RESULTS: In both the activations directly induced in motor cortex by TMS and the indirect activations in auditory cortex caused by the sound of the TMS coil firing, the BOLD-fMRI responses to multiple pulses were well described by a summation of single-pulse impulse functions. CONCLUSION: Up to 24 discrete pulses, BOLD-fMRI response to 1 Hz TMS in both motor cortex and auditory cortex were consistent with a linear increase in amplitude and length with train length, possibly suggesting that stimuli of 1 to 2 seconds may be too long to represent impulses.     

On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging

PURPOSE: To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS). MATERIALS AND METHODS: Distortions due to TMS pulses (0.25 msec, 2.0 T) were studied at 2.0 T before and during EPI. RESULTS: Best results were obtained if both the EPI section orientation and the frequency-encoding gradient were parallel to the plane of the TMS coil. Under these conditions, a TMS pulse caused image distortions when preceding the EPI sequence by less than 100 msec. Recordings with a magnetic field gradient pick-up coil revealed transient magnetic fields after TMS, which are generated by eddy currents in the TMS coil. TMS during image acquisition completely spoiled all transverse magnetizations and induced disturbances ranging from image corruption to mild image blurring, depending on the affected low and high spatial frequencies. Simultaneous TMS and radio-frequency (RF) excitation gave rise to T1-dependent signal changes that lasted for several seconds and yielded pronounced false-positive activations during functional brain mapping. CONCLUSION: To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging.     

New coil positioning method for interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) and its validation in a motor cortex study

Purpose

To develop and test a novel method for coil placement in interleaved transcranial magnetic stimulation (TMS)/functional MRI (fMRI) studies.

Materials and Methods

Initially, a desired TMS coil position at the subject's head is recorded using a neuronavigation system. Subsequently, a custom‐made holding device is used for coil placement inside the MR scanner. The parameters of the device corresponding to the prerecorded position are automatically determined from a fast structural image acquired directly before the experiment. The spatial accuracy of our method was verified on a phantom. Finally, in a study on five subjects, the coil was placed above the cortical representation of a hand muscle in M1 and the blood oxygenation level‐dependent (BOLD) responses to short repetitive TMS (rTMS) trains were assessed using echo‐planar imaging (EPI) recordings.

Results

The spatial accuracy of our method is in the range of 2.9 ± 1.3 (SD) mm. Motor cortex stimulation resulted in robust BOLD activations in motor‐ and auditoryrelated brain areas, with the activation in M1 being localized in the hand knob.

Conclusion

We present a user‐friendly method for TMS coil positioning in the MR scanner that exhibits good spatial accuracy and speeds up the setup of the experiment. The motor‐cortex study proves the viability of the approach and validates our interleaved TMS/fMRI setup. J. Magn. Reson. Imaging 2009;29:189–197. © 2008 Wiley‐Liss, Inc.     

Multi-gradient echo with susceptibility inhomogeneity compensation (MGESIC): Demonstration of fMRI in the olfactory cortex at 3.0 T

Short image acquisition times and sensitivity to magnetic susceptibility favor the use of gradient echo imaging methods in functional MRI (fMRI). However, magnetic susceptibility effects attributed to air-tissue interfaces also lead to severe signal loss in images of the large inferior frontal and lateral temporal cortices of the human brain, which renders these regions inaccessible to fMRI. The signal loss is caused by the local field gradients in the slice selection direction. A multigradient echo with magnetic susceptibility inhomogeneity compensation method (MGESIC) is proposed to overcome this problem. The MGESIC method effectively corrects the susceptibility artifacts and maintains the advantages of gradient echo methods to both BOLD sensitivity and fast image acquisition. The effectiveness of the MGESIC method is demonstrated by fMRI experimental results within the olfactory cortex.     

Metallic neurosurgical implants: evaluation of magnetic field interactions, heating, and artifacts at 1.5-Tesla

The purpose of this study was to use ex vivo testing to determine the magnetic resonance imaging (MRI) safety aspects for seven different metallic neurosurgical implants in association with the 1.5-T MR environment. Ex vivo testing was performed using previously-described techniques for the evaluation of magnetic field interactions (deflection angle and torque), heating (gel-filled phantom and fluoroptic thermometry; 15 minutes of MRI at a specific absorption rate [SAR] of 1.4 W/kg), and artifacts (using T1-weighted, spin-echo and gradient-echo pulse sequences). None of the metallic implants displayed interactions with the magnetic field. The highest temperature change was +0.6 degrees C for the representative implant that was evaluated. Artifacts were relatively minor. The lack of magnetic field interactions and negligible heating indicate that MR procedures may be conducted safely in patients with these neurosurgical implants using MR systems with static magnetic fields of 1.5-T or less. Furthermore, these implants may be considered for use in interventional MR procedures insofar as the MR safe qualities and relatively small artifacts would likely be desirable for such procedures.     

Motion Artifacts in fMRI: Comparison of 2DFT with PR and Spiral Scan Methods

Activation signals based on BOLD contrast changes consequent to neuronal stimulation typically produce cortical intensity differences of < 10% at 1.5T. Hemodynamically driven pulsation of the brain can cause highly pulsatile phase shifts, which in turn result in motion artifacts whose intensity is larger than the activation signals in 2DFT scan methods. This paper presents a theoretical and experimental comparison of the magnitude of such artifacts for 2DFT and two other methods using non-Cartesian k-space trajectories. It is shown that artifacts increase with TR for 2DFT methods, and that projection reconstruction (PR) and spiral methods have significantly reduced artifact intensities, because these trajectories collect low spatial frequencies with every view. The spiral technique is found to be superior in terms of efficiency and motion insensitivity.     

Spiral-in/out BOLD fMRI for increased SNR and reduced susceptibility artifacts.

BOLD fMRI is hampered by dropout of signal in the orbitofrontal and parietal brain regions due to magnetic field gradients near air-tissue interfaces. This work reports the use of spiral-in trajectories that begin at the edge of k-space and end at the origin, and spiral in/out trajectories in which a spiral-in readout is followed by a conventional spiral-out trajectory. The spiral-in trajectory reduces the dropout and increases the BOLD contrast. The spiral-in and spiral-out images can be combined in several ways to simultaneously achieve increased signal-to-noise ratio (SNR) and reduced dropout artifacts. Activation experiments employing an olfaction task demonstrate significantly increased activation volumes due to reduced dropout, and overall increased SNR in all regions.     

针刺太冲穴激活视皮层的脑功能磁共振研究

目的:应用血氧水平依赖性功能磁共振成像(BOLD-fMRI),探究针刺太冲穴治疗眼疾的现代科学机制.方法:30例健康志愿者接受交替针刺视觉相关穴位--太冲穴及假穴,1.5 T MRI采集数据,SPM 2软件处理后获得2个针刺任务的个体及组激活图;组激活图若见视皮层激活,计算激活点的信号强度,并采用SPSS 13.0软件进行Wilcoxon符号秩检验.结果:组激活图针刺太冲穴见视皮层激活,而假穴未见激活;针刺太冲穴双侧视皮层激活点信号强度均较强,且均为针刺侧激活强度强.个体激活图针刺太冲穴9例视皮层激活;假穴6例. 结论:BOLD-fMRI研究中发现针刺太冲穴激活双侧枕叶视皮层(BA18),负责较高级视觉过程,且针刺侧视皮层激活强度强,为中医针灸治疗眼疾提供了一种解释.     

Physiological origin of low‐frequency drift in blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI)

We investigated the biophysical mechanism of low‐frequency drift in blood‐oxygen‐level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) (0.00–0.01 Hz), by exploring its spatial distribution, dependence on imaging parameters, and relationship with task‐induced brain activation. Cardiac and respiratory signals were concurrently recorded during MRI scanning and subsequently removed from MRI data. It was found that the spatial distribution of low‐frequency drifts in human brain followed a tissue‐specific pattern, with greater drift magnitude in the gray matter than in white matter. In gray matter, the dependence of drift magnitudes on TE was similar to that of task‐induced BOLD signal changes, i.e., the absolute drift magnitude reached the maximum when TE approached T whereas relative drift magnitude increased linearly with TE. By systematically varying the flip angle, it was found that drift magnitudes possessed a positive dependence on image intensity. In phantom experiments, the observed drift was not only much smaller than that of human brain, but also showed different dependence on TE and flip angle. In fMRI studies with visual stimulation, a strong positive correlation between drift effects at baseline and task‐induced BOLD signal changes was observed both across subjects and across activated pixels within individual participants. We further demonstrated that intrinsic, physiological drift effects are a major component of the spontaneous fluctuations of BOLD fMRI signal within the frequency range of 0.0–0.1 Hz. Our study supports brain physiology, as opposed to scanner instabilities or cardiac/respiratory pulsations, as the main source of low‐frequency drifts in BOLD fMRI. Magn Reson Med 61, 2009. © 2009 Wiley‐Liss, Inc.     

Myoinositol content in the human brain is modified by transcranial direct current stimulation in a matter of minutes: A 1H‐MRS study

Brain content of myoinositol (mI) has been shown to be altered in several neuropsychiatric conditions. Likewise, various forms of electric currents have been applied to the human brain for therapeutic purposes in neuropsychiatric diseases. In this study we aimed to depict the effects of low‐power transcranial direct current stimulation (tDCS) on brain mI by proton magnetic resonance spectroscopy (¹H‐MRS). We studied two groups of five healthy subjects by ¹H‐MRS: the first group was studied before and after both anodal and sham (placebo) tDCS over the right frontal lobe, and the second group was studied at the same intervals without undergoing either sham or anodal tDCS. Anodal tDCS induced a significant increase of mI content at 30 min after stimulation offset (141.5 ± 16.7%, P < 0.001) below the stimulating electrode but not in distant regions, such as the visual cortex, whereas sham tDCS failed to induce changes in mI. Neither N‐acetyl‐aspartate (NAA) nor the other metabolite contents changed after anodal or sham stimulation. ¹H‐MRS represents a powerful tool to follow the regional effects of tDCS on brain mI and, possibly, on the related phosphoinositide system. Magn Reson Med 60:782–789, 2008. © 2008 Wiley‐Liss, Inc.     

Differences in the BOLD fMRI response to direct and indirect cortical stimulation in the rat.

Functional MRI (fMRI) exploits a relationship between neuronal activity, metabolism, and cerebral blood flow to functionally map the brain. We have developed a model of direct cortical stimulation in the rat that can be combined with fMRI and used to compare the hemodynamic responses to direct and indirect cortical stimulation. Unilateral electrical stimulation of the rat hindpaw motor cortex, via stereotaxically positioned carbon-fiber electrodes, yielded blood oxygenation level-dependent (BOLD) fMRI signal changes in both the stimulated and homotypic contralateral motor cortices. The maximal signal intensity change in both cortices was similar (stimulated = 3.7 +/- 1.7%; contralateral = 3.2 +/- 1.0%), although the response duration in the directly stimulated cortex was significantly longer (48.1 +/- 5.7 sec vs. 19.0 +/- 5.3 sec). Activation of the contralateral cortex is likely to occur via stimulation of corticocortical pathways, as distinct from direct electrical stimulation, and the response profile is similar to that observed in remote (e.g., forepaw) stimulation fMRI studies. Differences in the neuronal pool activated, or neurovascular mediators released, may account for the more prolonged BOLD response observed in the directly stimulated cortex. This work demonstrates the combination of direct cortical stimulation in the rat with fMRI and thus extends the scope of rodent fMRI into brain regions inaccessible to peripheral stimulation techniques.     

Interleaved spiral‐in/out with application to functional MRI (fMRI)

The conventional spiral‐in/out trajectory samples k‐space sufficiently in the spiral‐in path and sufficiently in the spiral‐out path to enable creation of separate images. We propose an “interleaved spiral‐in/out” trajectory comprising a spiral‐in path that gathers one half of the k‐space data, and a complimentary spiral‐out path that gathers the other half. The readout duration is thereby reduced by approximately half, offering two distinct advantages: reduction of signal dropout due to susceptibility‐induced field gradients (at the expense of signal‐to‐noise ratio [SNR]), and the ability to achieve higher spatial resolution when the readout duration is identical to the conventional method. Two reconstruction methods are described; both involve temporal filtering to remove aliasing artifacts. Empirically, interleaved spiral‐in/out images are free from false activation resulting from signal pileup around the air/tissue interface, which is common in the conventional spiral‐out method. Comparisons with conventional methods using a hyperoxia stimulus reveal greater frontal‐orbital activation volumes but a slight reduction of overall activation in other brain regions. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Magnetic resonance imaging and permanent cosmetics (tattoos): survey of complications and adverse events

PURPOSE: To use a survey to determine the incidence of complications and adverse events in individuals with permanent cosmetics (e.g., tattooed eyeliner, eyebrows, lips, cheeks, etc.) who underwent magnetic resonance (MR) imaging. MATERIALS and METHODS: A questionnaire was distributed to clients of cosmetic tattoo technicians. This survey asked study subjects for demographic data, information about their tattoos, and for their experiences during MR imaging procedures. RESULTS: Data obtained from 1032 surveys were tabulated. One hundred thirty-five (13.1%) study subjects underwent MR imaging after having permanent cosmetics applied. Of these, only two individuals (1.5%) experienced problems associated with MR imaging. One subject reported a sensation of "slight tingling" and the other subject reported a sensation of "burning"; both sensations were transient in nature. CONCLUSION: Based on these findings and information in the peer-reviewed literature, it appears that MR imaging may be performed in patients with permanent cosmetics without any serious soft tissue reactions or adverse events. Therefore, the presence of permanent cosmetics should not prevent a patient from undergoing MR imaging.     

Multislice interleaved excitation cycles (MUSIC): An efficient gradient-echo technique for functional MRI

A method providing improved slice efficiency for gradient-echo imaging requiring long echo times, such as in functional neuromagnetic resonance imaging, is presented. To enhance the volume coverage while maintaining the short imaging time of a conventional single-slice gradient-echo technique, an interleaved muttislice excitation is performed during the echo time. This technique allows detection of susceptibility changes, e.g., the acquisition of stimulated human cortical activation maps, on clinical MR instruments at multiple planes within total imaging times of a few seconds. The efficiency of the technique is demonstrated in the detection of temporary changes in T₂^* in functional MRI experiments of the human visual cortex at magnetic field strengths of 2 Tesla and 3 Tesla. Fourteen 128 × 128 slices can be acquired in 13 s to cover a large volume-of-interest in the same time that would be required for single-slice acquisition using the conventional technique.     

Decomposition of inflow and blood oxygen level-dependent (BOLD) effects with dual-echo spiral gradient-recalled echo (GRE) fMRI

Image contrast with gradient-recalled echo sequences (GRE) used for fMRI can have both blood oxygen level-dependent (BOLD) and inflow components, and the latter is often undesirable. A dual-echo technique can be used to differentiate these mechanisms, because modulation of signal from inflow is common to both echoes, whereas susceptibility and diffusion-related signal losses are larger in the second echo. An efficient dual-echo interleaved spiral sequence was developed for use with a conventional scanner. It uses a κ-space trajectory that spirals out from the origin while the first echo is collected, then spirals back in while collecting the second echo. Decomposition of the data provides separate images of the inflow and T₂*-weighted components. Results demonstrate the decomposition with phantom experiments and with photic stimulation in normal volunteers.     

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla

PURPOSE: To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). MATERIALS AND METHODS: Different configurations were evaluated for bilateral neurostimulators (Soletra Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98-3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07-0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. RESULTS: Using the body RF coil, the highest temperature changes ranged from 2.5 degrees-25.3 degrees C. Using the head RF coil, the highest temperature changes ranged from 2.3 degrees-7.1 degrees C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. CONCLUSION: The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system.     

Multislice first-pass myocardial perfusion imaging: Comparison of saturation recovery (SR)-TrueFISP-two-dimensional (2D) and SR-TurboFLASH-2D pulse sequences

PURPOSE: To compare signal-to-noise ratio (SNR), contrast-to-noise (CNR) ratio, and diagnostic accuracy of a newly developed saturation recovery (SR)-TrueFISP-two-dimensional (2D) sequence with an SR-TurboFLASH-2D sequence. MATERIALS AND METHODS: In seven healthy subjects and nine patients with coronary artery disease (CAD), contrast-enhanced perfusion imaging (with Gd-DTPA) was performed with SR-TrueFISP and SR-TurboFLASH sequences. Hypoperfused areas were assessed qualitatively (scale = 0-4). Furthermore, SNR and CNR were calculated and semiquantitative perfusion parameters were determined from signal intensity (SI) time curves. Standard of reference for patient studies was single-photon emission computer tomography (SPECT) and angiography. RESULTS: The perception of perfusion deficits was superior in TrueFISP images (2.6 +/- 1.0) than in TurboFLASH (1.4 +/- 0.6) (P < 0.001). Phantom measurements yielded increased SNR (143 +/- 34%) and CNR (158 +/- 64%) values for TrueFISP. In patient/volunteer studies SNR was 61% to 100% higher and signal enhancement was 110% to 115% higher with TrueFISP than with TurboFLASH. Qualitative and semiquantitative assessment of perfusion defects yielded higher sensitivities for detection of perfusion defects with TrueFISP (68% to 78%) than with TurboFLASH (44% to 59%). CONCLUSION: SR-TrueFISP-2D perfusion imaging provides superior SNR and CNR than TurboFLASH imaging. Moreover, the dynamic range of SIs was found to be higher with TrueFISP, resulting in an increased sensitivity for detection of perfusion defects.     

Artifacts in musculoskeletal magnetic resonance imaging: identification and correction

A large number of artifacts occur in magnetic resonance (MR) imaging of the musculoskeletal system. These artifacts may potentially affect the quality of MR images, and may also simulate pathologic conditions and produce pitfalls in interpretation. Motion artifacts may be periodic or random. Protocol-error artifacts include saturation, wraparound, radiofrequency (RF) interference, shading, and partial volume averaging artifacts. Truncation artifacts occur when the number of phase-encoding steps of high spatial frequencies is insufficient (or undersampled) for faithful reproduction of the true anatomic detail of the original image. Chemical shift artifacts are due to the protons in fat being mismapped relative to water protons. Susceptibility artifacts occur at the interfaces of structures with different magnetic susceptibilities. Artifacts special to the musculoskeletal system include the magic angle phenomenon and spurious signal induced at very short echo times, both of which affect anisotropic structures such as tendon, ligament, and cartilage. Recognition and, if possible, correction of these artifacts are an important aspect of practical musculoskeletal MR imaging. Received: 9 October 2000 Revision requested: 14 November 2000 Revision received: 8 January 2001 Accepted: 9 January 2001     

Postmortem computed tomographic (PMCT) and postmortem magnetic resonance imaging (PMMRI) demonstration of fatal massive retroperitoneal hemorrhage caused by abdominal aortic aneurysm (AAA) rupture

We report a case of fatal massive retroperitoneal homorrhage caused by the rupture of an abdominal aortic aneurysm (AAA) in which postmortem computed tomography (PMCT) and postmortem magnetic resonance imaging (PMMRI) provided clear delineation. In this case, the autopsy imaging system using PMCT and PMMRI was useful as a screening method to determine the cause of death as AAA rupture and became a guide for the subsequent autopsy.