Generating Motion- and Distortion-Free Local Field Map Using 3D Ultrashort TE MRI: Comparison with T₂* Mapping

PURPOSE: To generate phase images with free of motion-induced artifact and susceptibility-induced distortion using 3D radial ultrashort TE (UTE) MRI.MATERIALS AND METHODS: The field map was theoretically derived by solving Laplace's equation with appropriate boundary conditions, and used to simulate the image distortion in conventional spin-warp MRI. Manufacturer's 3D radial imaging sequence was modified to acquire maximum number of radial spokes in a given time, by removing the spoiler gradient and sampling during both rampup and rampdown gradient. Spoke direction randomly jumps so that a readout gradient acts as a spoiling gradient for the previous spoke. The custom raw data was reconstructed using a homemade image reconstruction software, which is programmed using Python language. The method was applied to a phantom and in-vivo human brain and abdomen. The performance of UTE was compared with 3D GRE for phase mapping. Local phase mapping was compared with T₂* mapping using UTE.RESULTS: The phase map using UTE mimics true field-map, which was theoretically calculated, while that using 3D GRE revealed both motion-induced artifact and geometric distortion. Motion-free imaging is particularly crucial for application of phase mapping for abdomen MRI, which typically requires multiple breathold acquisitions. The air pockets, which are caught within the digestive pathway, induce spatially varying and large background field. T₂* map, that was calculated using UTE data, suffers from non-uniform T₂* value due to this background field, while does not appear in the local phase map of UTE data.CONCLUSION: Phase map generated using UTE mimicked the true field map even when non-zero susceptibility objects were present. Phase map generated by 3D GRE did not accurately mimic the true field map when non-zero susceptibility objects were present due to the significant field distortion as theoretically calculated. Nonetheless, UTE allows for phase maps to be free of susceptibility-induced distortion without the use of any post-processing protocols.

Factors influencing the renal arterial Doppler waveform: a simulation study using an electrical circuit model (secondary publication)

PURPOSE: The goal of this study was to evaluate the effect of vascular compliance, resistance, and pulse rate on the resistive index (RI) by using an electrical circuit model to simulate renal blood flow. METHODS: In order to analyze the renal arterial Doppler waveform, we modeled the renal blood-flow circuit with an equivalent simple electrical circuit containing resistance, inductance, and capacitance. The relationships among the impedance, resistance, and compliance of the circuit were derived from well-known equations, including Kirchhoff's current law for alternating current circuits. Simulated velocity-time profiles for pulsatile flow were generated using Mathematica (Wolfram Research) and the influence of resistance, compliance, and pulse rate on waveforms and the RI was evaluated. RESULTS: Resistance and compliance were found to alter the waveforms independently. The impedance of the circuit increased with increasing proximal compliance, proximal resistance, and distal resistance. The impedance decreased with increasing distal compliance. The RI of the circuit decreased with increasing proximal compliance and resistance. The RI increased with increasing distal compliance and resistance. No positive correlation between impedance and the RI was found. Pulse rate was found to be an extrinsic factor that also influenced the RI. CONCLUSION: This simulation study using an electrical circuit model led to a better understanding of the renal arterial Doppler waveform and the RI, which may be useful for interpreting Doppler findings in various clinical settings.

Evaluation of Factors Affecting the Renal Doppler Waveform with the Use of an Electrical Circuit Model

PURPOSE: We have evaluated the influence of several variables such as vascular compliance and resistance and heart rate on the resistive index by the use of an electrical circuit model that simulates renal blood flow. MATERIALS AND METHODS: To analyze the renal artery Doppler waveform, we modeled the renal blood-flow circuit with an equivalent simple electrical circuit containing resistance, inductance and capacitance. The relationship between impedance and resistance and compliance of the circuit was derived from well-known equations including Kirchhoff's current law for an alternating current circuit. Simulated velocitytime profiles for pulsatile flow were generated by the use of Mathematica software (Wolfram Research, Champaign, IL USA) and the influence of resistance, compliance and pulse rate on waveforms and the resistive index were evaluated. RESULTS: Resistance and compliance altered the waveforms independently. The impedance of the circuit increased with increasing proximal compliance, proximal resistance and distal resistance. Impedance decreased with increasing distal compliance. The resistive index of the circuit decreased with increasing proximal compliance and resistance. The resistive index increased with increasing distal compliance and resistance. These results showed no tendency of a positive correlation between the impedance and resistive indices. The pulse rate is an extrinsic factor that also influences the resistive index. CONCLUSIONS: By the use of this simulation study using an electrical circuit model, a better understanding of the Doppler waveform and resistive index was achieved and the study findings may be useful in various clinical settings.

Dual-Modal Nanoprobes for Imaging of Mesenchymal Stem Cell Transplant by MRI and Fluorescence Imaging

OBJECTIVE: To determine the feasibility of labeling human mesenchymal stem cells (hMSCs) with bifunctional nanoparticles and assessing their potential as imaging probes in the monitoring of hMSC transplantation. MATERIALS AND METHODS: The T1 and T2 relaxivities of the nanoparticles (MNP@SiO2[RITC]-PEG) were measured at 1.5T and 3T magnetic resonance scanner. Using hMSCs and the nanoparticles, labeling efficiency, toxicity, and proliferation were assessed. Confocal laser scanning microscopy and transmission electron microscopy were used to specify the intracellular localization of the endocytosed iron nanoparticles. We also observed in vitro and in vivo visualization of the labeled hMSCs with a 3T MR scanner and optical imaging. RESULTS: MNP@SiO2(RITC)-PEG showed both superparamagnetic and fluorescent properties. The r1 and r2 relaxivity values of the MNP@SiO2(RITC)-PEG were 0.33 and 398 mM-1 s-1 at 1.5T, respectively, and 0.29 and 453 mM-1 s-1 at 3T, respectively. The effective internalization of MNP@SiO2(RITC)-PEG into hMSCs was observed by confocal laser scanning fluorescence microscopy. The transmission electron microscopy images showed that MNP@SiO2(RITC)-PEG was internalized into the cells and mainly resided in the cytoplasm. The viability and proliferation of MNP@SiO2(RITC)-PEG-labeled hMSCs were not significantly different from the control cells. MNP@SiO2(RITC)-PEG-labeled hMSCs were observed in vitro and in vivo with optical and MR imaging. CONCLUSION: MNP@SiO2(RITC)-PEG can be a useful contrast agent for stem cell imaging, which is suitable for a bimodal detection by MRI and optical imaging.

Multi-Component Relaxation Study of Human Brain Using Relaxographic Analysis

PURPOSE: To demonstrate that the relaxographic method provides additional information such as the distribution of relaxation times and water content which are poentially applicable to clinical medicine. MATERIALS AND METHODS: First, the computer simulation was performed with the generated relaxation data to verify the accuracy and reliablility of the relaxographic method (CONTIN). Secondly, in order to see how well the CONTIN quantifies and resolves the two different T1 environments, we calculated the oil to water peak area ratios and identified peak positions of T1-distribution curve of the phantom solutions, which consist of four centrifugal tubes (10ml) filled with the compounds of 0, 10, 20, 30% of corn oil and distilled water, using CONTIN. Finally, inversion recovery MR images for a volunteer are acquired for each TI ranged from 40 to 1160 msec with TR/TE=2200/20 msec. From the 3 different ROIs (GM, WM, CSF), CONTIN analysis was performed to obtain the T1-distribution curves, which gave peak positions and peak area of each ROI location. RESULTS: The simulation result shows that the errors of peak positions were less in the higher peak (centered T1=600 msec) than in the lower peak (centered T1=150 msec) for all SNR but the errors of peak areas were larger in the higher peak than in the lower peak. The CONTIN analysis of the measured relaxation data of phantoms revealed two peaks between 20 and 60 msec and between 500 and 700 msec. The analysis gives the peak area ratio as oil 10%: oil 20%: oil 30%=1:1.3:1.9, which is different from the exact ratio, 1:2:3. For human brain, in ROI 3 (CSF), only one component of -distributions was observed whereas in ROI 1 (GM) and in ROI 2 (WM) we observed two components of T1-distribution. For the WM and CSF there was great agreement between the observed T1-relaxation times and the reported values. CONCLUSION: we demonstrated that the relaxographic method provided additional information such as the distribution of relaxation times and water content, which were not available in the routine relaxometry and T1/T2 mapping techniques. In addition, these additional information provided by relaxographic analysis may have clinical importance.

Functional MRI in the Recovery of Hand Movement after Subcortical Stroke

OBJECTIVE: To investigate cortical reorganization of the brain during voluntary activities of the hand in patients with subcortical cerebral infarction. METHOD: Twelve patients with first-ever subcortical brain lesion causing hemiparesis had been evaluated with functional MRI. Bilateral hand clenching was done to test voluntary hand activities. Recovery period ranged from 2 to 36 months. RESULTS: During the unaffected hand movement, activation of contralateral primary sensorimotor cortex (SMC) were recorded in all cases and supplmentary motor area (SMA) in 1 case. The affected hand movement showed activation of the cotralateral SMC in all cases, ipsilateral SMC in 4 cases, SMA in 4 cases and contralateral prefrontal area in 2 cases. As for the contralateral SMC, affected hand movement showed more increased activation than the unaffected. For the bilateral SMC activation during movement of the affected hand, contralateral SMC activation was greater than the ipsilateral. CONCLUSION: Ipsilateral activation of the SMC, SMA, prefrontal area and increased activation of the contralateral SMC during affected hand movement suggest that these may play an important role in the reorganization of sensory and motor system in stroke patients with subcortical lesion. Functional MRI studies of patients who recovered from subcortical stroke provide evidence for several process that may be related to restoration of neurologic function.

Ipsilateral Motor Pathway Confirmed by Brain Mapping in a Patient with Traumatic Brain Injury: A case report

The aim of this study is to investigate the mechanism of motor recovery using both functional Magnetic Resonance Imaging (fMRI) and Transcranial Magnetic Stimulation (TMS) in a patient with hemorrhagic contusion on the right basal ganglia area. Functional MRI showed that the left primary sensorimotor cortex and the supplementary motor area were activated when the right fingers performed the flexion-extension exercise. On the other hand, the bilateral primary sensorimotor cortex and the left premotor area were activated with the excerise of left hand. Brain mapping for both abductor pollicis brevis muscles (APB) using TMS revealed that ipsilateral motor evoked potentials (MEPs) were obtained at left APB. Ipsilateral MEPs of left APB showed delayed latency and lower amplitude compared to that of right APB when stimulated at the left motor cortex. We concluded that ipsilateral motor pathway from undamaged motor cortex seems to contribute to the motor recovery in this patient and combining TMS with fMRI may provide a powerful tool for investigating the mechanism of motor recovery.

Functional-Magnetic Resonance Imaging and Transcranial Magnetic Stimulation in a Case of Schizencephaly

PURPOSE: This study was to present the functional brain mapping of both functional magnetic resonance imaging(MRI) and transcranial magnetic stimulation(TMS) in a case of schizencephaly. MATERIALS AND METHODS: A 28-year-old man, who had left hemiplegia and schizencephaly in right cerebral hemisphere, was examed with both functional MRI and TMS. Motor function of left hand was decreased whereas right hand was within normal limit. For functional MRI, gradient-echo echo planar imaging(TR/TE/alpha=1.2 sec/90 msec/90) was employed. The paradigm of motor task consisted of repetitive self-pased hand flexion-extension exercises with 1-2 Hz periods. An image set of 10 slices was repetitively acquired with 15 seconds alternating periods of task performance and rest and total 6 cycles( three ON periods and three OFF periods) were performed. In brain mapping, TMS was performed with the round magnetic stimulator(mean diameter; 90mm). The magnetic stimulation was done with 80% of maximal output. The latency and amplitude of motor evoked potential(MEP)s were obtained from both abductor pollicis brevis(APB) muscles. RESULTS: Functional MRI revealed activation of the left primary motor cortex with flexion-extension exercises of healthy right hand. On the other hand, the left primary motor cortex, left supplementary motor cortex, and left premotor areas were activated with flexion-extension exercises of left hand. In TMS, magnetic evoked potentials were induced in no areas of right cerebral hemisphere, but in 5 areas of left cerebral hemisphere from both abductor pollicis brevis. Latency, amplitude, and contour of response of the magnetic evoked potentials in both hands were similar. CONCLUSION: Functional MRI and TMS in a patient with schizencephaly were successfully used to localize cortical motor function. Ipsilateral motor pathway is thought to be secondary to reinforcement of the corticospinal tract of the ipsilateral motor cortex.

The Optimizations of Imaging Parameters in Abdominal and Peripheral Contrast Enhanced MR Angiography

PURPOSE: To investigate the relativethe signal difference. Also this optimal angle increased as TR increased. The optimal flip angles varied along with [Gd] concentration such that high concentration allowed larger flip angle. The experiment revealed similar results and confirmed the results obtained from simulation. Both simulation and experimental results suggest that the optimization scheme of imaging parameters for maximum signal difference, which gives best vessel visualization, is very close to that for maximizing blood signal alone for short TR application of contrast enhanced MRA. CONCLUSION: To improve the vessel visualization, it is necessary to optimize the imaging parameters. The expected optimal flip angle is very close to Ernst angle. Higher [Gd] concentration and longer TR, if allowed, will give rise to higher blood signal.

A case of pleomorphic adenoma of nasal tip / 대한이비인후과학회지

No abstract available.