Fast Low-angle Shot Imaging Research Articles

Spoiled FLASH MRI with slice selective excitation: Signal equation with a correction term

ABSTRACTSpoiled fast low‐angle shot (FLASH) magnetic resonance imaging (MRI) provides a simple contrast, largely independent of the spin‐spin relaxation time. Three‐dimensional FLASH imaging (3D FLASH) has gained importance due to its simplicity and its ability to produce isotropic, high‐resolution images. Nevertheless, 2D FLASH imaging can be more suited than 3D FLASH imaging in various specific applications requiring long repetition time, such as imaging of the proton density (PD), as it allows interleaved slice acquisition rather than consecutive slice acquisition. In practice, however, a slice‐selective excitation produces a nonuniform excitation profile, which needs to be taken into account to allow reliable quantitative data analysis. In this work, the influence of the nonuniform excitation profile on the detected signal is modeled as a dimensionless multiplicative correction, function of the flip angle, and the ratio of the repetition time to the spin‐lattice relaxation time T1. This model is validated experimentally by measuring the PD and T1 MR parameters in a phantom experiment on a 3T clinical scanner. A good accuracy in the estimation of T1 and in the reconstruction of the PD (weighted by the receiver sensitivity profile) is obtained, for example, a relative error of 3% in T1 within the range [500, 2000] ms and a precision of 1% in PD. It is also shown how the proposed theory can be extended to magnetization‐prepared 2D‐spoiled FLASH as well as other variations of the 2D‐spoiled FLASH sequence. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part A 42A: 89–100, 2013.

Antenatal architecture and activity of the human heart

We construct the components for a family of computational models of the electrophysiology of the human foetal heart from 60 days gestational age (DGA) to full term. This requires both cell excitation models that reconstruct the myocyte action potentials, and datasets of cardiac geometry and architecture. Fast low-angle shot and diffusion tensor magnetic resonance imaging (DT-MRI) of foetal hearts provides cardiac geometry with voxel resolution of approximately 100 µm. DT-MRI measures the relative diffusion of protons and provides a measure of the average intravoxel myocyte orientation, and the orientation of any higher order orthotropic organization of the tissue. Such orthotropic organization in the adult mammalian heart has been identified with myocardial sheets and cleavage planes between them. During gestation, the architecture of the human ventricular wall changes from being irregular and isotropic at 100 DGA to an anisotropic and orthotropic architecture by 140 DGA, when it has the smooth, approximately 120° transmural change in myocyte orientation that is characteristic of the adult mammalian ventricle. The DT obtained from DT-MRI provides the conductivity tensor that determines the spread of potential within computational models of cardiac tissue electrophysiology. The foetal electrocardiogram (fECG) can be recorded from approximately 60 DGA, and RR, PR and QT intervals between the P, R, Q and T waves of the fECG can be extracted by averaging from approximately 90 DGA. The RR intervals provide a measure of the pacemaker rate, the QT intervals an index of ventricular action potential duration, and its rate-dependence, and so these intervals constrain and inform models of cell electrophysiology. The parameters of models of adult human sinostrial node and ventricular cells that are based on adult cell electrophysiology and tissue molecular mapping have been modified to construct preliminary models of foetal cell electrophysiology, which reproduce these intervals from fECG recordings. The PR and QR intervals provide an index of conduction times, and hence propagation velocities (approx. 1-10 cm s(-1), increasing during gestation) and so inform models of tissue electrophysiology. Although the developing foetal heart is small and the cells are weakly coupled, it can support potentially lethal re-entrant arrhythmia.

Detection the changes of hippocampal volume and the lateral ventricle vein in the patients with depression with susceptibility weighted imaging and morphological measurement

Objective To study the hippocampal structure in the patients with depression in varying degrees of severity with the combination of the susceptibility weighted imaging (SWI) and morphological measurement. Methods Sixty patients (divided equally into mild,moderate and major group as the conditions of the severity of depression) and 20 healthy controls were scanned using the three dimensional fast low angle shot imaging sequence (3D-FLASH) and SWI.Then the morphological images were manually segmented and delineated by three doctors.The hippocampal volumes were calculated and standardized.The maximum diameter,length and the branching numbers of the Inferior ventricular vein which were clear demonstrated in the SWI were measured.The changes in the venous system and morphology among the three different degrees of severity in depression and the healthy controls were contrastively analyzed by the one-way analysis of variance. Results Left and right hippocampal volumes: the mild group were 2246±147,2271±151; the moderate group were 2028±65,2038±57; the major group were 1965±129,1962±110; the healthy control were 2287±160,2305±171.There were statistically significant differences among the four groups (F=7.45,8.55,P 0.05).The diameter,length and branch numbers of the lateral ventricle vein: the mild group were (0.94±0.09) mm,(12.0±1.07) mm and 3.67±1.03; the moderate group were (0.81±0.04) mm,(10.2±1.25) mm and 2.00±0.89; the major group were (0.70±0.08) mm,(8.6±1.40) mm and 1.83±0.75; the healthy control were (1.15±0.14) mm,(14.2±0.90) mm and 3.33±0.82.There were statistically significant differences among the four groups (F=17.07,25.20,6.65,P 0.05).As the progressive of the depression,the reduction in the length,diameter and branch numbers of the lateral ventricle vein showed more evident (theDunnett-t of the moderate were-10.20,-11.50,-2.88,P<0.05;the Dunnett-t of the major were-11.70,-14.69,-5.76,P<0.01). Conclusion The combination of the SWI and morphological measurement helps to prompt the regularity of the changes in hippocampal structure in patients with different severity of depression. Key words: Depressive disorder; Hippocampus; Magnetic resonance imaging

Multiple Sclerosis Lesions and Irreversible Brain Tissue Damage

In current clinical practice, T2-weighted magnetic resonance imaging (MRI) is commonly applied to quantify the accumulated multiple sclerosis (MS)lesion load, whereas T1-weighted sequences are used to differentiate edema, blood-brain barrier breakdown by contrast enhancement, and irreversible brain tissue damage(commonly called “black holes” owing to the loss of signal intensity in T1-weighted sequences). Black holes are histopathologically associated with axonal loss and severe tissue destruction. In addition, double inversion recovery techniques were developed to improve the sensitivity to cortical lesions. To demonstrate the potential of ultrahigh-field 3-dimensional T1-weighted imaging using magnetization-prepared rapid acquisition and multiple gradient echoes(MPRAGE) to detect and characterize white and gray matter pathology in MS. Comparative study. The patients with MS were recruited from the outpatient clinics of the Neuro Cure Clinical Research Center and underwent 7-T brain MRI at the Berlin Ultrahigh Field Facility, both of which are in Berlin, Germany. Twenty patients with relapsing-remitting MS and 14 healthy controls underwent 7-T brain MRI, using a 24-channel receive head coil, and a subgroup of 18 patients with relapsing-remitting MS also underwent 1.5-T brain MRI. The imaging protocol included 2-dimensional T2-weighted fast low-angle shot (FLASH) and turbo inversion recovery magnitude (TIRM) sequences. For 3-dimensional T1-weighted imaging, the MPRAGE sequence was used. Each sequence was initially examined independently in separate analyses by an investigator blinded to all other data. In a second study, all detected lesions were retrospectively analyzed in a side-by-side comparison of all sequences. By use of 7-T T2-weighted FLASH imaging, 604 cerebral lesions were detected in the patients with relapsing-remitting MS (mean, 30.2 lesions per patient[range, 2-107 lesions per patient]), but none were detected in healthy controls. Cortical pathology was visible in 10 patients (6 cortical lesions and 37 leukocortical lesions). Within the 7-T acquisitions, each lesion detected at T2-weighted sequences and/or double inversion recovery sequences was also clearly delineated on corresponding MPRAGE sequences in side-by-side analysis.However, at 1.5 T, the MPRAGE images depicted only 452 of 561 lesions visualized in T2-weighted sequences and/or double inversion recovery sequences. In contrast,when analyzing each sequence separately, we found that the 7-T MPRAGE depicted more lesions than the 7-TFLASH (728 lesions vs 584 lesions), and almost twice as many as the 1.5-T MPRAGE (399 lesions). The 7-TMPRAGE also improved the detection of cortical and leukocortical lesions (15 lesions vs 58 lesions). At ultrahigh-field strength, T1-weighted MPRAGE is highly sensitive in detecting MS plaques within the white and the gray brain parenchyma. Our results indicate structural damage beyond demyelination in every lesion depicted, which is in accordance with postmortem histopathological studies. The 7-T MPRAGE clearly delineated every cortical lesion that was visualized by any other MRI sequence at 1.5 or 7 T.

FLASH imaging: Rapid NMR imaging using low flip-angle pulses

A new method for rapid NMR imaging dubbed FLASH ( fast low-angle shot) imaging is described which, for example, allows measuring times of the order of 1 s (64 × 128 pixel resolution) or 6 s (256 × 256 pixels). The technique takes advantage of excitation pukes with small hip angles eliminating the need of waiting periods in between successive experiments. It is based on the acquisition of the free induction decay in the form of a gradient echo generated by reversal of the read gradient. The entire imaging time is only given by the number of projections desired times the duration of slice selection and data acquisition. The method results in about a 100-fold reduction in measuring time without sacrificing spatial resolution. Further advantages are an optimized signal-to-noise ratio, the applicability of commercial gradient systems, and the deposition of extremely low rf power. FLASH imaging is demonstrated on phantoms, animals, and human extremities using a 2.3 T 40 cm bore magnet system. 1H NMR images are obtained with variable relaxation time contrasts and without motional artifacts.

Correspondence of human visual areas identified using functional and anatomical MRI in vivo at 7 T

To study the correspondence of anatomically and functionally defined visual areas (primary visual cortex, V1, and motion selective area V5/human MT+) by using structural magnetic resonance imaging (MRI) and functional MRI (fMRI) in vivo at 7 T. Four subjects participated in this study. High-resolution (≈0.4 mm isotropic) anatomical MRI was used to identify cortical regions based on their distinct cortical lamination. The optimal contrast for identifying heavily myelinated layers within gray matter was quantitatively assessed by comparing T(1)-weighted magnetization-prepared rapid gradient echo (MPRAGE) and T(2)*-weighted, 3D fast-low angle shot (FLASH) imaging. Retinotopic mapping was performed using GE-based fMRI at 1.5 mm isotropic resolution to identify functional areas. T(2)*-weighted FLASH imaging was found to provide a significantly higher contrast-to-noise ratio, allowing visualization of the stria of Gennari in every slice of a volume covering the occipital cortex in each of the four subjects in this study. The independently derived boundary of V1, identified in the same subjects using retinotopic mapping by fMRI, closely matched the border of anatomically defined striate cortex in the human brain. Evidence of banding was also found within the functionally defined V5 area; however, we did not find a good correlation of this area, or the functionally identified subregion (MT), with the banded area. High-resolution T(2)*-weighted images acquired at 7 T can be used to identify myelinated bands within cortical gray matter in reasonable measurement times. Regions where a myelinated band was identified show a high degree of overlap with the functionally defined V1 area.

Investigation of motion artifacts associated with fat saturation technique in 3D flash imaging

Fast low-angle shot (FLASH) imaging is widely used in dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) because it permits fast and accurate T1 measurement in vivo. Suppression of the fat signal is necessary for most FLASH applications; otherwise, fat will appear hyperintense. The fat saturation technique is one popular method to reduce fat images on clinical scanners. However, fat saturation combined with the 3D FLASH sequence in breast DCE-MRI scans results in heavy ghosting artifacts caused by heartbeat. We used simulation and experimental scans to determine the cause of these artifact-enhancement phenomena. We simulated imaging of motion in the x, y, and z directions, with and without fat saturation, to investigate the origin of artifacts. Fourier transform (FT) of the whole field of view was used in the simulation, and we assumed that the uniform phantom was static during one TR. The amplitude of each echo was considered a factor in the FT data. Images were reconstructed using FT data from different phantom positions multiplied by the amplitude factor. Phantom experiments and volunteer studies were implemented to verify the conclusion. Both phantom and volunteer results showed artifacts similar to those in simulation images. We found that FLASH sequence without fat saturation is insensitive to motion. Fat saturation radiofrequency pulses placed before each group of echoes disrupted the steady state of the signal amplitude and produced a low-pass filter effect that enhanced the motion artifacts. We conclude that the low-pass filter effect associated with the fat saturation technique is responsible for dramatically increased motion artifacts.

7 Tesla (T) human cardiovascular magnetic resonance imaging using FLASH and SSFP to assess cardiac function: validation against 1.5 T and 3 T.

We report the first comparison of cardiovascular magnetic resonance imaging (CMR) at 1.5 T, 3 T and 7 T field strengths using steady state free precession (SSFP) and fast low angle shot (FLASH) cine sequences. Cardiac volumes and mass measurements were assessed for feasibility, reproducibility and validity at each given field strength using FLASH and SSFP sequences. Ten healthy volunteers underwent retrospectively electrocardiogram (ECG) gated CMR at 1.5 T, 3 T and 7 T using FLASH and SSFP sequences. B1 and B0 shimming and frequency scouts were used to optimise image quality. Cardiac volume and mass measurements were not significantly affected by field strength when using the same imaging sequence (P > 0.05 for all parameters at 1.5 T, 3 T and 7 T). SSFP imaging returned larger end diastolic and end systolic volumes and smaller left ventricular masses than FLASH imaging at 7 T, and at the lower field strengths (P < 0.05 for each parameter). However, univariate general linear model analysis with fixed effects for sequence and field strengths found an interaction between imaging sequence and field strength (P = 0.03), with a smaller difference in volumes and mass measurements between SSFP and FLASH imaging at 7 T than 1.5 T and 3 T. SSFP and FLASH cine imaging at 7 T is technically feasible and provides valid assessment of cardiac volumes and mass compared with CMR imaging at 1.5 T and 3 T field strengths.

MR-compatible Assistance System for Biopsy in a High-Field-Strength System: Initial Results in Patients with Suspicious Prostate Lesions

To examine the feasibility and safety of magnetic resonance (MR)-guided biopsy by using a transgluteal approach in patients with suspicious prostate lesions by using an MR-compatible robotic system and a 1.5-T MR system. The study was approved by the institutional review board of University Frankfurt, and informed consent was obtained from each patient. A total of 20 patients (age range, 57.8-71.9 years; mean age, 65.1 years) underwent biopsy in a closed-bore high-field-strength MR system. Biopsy was performed with an MR-compatible pneumatically driven robotic system. T1-weighted gradient-echo fast low-angle shot and T2-weighted true fast imaging with steady-state precession sequences were used to plan and guide the intervention with a transgluteal access on the external planning computer of the assistance system. The system calculated the trajectory and then moved the guiding arm to the insertion point. The cannula was advanced manually, and biopsies were performed with the coaxial technique by using a 15-gauge pencil tip needle. Intervention time, complications, and biopsy findings were documented. The MR-compatible robotic system did not interfere with image quality, nor did MR imaging cause dysfunction of the robot. In one patient, the interventionist caused a fail-safe system shutdown. This was due to inadvertent displacement of the guiding arm during cannula insertion. This problem was solved by increasing the displacement threshold. Accurate coaxial cannula biopsy could be performed in all subsequent patients. Sufficient histopathologic assessment was performed in 19 patients. Insufficient material was retrieved in the patient who experienced fail-safe system shutdown. The median intervention time was 39 minutes (23-65 minutes). No procedure-related complications were observed. Preliminary results indicate that MR-guided robot-assisted biopsy is feasible and can be performed safely with highly accurate cannula placement.

Usefulness of administration of SPIO prior to RF ablation for evaluation of the therapeutic effect: An experimental study using miniature pigs

Objectives To evaluate the usefulness of administration of superparamagnetic iron oxide (SPIO) and magnetic resonance (MR) imaging for assessing the efficacy of radiofrequency (RF) liver ablation. Material and methods Using a protocol approved by the animal research committee of our university, nine RF liver ablations were performed in three miniature pigs. Six ablations were performed after administration of SPIO in two pigs (group A). Three ablations were performed in the other pig without administration of SPIO (group B). All pigs were sacrificed 4 days after the procedure. Harvested livers were scanned with a 1.5 T MR system before and after fixation with 10% buffered formalin, and MR images were precisely compared with histological specimens. Results There were no histological differences between the two groups. All ablated liver lesions showed coagulation necrosis at the external layer. There were no viable cells inside the coagulation necrosis. All ablated lesions had a hypointense rim on fast low angle shot (FLASH) images. The rims of group A were thicker than those of group B. The rims of group B corresponded histologically to congestion and hemorrhagic necrosis area. The rims of the group A corresponded to hemorrhagic necrosis and coagulation necrosis areas. In group A, the hypointense rim reflected necrotic Kupffer cells that took up SPIO before RF liver ablation. Conclusion Administration of SPIO made it possible to precisely evaluate ablated liver parenchyma by hypointense rim on FLASH images. This method is helpful for the evaluation of safety margin after RF ablation for liver tumors.

Ideal combination of sequences for perianal fistula classification and evaluation of additional findings for readers with varying experience

The aim of our study was to assess the contribution of various magnetic resonance imaging (MRI) sequences in determining the type of perianal fistula and in obtaining critical information for surgical decisions, as well as to define the optimal combination of sequences for readers with varying levels of experience. The study included 33 MRI examinations in 26 patients with suspected perianal fistula. The following sequences were obtained in both the coronal and axial planes: thin slice, high resolution T1-weighted (W) spin echo; T2-weighted turbo spin echo; short tau inversion recovery (STIR); and native and contrast enhanced T1-weighted gradient echo fast low-angle shot (FLASH) images with fat suppression (FS-CE-T1W-GRE). The examinations were interpreted by three radiologists with varying degrees of experience in two different sessions, and the inter-reader agreement was assessed. Seventeen of the patients underwent surgery. The agreement between the surgical findings and the MRI results were evaluated. A statistically significant concordance between the fistula classification and surgery was achieved with the FS-CE-T1W-GRE sequence for Reader 1 (Cramer's V=0.701, P = 0,022) and Reader 3 (Cramer's V=0.716, P = 0,043). For Reader 2, statistically significant concordance between fistula classification and surgery was achieved with the FS-CE-T1W-GRE (Cramer's V=0.703, P = 0,011) and the T2W images (Cramer's V=0.648, P = 0,027). For all sequences, there was statistically significant agreement between readers for fistula classification, internal opening location, and the presence of sinus tracts, abscess, a horseshoe component, and inflammation. For experienced readers, combining FS-CE-T1W-GRE images with either T2W or STIR images collected in both the coronal and axial planes was sufficient to make an assessment before deciding the surgical extent of the procedure.

Evaluation of flow measurement from the first pass bolusT1weighted images using inversion recovery sequence

Previous studies have shown that the organ blood flows (OBFs) calculated using the T(1) weighted MRI technique were lower than the expected values. The aim of this study was a flow measurement comparison between the theoretical and experimental flows based on the technique before and after corrections (coil non-uniformity and inflow) using a flow phantom at two different concentrations (0.8 and 1.2 mmol l(-1)). A flow phantom was designed to produce three different flow rates at the same time. Theoretical flow was calculated by measuring the volumes of the phantom and dividing them by the time taken to fill these volumes. T(1) weighted turbo fast low-angle shot images were used to measure signal intensity (SI) change during the first bolus passage of the contrast medium through the phantom using linear phase-encoding acquisition. The corrected experimental flow based on the technique shows a good agreement with the theoretical flow, where the flow rate is low at the two concentrations. The T(1) weighted MRI technique after the two correction factors can be used to measure the absolute flow where the flow rate is low, such as in the capillaries. For measuring high flow rate (e.g. artery), additional correction factors should be considered.

Comparison between pig lumbar zypapophyseal joint cartilage acquired from multiple magnetic resonance image sequences and gross specimens

To evaluate the capability and limitation of magnetic resonance image(MRI)for Lumbar zygapophyseal joint cartilage through comparing pig lumbar zygapophyseal joint cartilage acquired from multiple MRI sequences of a 1.5 Tesla MR and gross specimens. Six fresh lumbar spines from adult pigs were sagittaly scanned by Siemens 1.5 Tesla MR. The scan sequences included fast spin echo T1-weighted imaging (FSE T1WI), fast spin echo T2-weighted imaging (FSE T2 weighted T2WI), fat saturation proton density-weighted imaging (FS PDWI), 3-dimensional fast low angle shot imaging (3D-FLASH), and water excitation 3-dimensional fast low angle shot imaging (WE 3D-FLASH). Each scan sequence acquired images from the same layer. The signal-noise ratio (SNR) for articular cartilage, contrast-noise ratio (CNR) for cartilage versus bone cortex, cartilage versus bone marrow, and cartilage versus saline were calculated. Right after the scanning, the lumbar spines were snap-frozen, incised sagittally along the midline lumbar zypapophyseal joints, and photographed to compare the gross specimens with corresponding MRIs. The thickness of sagittal midline center of 6 pairs of lumbar(L₃/L₄) zypapophyseal joint cartilage was measured by vernier caliper. The thickness of the back ventral articular cartilage was added and then compared with corresponding MR images. 3D-FLASH (FA 20°) and WE 3D-FLASH (FA 20°) sequences had significant advantages compared with other sequences in imaging lumbar zypapophyseal joint cartilage, and were mostly close to the real thickness.(1) Comparison of the 4 flip angle (FA 10°, FA 20°, FA 30°, and FA 40°) 3D-FLASH sequences:The highest cartilage SNR and best CNR of cartilage versus bone cortex were both found in the 3D-FLASH(FA 20°) sequence, which was significantly different from the other three 3D-FLASH sequences.The satisfactory CNR of cartilage versus bone marrow, cartilage versus saline were found more in the 3D-FLASH(FA 20°) sequence. (2) Comparison of the 4 flip angle(FA 10°, FA 20°, FA 30°, and FA 40°) WE 3D-FLASH sequences: the highest cartilage SNR,best CNR of cartilage versus bone cortex,and best CNR of cartilage versus bone marrow were found in the WE 3D-FLASH (FA 20°) sequence, which was significantly different from the other three 3D-FLASH sequences. The CNR of cartilage versus saline was found more satisfactory in the WE 3D-FLASH (FA 20°) sequence. (3) The highest cartilage SNR and best CNR of cartilage versus bone cortex were both found in the 3D-FLASH (FA 20°) sequence, which was significantly different from those in the PDWI, FSE T1WI,and FSE T2WI sequences (P<0.05), but with no significance (P>0.05) in the WE 3D-FLASH (FA 20°) sequence. The highest CNR of cartilage versus bone marrow was seen in WE 3D-FLASH (FA 20°) sequence. It was statistically significant compared with that in FS PDWI,FSE T1WI, and T2WI sequences respectively, but the difference was not significant compared with 3D-FLASH (FA 20°) sequence (P>0.05). Both the FS PDWI and T2WI sequences displayed ideal CNR of cartilage versus saline, with no significant difference (P>0.05). The lower SNR of cartilage versus saline was shown in 3D-FLASH (FA 20°) and WE 3D-FLASH (FA 20°) sequence, and the difference was not significant (P>0.05). However, they were significantly different compared with FS PDWI and T2WI sequences (P<0.05). (4) WE 3D-FLASH (FA 20°) and 3D-FLASH (FA 20°) sequences were relatively better than the FS PDWI when comparing the thickness of articular cartilage, which was significantly different from the FS PDWI sequence (P<0.05). The 3D-FLASH sequence and derived WE 3D-FLASH sequence have better definition of cartilage images and are mostly close to the real thickness, which possibly are the optimal scanning sequences for lumbar zypapophyseal joint articular cartilage MR imaging.

Gadolinium toxicity: epidermis thickness measurement by magnetic resonance imaging at 500 MHz

Regional epidermal thickening and hair follicle width measurement by delayed gadolinium contrast magnetic resonance imaging (MRI) may assess the contrast agent gadolinium toxicity on mice skin. Delayed contrast in vivo MRI was performed in mice. Six mice skin samples were removed and exposed to a gadolinium contrast agent at different times after 2, 4, 6 and 8 h. The relaxation constants of each skin structure were measured. The thickness of the epidermis and hair follicle on follow-up ex vivo delayed-contrast MRI served as an index of gadolinium toxicity on the skin. In vivo MRI by fast low-angle shot imaging technique showed distinct skin layers. High-resolution gradient echo T1-weighted and multislice multiecho proton density-weighted MRI intensities in the epidermis and hair follicle showed a positive correlation with delayed gadolinium-enhanced MRI hyperintensities (Pearson's correlation coefficient r(2)=0.81, P<0.0001) in the excised mice skin tissues. Delayed contrast-enhanced mice skin MRI after 2-4 h showed epidermis swelling and hair follicle regions with a size measurement accuracy of 65%, a sensitivity of 95%, a specificity of 25%, a positive predictive value of 65% and a negative predictive value of 65%. Areas under the receiver operating characteristic curves by MRI were 0.92-0.94 for hair and epidermis as good discriminators. MRI visualized distinct relaxation constants of the epidermis, sebaceous gland, skin papillary and reticular dermis layers and hair follicle. Gadolinium contrast-enhanced MRI may visualize the thickening of the epidermis wall and hair follicle as an index of viable mice skin. Gadolinium enhanced the MRI visibility of skin structures. Gadolinium treatment showed skin toxicity as epidermis thickening the first time due to the undesirable use of high concentrations of gadolinium in microimaging.

Cardiovascular Magnetic Resonance

Cardiovascular magnetic resonance (CMR) creates images from atomic nuclei with uneven spin using radiowaves in the presence of a magnetic field. Full details of the physical principles can be found elsewhere.1 For clinical purposes, MR is performed using hydrogen-1, which is abundant in water and fat. Radiofrequency waves excite the area of interest to create tissue magnetization that decays (relaxation) and after a short period is induced to release energy as a radio signal. These echoes are converted with Fourier transformation into images of spatially resolved radio signals. Relaxation is quantified in spatially orthogonal directions as T1 and T2, which allows tissue characterization to serve as a powerful clinical tool. A CMR scanner consists of a superconducting magnet, a radiofrequency transmitter and receiver connected to radio aerials, and gradient coils driven by powerful pulses of electricity to create transient magnetic fields. The imaging computer triggers to the ECG and runs scanning sequences that coordinate the complex processes. The spin-echo sequence yields static images with black blood, whereas the gradient-echo sequence usually acquires multiple images through the cardiac cycle that display cardiovascular function as cine loops. Preparation pulses can be used to display T1 or T2 contrast for tissue characterization. Fast and real-time sequences include fast low-angle shot, steady-state free-precession, spiral, and echo-planar imaging. Velocity and flow can be quantified with gradient-echo sequences with encoding of velocity in the signal phase.2 CMR is extremely safe, which is unequivocally advantageous compared with x-rays, but special cautions apply. Ferromagnetic items become potentially dangerous projectiles in the scanner room. Although most metallic medical implants, including all prosthetic cardiac valves and coronary stents, are MR compatible, care is required with cerebrovascular clips. Pacemakers and defibrillators present problems, in particular related to wire heating, and are a contraindication for CMR, although specialist centers have …

An expandable catheter loop coil for intravascular MRI in larger blood vessels

The present study proposes a catheter system with an expandable coil etched on a polyimide foil. The catheter system combines the advantages of a small insertion diameter when the coil is rolled up in a protective carrier sheath with an increased signal-to-noise ratio (SNR) and penetration depth when the coil is pushed out. After imaging, the coil can be retracted into the sheath and folded back into the initial rolled-up configuration due to the tapered geometry of the carrier foil. The catheter system was tested on two healthy anesthetized pigs, including tracking and high-resolution intravascular imaging. To reduce artifacts in high-resolution images induced by catheter motion in the pulsatile blood flow, a motion-gating method was implemented that combines a flow-compensated two-dimensional fast low angle shot (FLASH) imaging sequence with the acquisition of projection data for retrospective gating. Using the projection data for motion detection, image SNR was increased by up to 500% over uncorrected images, and anatomic structures of 150 microm size could be differentiated in the aorta.

Age-related Iron Deposition in the Basal Ganglia: Quantitative Analysis in Healthy Subjects

To determine the values of iron accumulation in the basal ganglia of healthy volunteers of different ages with R2* and raw signal intensity measurements from T1-weighted magnetic resonance (MR) images, supported by voxel-based relaxometry (VBR), and to compare them with previously reported iron concentrations found in autopsy material. The ethics committee approved the study, and the participants or their parents gave written informed consent. Eighty subjects (41 female and 39 male subjects; age range, 1-80 years) were examined at 1.5 T. For each subject, R2* values were calculated. Curves for R2* versus age were obtained for globus pallidus (GP), putamen, caudate nucleus, substantia nigra (SN), and frontal white matter (FWM). To highlight possible differences in iron concentration among the age decades, VBR was applied. Signal intensity values were estimated on T1-weighted fast low-angle shot images, and regions of interest were drawn in each nucleus. R2* values were also compared with iron concentrations reported in a postmortem study. Statistical analysis was performed (t test), and a difference with P < .05 (FDR corrected) was significant. The curves for R2* versus age showed an exponential increase with increasing age in all the basal ganglia. VBR demonstrated significant differences (P < .05, corrected) in the comparison between the 2nd and the following decades for lenticular nuclei. Good correlation coefficients were found for GP (R(2) = 0.64), putamen (R(2) = 0.51), and SN (R(2) = 0.53) when compared with findings in the postmortem study. Signal intensity curves were similar to the R2* curves. R2* measurements can be used to quantify brain iron accumulation and thus may allow better evaluation of neurodegenerative diseases associated with iron deposition.

Increased SNR and reduced distortions by averaging multiple gradient echo signals in 3D FLASH imaging of the human brain at 3T

To demonstrate how averaging of multiple gradient echoes can improve high-resolution FLASH (fast low angle shot) magnetic resonance imaging (MRI) of the human brain. 3D-FLASH with multiple bipolar echoes was studied by simulation and in three experiments on human brain at 3T. First, the repetition time (TR) was increased by the square of the flip angle to maintain contrast as derived by theory. Then the number of echoes was increased at constant TR with bandwidths between 110 and 1370 Hz/pixel. Finally, signals of a 12-echo acquisition train (echo times 4.9-59 msec) were averaged consecutively to study the increase in SNR. At unchanged contrast, the signal increased proportionally with flip angle and sqrt(TR). Increasing the bandwidth improved delineation of the basal cortex and vessels, while most of the loss in the signal-to-noise ratio (SNR) was recovered by averaging. Consecutive averaging increased the SNR to reach maximum efficiency at an echo train length corresponding roughly to T(2) (*). SNR is gained efficiently by acquiring additional echoes and increasing TR (and flip angle accordingly to maintain contrast) until the associated T(2) (*) loss in the averaged signal consumes the sqrt(TR) increase in the steady state. A bandwidth of 350 Hz/pixel or higher and echo trains shorter than T(2) (*) are recommended.

Functional MRI: Evaluation of Chronic Kidney Disease with Perfusion Imaging

To evaluate the functional alterations of chronic kidney disease (CKD) with magnetic resonance dynamic perfusion imaging. Twenty-one healthy subjects (42 kidneys) and 20 CKD patients (40 kidneys) underwent routine scans with fat-saturated T1-weighted fast low angle shot (FLASH) and true-fast imaging with steady-state precession (FISP) sequences followed by dynamic perfusion scans using a turbo-FLASH T1-weighted sequence. Signal intensity (SI) of the cortex and medulla on images was measured and plotted as a function of time. Peak height (P) and time to peak (T) of the cortex and medulla SI were estimated, and P/T ratio and the area under the time-intensity curves were calculated. We also tested the correlation between these data and serum creatinine (sCr) levels in patients. P, P/T ratio, and the area under the curve of patients' cortex and medulla were significantly decreased compared to control subjects, and T was delayed. In patients, P and P/T ratio of the cortex and P of the medulla were negatively correlated with sCr levels (r = -0.469, r = -0.419, and r = -0.423, respectively; P < 0.01). Renal dysfunction in CKD can be evaluated by magnetic resonance dynamic perfusion imaging.

Feasibility of Stent-Graft Placement with Real-Time MR Fluoroscopy in a Nonrigid Aortic Phantom

To evaluate the feasibility of using real-time magnetic resonance (MR) fluoroscopic guidance to place a stent-graft mounted on a guide wire in a nonrigid aortic phantom. Real-time fast low-angle shot and true fast imaging with steady-state precession MR imaging sequences were used for device tracking. A modified fiber-optic guide wire and catheter embedded with titanium oxide in predefined positions were used for navigation in a homemade silicone thoracic aortic phantom. Susceptibility artifacts caused by the modified guide wire and catheters mounted in the descending thoracic aorta of the phantom were found to enable adequate determination of the guide wire position in relation to the surrounding anatomy and to cause no image distortion. Real-time MR imaging enabled visualization of both the vessel lumen and the delivery system with the mounted stent-graft, providing an image quality sufficient for successful localization of the lesion and deployment of the stent-graft. The results of this study prove the possibility of passive guidance in MR imaging-guided stent placement in vitro. The modified guide wire can be used with interventional commercial catheters and recent implant devices with selective tracking in the surrounding anatomy.