Echo Train Length Research Articles

Optimizing isotropic three-dimensional fast spin-echo methods for imaging the knee.

To optimize acquisition parameters for three dimensional fast spin-echo (3D FSE) imaging of the knee. The knees of eight healthy volunteers were imaged in a 3 Tesla MRI scanner using an eight-channel knee coil. A total of 146 intermediate weighted isotropic resolution 3D FSE (3D-FSE-Cube)images with varied acquisition parameter settings were acquired with an additional reference scan performed for subjective image quality assessment. Images were graded for overall quality, parallel imaging artifact severity and blurring. Cartilage, muscle, and fluid signal-to-noise ratios and fluid-cartilage contrast-to-noise ratios were quantified by acquiring scans without radio frequency excitation and custom-reconstructing the k-space data.Mixed effects regression modeling was used to determine statistically significant effects of different parameters on image quality. Changes in receiver bandwidth, repetition time and echo train length significantly affected all measurements of image quality (P < 0.05). Reducing band width improved all metrics of image quality with the exception of blurring. Reader agreement was slight to fair for subjective metrics, but overall trends in quality ratings were apparent. We used a systematic approach to optimize 3D-FSE-Cube parameters for knee imaging. Image quality was overall improved using a receiver bandwidth of 631.25 kHz, and blurring increased with lower band width and longer echo trains.

Application of PINS radiofrequency pulses to reduce power deposition in RARE/turbo spin echo imaging of the human head

To explore the use of PINS radiofrequency (RF) pulses to reduce RF power deposition in multiband/simultaneous multislice imaging with the RARE/turbo spin echo (TSE) sequence at 3T and 7T. A PINS-TSE sequence was implemented and combined with blipped CAIPI to improve the reconstruction of superposed slices. Whole brain imaging of healthy volunteers was performed at both 3T and 7T using a 32-channel coil for signal reception. A considerable reduction in power deposition was achieved compared with a standard sequence of the manufacturer. At 3T, the reduction in specific absorption rate (SAR) made short pulse repetition times (TRs) possible, however, in order to obtain a good T2 contrast, it is advisable to maintain TR while extending the echo train length. At 7T, whole brain coverage with a spatial resolution of 1 × 1 × 2 mm(3) was achieved in an acquisition time of 150 s. Furthermore, it could be shown that pulse sequences that use PINS pulses do not suffer from having additional magnetization transfer contrast. PINS RF pulses combined with multiband imaging reduce SAR sufficiently to enable routine TSE imaging at 7T within clinically acceptable acquisition times. In general, the combination of multiband imaging with PINS RF pulses represents a method to reduce total RF power deposition.

Coronary Edema Demonstrated by Cardiovascular Magnetic Resonance in Patients With Peri-Stent Inflammation and Aneurysm Formation After Treatment by Drug-Eluting Stents

Drug-eluting stents (DES) minimized the occurrence of in-stent restenosis but with a penalty of increased risk of late stent thrombosis.1 In patients with late stent thrombosis, adverse vessel wall reactions such as uncovered and malapposed stent struts, coronary aneurysms, and accelerated neoatherosclerosis2 have been identified. The stent polymer is suspected to induce an inflammatory response,3 resulting in vessel wall abnormalities. Identification of patients with an inflammatory response to DES might be essential in preventing late stent thrombosis. Inflammatory processes increase vessel wall permeability, leading to edema, and both native and peri-stent vessel wall inflammation may be associated with tissue edema. Cardiovascular magnetic resonance (CMR) performed with a T2-weighted short-τ inversion recovery sequence (T2-STIR) has recently been shown to identify localized coronary edema in the culprit artery of patients with acute myocardial infarction.4 Parameters for the ECG-triggered, navigator-gated, dark-blood, T2-STIR fast-spin-echo sequence were as follows: repetition time, 2 RR intervals; echo time, 100 milliseconds; echo train length, twenty 0.68×0.68×8-mm3 voxels; and 2 signal averages. Here we present 3 patients treated with DES (Cypher Select+, Cordis, Johnson & Johnson, Miami Lakes, FL) who demonstrate peri-stent edema as a possible marker of stent-induced coronary inflammation. A 41-year-old woman experienced very late stent thrombosis 15 months after treatment with DES in the proximal left anterior descending …

Transverse relaxometry with reduced echo train lengths via stimulated echo compensation

Transverse relaxation (T2 ) mapping has many applications, including imaging of iron accumulation in grey matter. Using the typical multiecho spin-echo sequence with long echo trains, stimulated echo compensation can enable T2 fitting under conditions of variable radio frequency homogeneity arising from slice profile and in-plane radio frequency variation. Substantial reduction in the number of refocusing pulses could enable use at high magnetic fields where specific absorption rate is a major limitation, and enable multislice use with reduced incidental magnetization transfer at all field strengths. We examine the effect of reduced echo train lengths and multislice imaging on T2 fitting using stimulated echo compensation applied to iron-rich subcortical grey matter in human brain at 4.7 T. Our findings indicate that reducing from 20 echoes to as few as four echoes can maintain consistent T2 values when using stimulated echo compensation in grey and white matter, but not for cerebrospinal fluid. All territories produce marginal results when using standard exponential fitting. Savings from reduced echoes can be used to substantially increase slice coverage. In multislice mode, the resulting incidental magnetization transfer decreased brain signal but had minimal effect on measured T2 values.

Defining a magnetic resonance scan sequence for permanent seed prostate brachytherapy postimplant assessment

We describe a magnetic resonance (MR) scan sequence for prostate brachytherapy postimplant assessment. One brachytherapy team at the British Columbia Cancer Agency has incorporated MR-CT fusion into their permanent seed prostate brachytherapy quality assurance procedure. Several attempts were required to ensure that the diagnostic MR scanner at the adjoining general hospital performed the desired sequence, providing many examples of suboptimal scans and underlining the pitfalls for a center trying to incorporate the use of MR scanning into their brachytherapy program. The recommended sequence (Fast Spin Echo T2-weighted, repetition time [TR]/echo time [TE] 4500/90, echo train length [ETL] 10, 20×20 field of view [FOV], 80 bandwidth [BW]) is associated with superior edge detection when compared with those images in which a typical diagnostic sequence was used. The use of a low bandwidth sequence does not compromise edge detection or seed identification when compared with a higher bandwidth. We have defined a magnetic resonance imaging sequence, which appears to optimize both prostate delineation and identification of seeds, lending itself to straightforward fusion with CT images and allowing for less uncertainty in permanent seed prostate brachytherapy quality assurance.

Single‐shot echo‐planar imaging with Nyquist ghost compensation: Interleaved dual echo with acceleration (IDEA) echo‐planar imaging (EPI)

Echo planar imaging (EPI) is most commonly used for blood oxygen level-dependent fMRI, owing to its sensitivity and acquisition speed. A major problem with EPI is Nyquist (N/2) ghosting, most notably at high field. EPI data are acquired under an oscillating readout gradient and hence vulnerable to gradient imperfections such as eddy current delays and off-resonance effects, as these cause inconsistencies between odd and even k-space lines after time reversal. We propose a straightforward and pragmatic method herein termed "interleaved dual echo with acceleration (IDEA) EPI": two k-spaces (echoes) are acquired under the positive and negative readout lobes, respectively, by performing phase encoding blips only before alternate readout gradients. From these two k-spaces, two almost entirely ghost free images per shot can be constructed, without need for phase correction. The doubled echo train length can be compensated by parallel imaging and/or partial Fourier acquisition. The two k-spaces can either be complex averaged during reconstruction, which results in near-perfect cancellation of residual phase errors, or reconstructed into separate images. We demonstrate the efficacy of IDEA EPI and show phantom and in vivo images at both 3 T and 7 T.

Improved motion correction capabilities for fast spin echo T1 FLAIR propeller using non‐Cartesian external calibration data driven parallel imaging

Patient motion is a common challenge in the clinical setting and fast spin echo longitudinal relaxation time fluid attenuating inversion recovery imaging method with motion correction would be highly desirable. The motion correction provided by transverse relaxation time- and diffusion-weighted periodically rotated overlapping parallel lines with enhanced reconstruction methods has seen significant clinical adoption. However, periodically rotated overlapping parallel lines with enhanced reconstruction with fast spin echo longitudinal relaxation time fluid attenuating inversion recovery-weighting has proved challenging since motion correction requires wide blades that are difficult to acquire while also maintaining short echo train lengths that are optimal for longitudinal relaxation time fluid attenuating inversion recovery-weighting. Parallel imaging provides an opportunity to increase the effective blade width for a given echo train lengths. Coil-by-coil data-driven autocalibrated parallel imaging methods provide greater robustness in the event of motion compared to techniques relying on accurate coil sensitivity maps. However, conventional internally calibrated data-driven parallel imaging methods limit the effective acceleration possible for each blade. We present a method to share a single calibration dataset over all imaging blades on a slice by slice basis using the APPEAR non-cartesian parallel imaging method providing an effective blade width increase of 2.45×, enabling robust motion correction. Results comparing the proposed technique to conventional cartesian and periodically rotated overlapping parallel lines with enhanced reconstruction methods demonstrate a significant improvement during subject motion and maintaining high image quality when no motion is present in normal and clinical volunteers.

Isotropic submillimeter fMRI in the human brain at 7 T: Combining reduced field‐of‐view imaging and partially parallel acquisitions

Echo-planar imaging is the most widely used imaging sequence for functional magnetic resonance imaging (fMRI) due to its fast acquisition. However, it is prone to local distortions, image blurring, and signal voids. As these effects scale with echo train length and field strength, it is essential for high-resolution echo-planar imaging at ultrahigh field to address these problems. Partially parallel acquisition methods can be used to improve the image quality of echo-planar imaging. However, partially parallel acquisition can be affected by aliasing artifacts and noise enhancement. Another way to shorten the echo train length is to reduce the field-of-view (FOV) while maintaining the same spatial resolution. However, to achieve significant acceleration, the resulting FOV becomes very small. Another problem occurs when FOV selection is incomplete such that there is remaining signal aliased from the region outside the reduced FOV. In this article, a novel approach, a combination of reduced FOV imaging with partially parallel acquisition, is presented. This approach can address the problems described above of each individual method, enabling high-quality single-shot echo-planar imaging acquisition, with submillimeter isotropic resolution and good signal-to-noise ratio, for fMRI at ultrahigh field strength. This is demonstrated in fMRI of human brain at 7T with an isotropic resolution of 650 μm.

Imaging Characteristics of Two-Dimensional Spin Echo T1-weighted Image in Carotid Artery Plaque

Recently, T₁ weighted image (T₁WI) has proven to be useful for diagnosing carotid plaque. This time, the image parameter of two-dimensional spin echo (2D SE) T₁WI was examined. Phantoms that imitated muscle and carotid plaque were made. Signal noise ratio (SNR) and the contrast of phantoms were examined when the flip angle (FA) of radio frequency (RF) pulse, repetition time (TR), and echo train length (ETL) was changed. A visual evaluation was done in a clinical case. Both SE and fast spin echo (FSE) SNR improved according to the extension of TR, and the contrast decreased. Moreover, the contrast improved when there was a lot of ETL and the FA of RF pulse. It is thought that this is because SNR and the contrast depend on the interrelation of TR, T₁ value, and the FA of RF pulse. When the FA of RF pulse was set to 70 degrees and the TR was set to 400 ms resulting from the phantom experiment, clinical cases obtained great results. This examination confirmed the utility of 2D SE in carotid plaque inspection.

Loss of fine structure and edge sharpness in fast‐spin‐echo carotid wall imaging: Measurements and comparison with multiple‐spin‐echo in normal and atherosclerotic subjects

To test whether the k-space acquisition strategy used by fast-spin-echo (FSE) is a major source of blurring in carotid wall and plaque imaging, and investigate an alternative acquisition approach. The effect of echo train length (ETL) and T(2) on the amount of blurring was studied in FSE simulations of vessel images. Edge sharpness was measured in black-blood T(1) W and proton density-weighted (PDW) carotid images acquired from 5 normal volunteers and 19 asymptomatic patients using both FSE and multiple-spin-echo (Multi-SE) sequences at 3 Tesla (T). Plaque images were classified and divided in group α (tissues' average T(2) ∼ 40-70 ms) and group β (plaque components with shorter T(2) ). Simulations predicted 26.9% reduction of vessel edge sharpness from Multi-SE to FSE images (ETL = 9, T(2) = 60 ms). This agreed with in vivo measurements in normal volunteers (27.4%) and in patient group α (26.2%), while in group β the loss was higher (31.6%). FSE significantly reduced vessel edge sharpness along the phase-encoding direction in T(1) W and PDW images. Blurring was stronger in the presence of plaque components with short T(2) times. This study shows a limitation of FSE and the potential of Multi-SE to improve the quality of carotid imaging.

Precise delineation of ureterocele anatomy: virtual magnetic resonance cystoscopy

To evaluate the application of virtual magnetic resonance cystoscopy (VMRC) in precise detection of ureterocele extension compared with conventional cystoscopy (as the gold standard) and other imaging modalities. Medical records of 55 patients, with confirmed diagnosis of ureterocele during endoscopic surgery, were reviewed retrospectively. Thirty-two of them (14 girls and 18 boys, age range: 4-22 months) whom underwent investigation with magnetic resonance urography, voiding cystourethrography, and ultrasonography were included. Images were obtained using T1-scanner with pulse sequences (TR = 30-50 ms, TE = 2-8 ms, echo train length = 8 moderate, filip angel = 40°). Using multiplanar reformation from source images, the bladder was evaluated to determine ureterocele anatomy. Virtual findings were compared with other imaging studies. Cystoscopy confirmed 3(9%) bilateral and 9(28%) unilateral intravesical ureteroceles, and 20(63%) ectopic type. VMRC was the most sensitive method in detection of ureterocele anatomy and its extension (detection rate: 94%) It detected one case of bilateral intravesical and three ectopic types, which were not detected in other imaging modalities. VMRC is a promising non-invasive technique in exact detection of ureterocele extension prior to endoscopic operation. It permits evaluation of concurrent genitourinary abnormalities and facilitates preoperative planning especially in complex ectopic ureteroceles. It may be indicated as a clinical routine, when conventional cystoscopy is contraindicated.

Optimization of multiply acquired magnetic flux density Bz using ICNE-Multiecho train in MREIT

The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, Bz, for MREIT in the signal-to-noise ratio (SNR). However, the ICNE method results in undesirable side artifacts, such as blurring, chemical shift and phase artifacts, due to the long data acquisition under an inhomogeneous static field. In this paper, we apply the ICNE method to a gradient and spin echo (GRASE) multi-echo train pulse sequence in order to provide the multiple k-space lines during a single RF pulse period. We analyze the SNR of the measured multiple Bz data using the proposed ICNE-Multiecho MR pulse sequence. By determining a weighting factor for Bz data in each of the echoes, an optimized inversion formula for the magnetic flux density data is proposed for the ICNE-Multiecho MR sequence. Using the ICNE-Multiecho method, the quality of the measured magnetic flux density is considerably increased by the injection of a long current through the echo train length and by optimization of the voxel-by-voxel noise level of the Bz value. Agarose-gel phantom experiments have demonstrated fewer artifacts and a better SNR using the ICNE-Multiecho method. Experimenting with the brain of an anesthetized dog, we collected valuable echoes by taking into account the noise level of each of the echoes and determined Bz data by determining optimized weighting factors for the multiply acquired magnetic flux density data.

Isotropic 3-Dimensional Fast Spin Echo Imaging Versus Standard 2-Dimensional Imaging at 3.0 T of the Knee

To compare different fat-saturated (FS) 3-dimensional (3D) intermediate-weighted (IM-w) fast spin echo (FSE) sequences with a standard FS 2-dimensional (2D) IM-w FSE sequence using a porcine in vitro model with artificially created cartilage and meniscus lesions. Using a ceramic scalpel, cartilage lesions with different depths and sizes were created in porcine knee specimens at the patella as well as the medial and lateral femoral and tibial cartilage. In addition, lateral and medial meniscal lesions were produced. Magnetic resonance imaging was performed at 3.0 T in sagittal plane using an 8-channel knee coil. A standard FS 2D IM-w FSE sequence and 3 newly developed isotropic 3D FSE sequences: (i) non-FS echo train length (ETL): 78, (ii) FS ETL: 44, and (iii) FS ETL: 44, were used. The images were independently analyzed by 4 radiologists concerning image quality (1 = optimal image quality, 4 = substantially limited quality) and absence or presence of lesions using a 5-level confidence score (1 = definite no presence of abnormality, 5 = definite presence of abnormality). Radiologists were also asked to measure diameter and categorize the depth of cartilage lesions using a modified Noyes classification. Average scores for image quality, confidence of diagnosis, and sensitivity, specificity, and accuracy were calculated. In addition, contrast-to-noise ratios were calculated. Image quality was significantly (P < 0.05) lower on the 3D FSE images than on the 2D FSE images [3D (i): 1.6 (SD, 0.43); 3D (ii): 2.35 (SD, 0.7); 3D (iii): 2.35 (SD, 0.5); 2D: 1.3 (SD, 0.35)]. No significant differences in diagnostic performance were found between 3D (i) and 2D FSE sequences. However, 16% fewer lesions were correctly detected with the 3D (ii) and (iii) sequences. Sensitivity was highest for the 2D sequence, and specificity was highest for the 3D (i) sequence. Confidence scores were higher for the 3D (i) sequence than for the 2D sequence. A significant increase (P < 0.05) in correctly measured cartilage lesions size and depth was found for the 3D (i) sequence over the standard 2D FSE sequence. Although the 3D FSE sequence performed better in depiction and characterization of cartilage abnormalities than the standard 2D FSE sequence, we currently do not recommend to use it as substitute. For the diagnosis of meniscal defects, however, no significant improvement was found.

Minimizing Ferromagnetic Artefact With Metallic Lumbar Total Disc Arthroplasty Devices at Adjacent Segments

Technical report. To minimize the gross artefact associated with a conventional metallic lumbar total disc arthroplasty (TDA) device on a conventional high field-strength (1.5 Tesla [T]) magnetic resonance imaging (MRI) scanner. Gross artefact is often apparent with ferromagnetic metallic TDA devices on conventional high field-strength MRI scanners. Such artefact completely obliterates MRI assessment at the operated level, and usually obscures visualization of adjacent segments. Because adjacent segment preservation is a raison d'être of spinal TDA, clarity of imaging at this latter level is imperative. A failure to image adjacent segments may presage investigations, which are either invasive (e.g., myelography) or associated with significant radiation hazard (e.g., computed tomography), both with significantly less diagnostic sensitivity. This could negatively direct TDA choice with certain TDAs. Progressive modifications to specific imaging parameter settings were sought on a conventional high field-strength (1.5T) closed-bore scanner to match the minimal artefact previously observed on a lower field-strength (0.3T) open scanner. Direct comparisons were made between each modified protocol image obtained initially using a phantom; however, routine postoperative MRIs were subsequently obtained in n = 40 patients following lumbar TDA insertion. Key parameter modifications were required in the receive bandwidth, the strength of the frequency encoding gradient, as well as in the echo train length. The use of higher specification "focused gradients" was also avoided. The overall effect was to reduce the slew rate of the gradients, which limited artefact due to a decrease in phase dispersion. Such appearances effectively matched with those previously obtained on the low field-strength (0.3T) open scanner in n = 40 patients. Relatively simple modifications to MRI parameter settings can be made on conventional high field-strength (1.5T) closed-bore scanners, which minimize metal artefact and enhance imaging of adjacent segments with ferromagnetic TDA devices. Such modifications effectively match appearances to those obtained with outmoded low field-strength (0.3T) open-bore scanners.

MR Imaging at 3.0 T in Children: Technical Differences, Safety Issues, and Initial Experience

The high signal-to-noise ratio and contrast-to-noise ratio of 3.0-T magnetic resonance (MR) imaging can be used to obtain high-resolution thin-section images in a short acquisition time. These advantages are associated with an increased specific absorption rate (SAR) and more artifacts owing to B(1) inhomogeneity and increased susceptibility and chemical shift. Potential advantages of 3-T imaging in children include acquisition of good-quality images even with a small field of view (FOV). The shorter overall acquisition time of 3-T imaging is useful in children, who may not be able to cooperate for long. Shorter acquisition times also improve safety by reducing patient monitoring time within the enclosed bore of an MR imaging unit. SAR-related issues and dielectric artifacts are less problematic with a small FOV. Parallel imaging helps reduce SAR, susceptibility artifacts, and blurring of T2-weighted fast spin-echo (FSE) and single-shot FSE images by reducing the echo train length.

Clinical application of respiratory navigator echo triggered black blood contrast cardiac MRI

Objective To investigate the application of respiratory navigator echo triggered black blood contrast FSE in cardiac MRI. Methods The respiratory navigator echo trigger technique combining with black blood FSE (NAV-FSE) was tested on 11 volunteers and 5 patients in free breathing,using breath-hold FSE (BH-FSE) with the same imaging protocals as control. The imaging efficiency and the image sharpness were compared between NAV-FSE and BH-FSE and t-test was used for the statistics. Results All NAV-FSE acquisitions were completed in sixteen subjects while 4 BH-FSE acquisitions failed because of poor breath holding. The efficiencies of NAV-FSE were (42. 95±11.50)%, (56. 14±11.40)% and (55.25± 14. 70)% when echo train length (ETL) were 24, 16 and 8, respectively. When ETL were 16 and 24, the sharpness of NAV-FSE ( 0. 43±0. 02 vs 0. 36±0. 02 ) and BH-FSE ( 0. 36±0. 03 vs 0. 35±0. 02 ) were statistically different (t =4. 26, 5. 53 ,respectively; P <0. 05). NAV-FSE could have a shorter ETL setting without consideration of breath holding. Conclusion The navigator echo trigger technique could be compatible with black blood contrast FSE to image the heart without the restriction of breath holding and it allows to optimize the parameters to improve the image quality. Key words: Heart; Magnetic resonance imaging; Image processing, computer-assisted

3T Magnetic Resonance Imaging of Ankle and Hindfoot Tendon Pathology

Ankle tendon pathology is relatively common in the active adult population. Magnetic resonance imaging is often the preferred advanced imaging option for the evaluation of tendon pathology of the hindfoot and ankle. The almost linear increase in signal-to-noise ratio provided by higher field strength imaging allows for improved image resolution and decreased scan times. Newer systems with faster gradients allow for optimal fast spin-echo imaging with lower echo spacing for longer echo train lengths and minimal image blurring. The ability to comfortably scan the ankle within the magnet isocenter using high-field strength-compatible extremity coils further maximizes the image resolution. It is imperative for the radiologist to be aware of necessary protocol adjustments and potential imaging artifacts unique to high-field strength imaging of the ankle. Our review outlines high-field strength magnetic resonance imaging technique and artifacts and also details the specifics of our own methods of ankle imaging.

The Accuracy of Virtual Magnetic Resonance Cystoscopy in Detection of Uretererocele Anatomy

Purpose To evaluate the accuracy of Virtual Magnetic Resonance Cystoscopy (VMRC) in confirmation of ureterocele and the precise anatomy of ectopic ureterocele in children. Material and Methods Thirty children with duplex collecting system and suspicious ureterocele on ultrasonography were enrolled in this study. The children were examined with a three-dimensional T1-weighted spoiled grass echo (TR=30-50 ms, TE= 2-8 ms, echo train length 8). The virtual cystoscopic and multiplanar reconstructions were performed. The accurate anatomy of upper and lower urinary tracts systems was observed in all children. The VMRC findings were compared with conventional imaging studies and cystoscopy. Results Virtual MR Cystoscopy was the most sensitive method in detection of ureterocele anatomy (97.7%). The role of this method was significantly enhanced in confirmation of bilaterally, and the anatomy of ureterocele in contrast to the other imaging methods. Conclusions Virtual MR Cystoscopy provides a promising non-invasive and safe technique in detection of ureterocele, and delineates the ectopic type in children prior to surgery. It could have a considerable role in precise anatomy of the ureter especially the distal site and it permits whole evaluation of the following abnormalities in genitourinary system. This adjuvant technique could facilitate the preoperative planning, especially in poor functioning upper moiety, ectopic vaginal ureter in contra-lateral system and cecoureterocele.

Autoregressive moving average modeling for spectral parameter estimation from a multigradient echo chemical shift acquisition

The authors investigated the performance of the iterative Steiglitz-McBride (SM) algorithm on an autoregressive moving average (ARMA) model of signals from a fast, sparsely sampled, multiecho, chemical shift imaging (CSI) acquisition using simulation, phantom, ex vivo, and in vivo experiments with a focus on its potential usage in magnetic resonance (MR)-guided interventions. The ARMA signal model facilitated a rapid calculation of the chemical shift, apparent spin-spin relaxation time (T2*), and complex amplitudes of a multipeak system from a limited number of echoes (< or equal 16). Numerical simulations of one- and two-peak systems were used to assess the accuracy and uncertainty in the calculated spectral parameters as a function of acquisition and tissue parameters. The measured uncertainties from simulation were compared to the theoretical Cramer-Rao lower bound (CRLB) for the acquisition. Measurements made in phantoms were used to validate the T2* estimates and to validate uncertainty estimates made from the CRLB. We demonstrated application to real-time MR-guided interventions ex vivo by using the technique to monitor a percutaneous ethanol injection into a bovine liver and in vivo to monitor a laser-induced thermal therapy treatment in a canine brain. Simulation results showed that the chemical shift and amplitude uncertainties reached their respective CRLB at a signal-to-noise ratio (SNR) > or =5 for echo train lengths (ETLs) > or =4 using a fixed echo spacing of 3.3 ms. T2* estimates from the signal model possessed higher uncertainties but reached the CRLB at larger SNRs and/or ETLs. Highly accurate estimates for the chemical shift (<0.01 ppm) and amplitude (<1.0%) were obtained with > or =4 echoes and for T2*(<1.0%) with > or =7 echoes. We conclude that, over a reasonable range of SNR, the SM algorithm is a robust estimator of spectral parameters from fast CSI acquisitions that acquire < or =16 echoes for one- and two-peak systems. Preliminary ex vivo and in vivo experiments corroborated the results from simulation experiments and further indicate the potential of this technique for MR-guided interventional procedures with high spatiotemporal resolution approximately 1.6 x 1.6 x 4 mm3 in < or =5 s.

Coronary Vessel Wall Evaluation by Magnetic Resonance Imaging in the Multi-Ethnic Study of Atherosclerosis

Coronary artery wall magnetic resonance imaging (MRI) has been developed to assess coronary lumen diameter and wall thickness. The purpose of this study was to evaluate the physiological parameters that affect the measures of coronary wall thickness using black-blood MRI pulse sequences. Eighty-seven participants (38 men and 49 women) of the Multi-Ethnic Study of Atherosclerosis were enrolled in the coronary artery wall MRI study. Cine 4-chamber imaging was used to determine the coronary artery rest period. Free-breathing whole-heart magnetic resonance angiography with motion adaptor navigator was performed to localize the coronary arteries in 64 participants. Cross-sectional free-breathing black-blood images were acquired using electrocardiogram-gated, turbo spin echo sequence. Imaging parameters were as follows: repetition time = 2 R-R intervals, time to echo = 33 milliseconds, echo train length = 13, bandwidth = 305 Hz/pixel, matrix = 416 x 416, field of view = 420 x 420 mm, and slice thickness = 4 to 5 mm. Imaging was completed in 215 (92%) of 234 coronary segments; 9 participants had incomplete scans. Mean age was 62.6 +/- 8.4 years (range, 45-81 years). Mean body mass index was 29.2 +/- 5.9 kg/m2. A higher proportion of images with quality of "good" was seen in the right coronary artery (40.5%) compared to the left main and left anterior descending coronary arteries (31.9% and 26.4%, respectively). There was a very good agreement between observers in the image quality scores (kappa = 0.79, P < 0.001). Lower heart rate, male sex, and longer coronary rest period were associated with higher image quality score (P < 0.05). Signal-to-noise ratio was higher in participants with Agatston calcium score of more than 10 in the right coronary and left main arteries (48.5 vs 69.7, P = 0.001; and 53.4 vs 61.6, P = 0.032, respectively). Improved depiction of the coronary artery wall with MRI is related to coronary rest period and atherosclerotic plaque burden as measured by calcium score and inversely related to heart rate. Because longer coronary artery rest periods are associated with improved image quality both for angiography with MRI and coronary artery wall imaging, heart rate-lowering methods in association with these techniques appear to be a logical application.