The improvement of MRI speed with parallel acquisition is ultimately an SNR-limited process. To offset acquisition- and reconstruction-related SNR losses, practical parallel imaging at high accelerations should include the use of a many-element array with a high intrinsic signal-to-noise ratio (SNR) and spatial-encoding capability, and an advantageous imaging paradigm. We present a 32-element receive-coil array and a volumetric paradigm that address the SNR challenge at high accelerations by maximally exploiting multidimensional acceleration in conjunction with noise averaging. Geometric details beyond an initial design concept for the array were determined with the guidance of simulations. Imaging with the support of 32-channel data acquisition systems produced in vivo results with up to 16-fold acceleration, including images from rapid abdominal and MRA studies.
parallel imaging; volumetric imaging; coil design; high acceleration; abdominal imaging; MRA
To compare the performance of the conventional diaphragm navigator (DNAV) and the recently developed cardiac fat navigator (FatNAV) in suppressing respiration-induced cardiac motion in free-breathing 3D balanced steady-state free precession coronary MRA (SSFP CMRA).
Materials and Methods
In sixteen healthy volunteers, the right coronary artery (RCA) was imaged at 1.5T using a navigator-gated 3D SSFP CMRA sequence. DNAV and FatNAV gating were performed in random order. Image quality difference was scored by three experienced readers blinded to the gating technique. Blood signal-to-noise ratio (SNR), blood-to-myocardium contrast-to-noise ratio (CNR), and navigator efficiency were calculated.
Diagnostically interpretable CMRA was obtained successfully in all sixteen subjects with FatNAV gating (0% failure rate) and only fourteen subjects with DNAV gating (12% failure rate). Compared to DNAV gating, FatNAV gating provided similar SNR and CNR, better image quality (P < 0.01), and 28% improvement in navigator efficiency (P = 0.002).
FatNAV gating provides more effective motion suppression and better image quality than DNAV gating for free-breathing 3D SSFP CMRA of the RCA in healthy subjects.
coronary magnetic resonance angiography; motion artifacts; diaphragm navigator; cardiac fat navigator; steady-state free precession
A new four-dimensional magnetic resonance angiograpy (MRA) technique called contrast-enhanced angiography with multiecho and radial k-space is introduced, which accelerates the acquisition using multiecho while maintaining a high spatial resolution and increasing the signal-to-noise ratio. An acceleration factor of approximately 2 is achieved without parallel imaging or undersampling by multiecho (i.e., echo-planar imaging) acquisition. SNR is gained from (1) longer pulse repetition times, which allow more time for T1 regrowth; (2) decreased specific absorption rate, which allows use of flip angles that maximize contrast at high field; and (3) minimized effects of a transient contrast bolus signal with a shorter temporal footprint. Simulations, phantom studies, and in vivo scans were performed. Contrast-enhanced angiography with multiecho and radial k-space can be combined with parallel imaging techniques such as Generalized Autocalibrating Partially Parallel Acquisitions to provide additional 2-fold acceleration in addition to higher SNR to trade off for parallel imaging. This technique can be useful in diagnosing vascular lesions where accurate dynamic information is necessary.
multiecho; radial; MRA; time-resolved; 4D; contrast-enhanced; CAMERA
The purpose of this study was to determine the image quality and diagnostic accuracy of three-dimensional (3D) unenhanced steady state free precession (SSFP) magnetic resonance angiography (MRA) for the evaluation of thoracic aortic diseases.
Fifty consecutive patients with known or suspected thoracic aortic disease underwent free-breathing ECG-gated unenhanced SSFP MRA with non-selective radiofrequency excitation and contrast-enhanced (CE) MRA of the thorax at 1.5 T. Two readers independently evaluated the two datasets for image quality in the aortic root, ascending aorta, aortic arch, descending aorta, and origins of supra-aortic arteries, and for abnormal findings. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were determined for both datasets. Sensitivity, specificity, and diagnostic accuracy of unenhanced SSFP MRA for the diagnosis of aortic abnormalities were determined.
Abnormal aortic findings, including aneurysm (n = 47), coarctation (n = 14), dissection (n = 12), aortic graft (n = 6), intramural hematoma (n = 11), mural thrombus in the aortic arch (n = 1), and penetrating aortic ulcer (n = 9), were confidently detected on both datasets. Sensitivity, specificity, and diagnostic accuracy of SSFP MRA for the detection of aortic disease were 100% with CE-MRA serving as a reference standard. Image quality of the aortic root was significantly higher on SSFP MRA (P < 0.001) with no significant difference for other aortic segments (P > 0.05). SNR and CNR values were higher for all segments on SSFP MRA (P < 0.01).
Our results suggest that free-breathing navigator-gated 3D SSFP MRA with non-selective radiofrequency excitation is a promising technique that provides high image quality and diagnostic accuracy for the assessment of thoracic aortic disease without the need for intravenous contrast material.
SSFP MR angiography; Unenhanced MRA; Thoracic aorta; Contrast; Enhanced MRA; Steady state free precession
Sensitivity in BOLD fMRI is characterized by the Signal to Noise Ratio (SNR) of the time-series (tSNR), which contains fluctuations from thermal and physiological noise sources. Alteration of an acquisition parameter can affect the tSNR differently depending on the relative magnitude of the physiological and thermal noise, therefore knowledge of this ratio is essential for optimizing fMRI acquisitions. In this study, we compare image and time-series SNR from array coils at 3T with and without parallel imaging (GRAPPA) as a function of image resolution and acceleration. We use the “absolute unit” SNR method of Kellman and McVeigh to calculate the image SNR (SNR0) in a way that renders it comparable to tSNR, allowing determination of the thermal to physiological noise ratio, and the pseudo-multiple replica method to quantify the image noise alterations due to the GRAPPA reconstruction. The Kruger and Glover noise model, in which the physiological noise standard deviation is proportional to signal strength, was found to hold for the accelerated and non-accelerated array coil data. Thermal noise dominated the EPI time-series for medium to large voxel sizes for single-channel and 12-channel head coil configurations, but physiological noise dominated the 32-channel array acquisition even at 1mm × 1mm × 3mm resolution. At higher acceleration factors, image SNR is reduced and the time-series becomes increasingly thermal noise dominant. However, the tSNR reduction is smaller than the reduction in image SNR due to the presence of physiological noise.
physiological noise; parallel imaging; array coils; fMRI; GRAPPA; SNR; 32-channel coil; resolution
To propose an improved motion-sensitized driven-equilibrium (iMSDE) pulse sequence to enhance the tissue signal-to-noise ratio (SNR) while maintaining the same flow suppression capability in black-blood carotid artery MRI.
Materials and Methods
Compared to the traditional MSDE sequence, the iMSDE sequence uses an extra refocusing pulse and two extra gradients to achieve SNR improvement. Computer simulation and phantom studies were used to evaluate both eddy currents and local B1 inhomogeneity effects on SNR behaviors on both MSDE and iMSDE images. To further assess the SNR improvements brought by iMSDE in vivo, 5 healthy volunteers were also scanned with both sequences. Paired t-test was used for statistical comparison.
Both simulations and phantom studies demonstrated that eddy currents and local B1 inhomogeneity will cause image SNR reduction in the MSDE sequence, and that these factors can be partially compensated for with the iMSDE sequence. In vivo comparison showed that the iMSDE sequence significantly improved the tissue-lumen contrast-to-noise ratio (CNR) and static tissue SNR (p<0.001 for both), while maintaining low lumen SNR in carotid MRI.
Compared to the traditional MSDE sequence, the iMSDE sequence can achieve improved soft tissue SNR and CNR in carotid artery MRI without sacrificing flow suppression capability and time efficiency.
Black-Blood; Carotid MRI; MSDE; iMSDE; Flow Suppression
To model the theoretical signal-to-noise ratio (SNR) behavior of 3-point chemical shift-based water-fat separation, using spectral modeling of fat, with experimental validation for spin-echo and gradient-echo imaging. The echo combination that achieves the best SNR performance for a given spectral model of fat was also investigated.
Materials and Methods
Cramér-Rao bound analysis was used to calculate the best possible SNR performance for a given echo combination. Experimental validation in a fat-water phantom was performed and compared with theory. In vivo scans were performed to compare fat separation with and with out spectral modeling of fat.
Theoretical SNR calculations for methods that include spectral modeling of fat agree closely with experimental SNR measurements. Spectral modeling of fat more accurately separates fat and water signals, with only a slight decrease in the SNR performance of the water-only image, although with a relatively large decrease in the fat SNR performance.
The optimal echo combination that provides the best SNR performance for water using spectral modeling of fat is very similar to previous optimizations that modeled fat as a single peak. Therefore, the optimal echo spacing commonly used for single fat peak models is adequate for most applications that use spectral modeling of fat.
chemical shift imaging; water-fat separation; IDEAL; spectral modeling of fat; Cramér-Rao bound
A 128-channel receive-only array coil is described and tested for cardiac imaging at 3T. The coil is closely contoured to the body with a “clam-shell” geometry with 68 posterior and 60 anterior elements, each 75 mm in diameter, and arranged in a continuous overlapped array of hexagonal symmetry to minimize nearest neighbor coupling. Signal-to-noise ratio (SNR) and noise amplification for parallel imaging (G-factor) were evaluated in phantom and volunteer experiments. These results were compared to those of commercially available 24-channel and 32-channel coils in routine use for cardiac imaging. The in vivo measurements with the 128-channel coil resulted in SNR gains compared to the 24-channel coil (up to 2.2-fold in the apex). The 128- and 32-channel coils showed similar SNR in the heart, likely dominated by the similar element diameters of these coils. The maximum G-factor values were up to seven times better for a seven-fold acceleration factor (R = 7) compared to the 24-channel coil and up to two-fold improved compared to the 32-channel coil. The ability of the 128-channel coil to facilitate highly accelerated cardiac imaging was demonstrated in four volunteers using acceleration factors up to seven-fold (R = 7) in a single spatial dimension.
MR array coil; cardiac MRI; parallel imaging
The quality of an RF detector coil design is commonly judged on how it compares with other coil configurations. The aim of this article is to develop a tool for evaluating the absolute performance of RF coil arrays. An algorithm to calculate the ultimate intrinsic signal-to-noise ratio (SNR) was implemented for a spherical geometry. The same imaging tasks modeled in the calculations were reproduced experimentally using a 32-element head array. Coil performance maps were then generated based on the ratio of experimentally measured SNR to the ultimate intrinsic SNR, for different acceleration factors associated with different degrees of parallel imaging. The relative performance in all cases was highest near the center of the samples (where the absolute SNR was lowest). The highest performance was found in the unaccelerated case and a maximum of 85% was observed with a phantom whose electrical properties are consistent with values in the human brain. The performance remained almost constant for 2-fold acceleration, but deteriorated at higher acceleration factors, suggesting that larger arrays are needed for effective highly-accelerated parallel imaging. The method proposed here can serve as a tool for the evaluation of coil designs, as well as a tool to guide the development of original designs which may begin to approach the optimal performance.
Magnetic Resonance Imaging; RF coils; phased-array; coil performance; coil design; parallel imaging; electrodynamics; ultimate intrinsic SNR
To investigate the signal-to-noise-ratio (SNR) and data quality of time-reduced 1H 3D-MRSI techniques in the human brain at 3T.
Materials and Methods
Techniques that were investigated included ellipsoidal k-space sampling, parallel imaging, and EPSI. The SNR values for NAA, Cho, Cre, and lactate or lipid peaks were compared after correcting for effective spatial resolution and acquisition time in a phantom and in the brains of human volunteers. Other factors considered were linewidths, metabolite ratios, partial volume effects, and subcutaneous lipid contamination.
In volunteers, the median normalized SNR for parallel imaging data decreased by 34–42%, but could be significantly improved using regularization. The normalized signal to noise loss in flyback EPSI data was 11–18%. The effective spatial resolutions of the traditional, ellipsoidal, SENSE, and EPSI data were 1.02, 2.43, 1.03, and 1.01cm3, respectively. As expected, lipid contamination was variable between subjects but was highest for the SENSE data. Patient data obtained using the flyback EPSI method were of excellent quality.
Data from all 1H 3D-MRSI techniques were qualitatively acceptable, based upon SNR, linewidths, and metabolite ratios. The larger FOV obtained with the EPSI methods showed negligible lipid aliasing with acceptable SNR values in less than 9.5 minutes without compromising the PSF.
Spectroscopic Imaging; EPSI; Parallel Imaging; SNR
This study reports quantitative comparisons of signal-to-noise ratio (SNR) at 1.5 and 3 T from images of carotid atheroma obtained using a multicontrast, cardiac-gated, blood-suppressed fast spin echo protocol.
18 subjects, with carotid atherosclerosis (>30% stenosis) confirmed on ultrasound, were imaged on both 1.5 and 3 T systems using phased-array coils with matched hardware specifications. T1 weighted (T1W), T2 weighted (T2W) and proton density-weighted (PDW) images were acquired with identical scan times. Multiple slices were prescribed to encompass both the carotid bifurcation and the plaque. Image quality was quantified using the SNR and contrast-to-noise ratio (CNR). A phantom experiment was also performed to validate the SNR method and confirm the size of the improvement in SNR. Comparisons of the SNR values from the vessel wall with muscle and plaque/lumen CNR measurements were performed at a patient level. To account for the multiple comparisons a Bonferroni correction was applied.
One subject was excluded from the protocol owing to image quality and protocol failure. The mean improvement in SNR in plaque was 1.9, 2.1 and 2.1 in T1W, T2W and PDW images, respectively. All plaque SNR improvements were statistically significant at the p<0.05 level. The phantom experiment reported an improvement in SNR of 2.4 for PDW images.
Significant gains in SNR can be obtained for carotid atheroma imaging at 3 T compared with 1.5 T. There was also a trend towards increased CNR. However, this was not significant after the application of the Bonferroni correction.
Sufficient temporal resolution is required to image the dynamics of blood flow, which may be critical for accurate diagnosis and treatment of various intracranial vascular diseases, such as arteriovenous malformations (AVMs) and aneurysms. Highly-constrained projection reconstruction (HYPR) has recently become a technique of interest for high-speed contrast-enhanced magnetic resonance angiography (CE-MRA). HYPR provides high frame rates by preferential weighting of radial projections while maintaining signal-to-noise ratio (SNR) by using a high SNR composite. An analysis was done to quantify the effects of HYPR on SNR, contrast-to-noise ratio (CNR), and temporal blur compared to the previously developed radial sliding-window technique using standard filtered backprojection or regridding methods. Computer simulations were performed to study the effects of HYPR processing on image error and the temporal information. Additionally, in vivo imaging was done on patients with angiographically confirmed AVMs to measure the effects of alteration of various HYPR parameters on SNR as well as the fidelity of the temporal information. The images were scored by an interventional radiologist in a blinded read and were compared with X-ray digital subtraction angiography (DSA). It was found that with the right choice of parameters, modest improvements in both SNR and dynamic information can be achieved as compared to radial sliding-window MRA.
HYPR; radial imaging; MRA; time-resolved; arteriovenous malformation; AVM
To investigate the magnetic field dependence of the signal-to-noise ratio (SNR) for carotid vessel wall magnetic resonance imaging (MRI) using phased-array (PA) surface coils by comparing images obtained at 1.5 T and 3 T, and to determine to what extent the improved SNR at the higher field can be traded for improved spatial resolution.
Materials and methods
Two pairs of dual-element PA coils were constructed for operation at the two field strengths. The individual elements of each PA were matched to 50 Ω impedance on the neck and tuned at the respective frequencies. The coils were evaluated on a cylindrical phantom positioned with its axis parallel to the main field, and with the coils placed on either side of the phantom parallel to the sagittal plane. In-vivo MR images of the carotid arteries were obtained in five subjects at both field strengths with a fast spin-echo double-inversion black-blood pulse sequence with fat saturation. SNR was measured at both field strengths using standard techniques.
At a depth corresponding to the average location of the carotid arteries in the study subjects, mean phantom SNR for the two coils was higher at 3 T by a factor of 2.5. The greater than linear increase is due to only partial coil loading of these relatively small coils. The practically achievable average SNR gain in vivo was 2.1. The lower in vivo SNR gain is attributed to a reduction in T2 and prolongation of T1 at the higher field strength and, to a lesser extent, the requirement for reduced refocusing pulse flip angle to operate within specific absorption ratio limits. The superior SNR at 3 T appears to provide considerably improved vessel wall delineation.
Carotid artery vessel wall MRI using phased-array surface coils provides a considerable increase in SNR when field strength is raised from 1.5 T to 3 T. This increase can be traded for enhanced in-plane resolution.
high field MRI; 3 T; carotid arteries; phased array coils; SNR
To design and compare an eight-channel phased array (PA) coil for carotid imaging to an established four-channel PA design at 3T.
Materials and Methods
An eight-channel PA (8PA) coil was designed specifically for imaging the carotid bifurcation and compared with the existing four-channel (4PA) design using a phantom and by in vivo black-blood magnetic resonance imaging (MRI). The 8PA and 4PA were compared in terms of coverage, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR).
The 8PA showed up to 1.7-fold improvement in SNR at a depth of 3.5 cm and greater longitudinal coverage at a given SNR on a phantom. The 8PA showed improved vessel wall SNR for high spatial resolution (0.63 mm2) PD, T1, and T2 (1.7, 1.7, 1.6 times, respectively; P ≤ 0.002) and improved CNR (1.7, 1.6, 1.5 times, respectively; P ≤ 0.002). Ultrahigh-resolution (0.27 mm2) T1-weighted images showed better SNR and CNR (1.4 times, P ≤ 0.0001) on 8PA compared to 4PA.
Carotid imaging studies may benefit from the improved SNR and larger coverage provided by use of the 8PA.
atherosclerosis; phased array coil; black-blood MRI; vessel wall imaging; carotid bifurcation; vulnerable atheroma
The benefits and challenges of highly parallel array coils for head imaging were investigated through the development of a 3T receive-only phased-array head coil with 96 receive elements constructed on a close-fitting helmet-shaped former. We evaluated several designs for the coil elements and matching circuitry, with particular attention to sources of signal-to-noise ratio (SNR) loss, including various sources of coil loading and coupling between the array elements. The SNR and noise amplification (g-factor) in accelerated imaging were quantitatively evaluated in phantom and human imaging and compared to a 32-channel array built on an identical helmet-shaped former and to a larger commercial 12-channel head coil. The 96-channel coil provided substantial SNR gains in the distal cortex compared to the 12- and 32-channel coils. The central SNR for the 96-channel coil was similar to the 32-channel coil for optimum SNR combination and 20% lower for root-sum-of-squares combination. There was a significant reduction in the maximum g-factor for 96 channels compared to 32; for example, the 96-channel maximum g-factor was 65% of the 32-channel value for acceleration rate 4. The performance of the array is demonstrated in highly accelerated brain images.
phased array; 96-channel; RF coil design; SNR; g-factor
To compare between the American College of Radiology (ACR) accreditation phantom and digital mammography accreditation phantom in assessing the image quality in full-field digital mammography (FFDM).
Materials and Methods
In each week throughout the 42-week study, we obtained phantom images using both the ACR accreditation phantom and the digital mammography accreditation phantom, and a total of 42 pairs of images were included in this study. We assessed the signal-to-noise ratio (SNR) in each phantom image. A radiologist drew a square-shaped region of interest on the phantom and then the mean value of the SNR and the standard deviation were automatically provided on a monitor. SNR was calculated by an equation, measured mean value of SNR-constant coefficient of FFDM/standard deviation. Two breast radiologists scored visible objects (fibers, specks, and masses) with soft-copy images and calculated the visible rate (number of visible objects/total number of objects). We compared SNR and the visible rate of objects between the two phantoms and calculated the k-coefficient for interobserver agreement.
The SNR of the ACR accreditation phantom ranged from 42.0 to 52.9 (Mean, 47.3 ± 2.79) and that of Digital Phantom ranged from 24.8 to 54.0 (Mean, 44.1 ± 9.93) (p = 0.028). The visible rates of all three types of objects were much higher in the ACR accreditation phantom than those in the digital mammography accreditation phantom (p < 0.05). Interobserver agreement for visible rates of objects on phantom images was fair to moderate agreement (k-coefficients: 0.34-0.57).
The ACR accreditation phantom is superior to the digital mammography accreditation phantom in terms of SNR and visibility of phantom objects. Thus, ACR accreditation phantom appears to be satisfactory for assessing the image quality in FFDM.
Breast; Mammography; Comparative study; Phantoms; Imaging
To optimize the MR imaging protocol for coronary arterial wall depiction in vitro and characterize the coronary atherosclerotic plaques.
Materials and Methods
MRI examination was prospectively performed in ten porcine hearts in order to optimize the MR imaging protocol. Various surface coils were used for coronary arterial wall imaging with the same parameters. Then, the image parameters were further optimized for high-resolution coronary wall imaging. The signal-noise ratio (SNR) and contrast-noise ratio (CNR) of images were measured. Finally, 8 human cadaver hearts with coronary atherosclerotic plaques were prospectively performed with MRI examination using optimized protocol in order to characterize the coronary atherosclerotic plaques.
The SNR and CNR of MR image with temporomandibular coil were the highest of various surface coils. High-resolution and high SNR and CNR for ex vivo coronary artery wall depiction can be achieved using temporomandibular coil with 512 × 512 in matrix. Compared with histopathology, the sensitivity and specificity of MRI for identifying advanced plaques were: type IV-V (lipid, necrosis, fibrosis), 94% and 95%; type VI (hemorrhage), 100% and 98%; type VII (calcification), 91% and 100%; and type VIII (fibrosis without lipid core), 100% and 98%, respectively.
Temporomandibular coil appears to be dramatically superior to eight-channel head coil and knee coil for ex vivo coronary artery wall imaging, providing higher spatial resolution and improved the SNR. Ex vivo high-resolution MRI has capability to distinguish human coronary atherosclerotic plaque compositions and accurately classify advanced plaques.
Coronary vessels; Magnetic resonance imaging; Atherosclerosis; Pathology
To develop an experimental protocol to calculate the precision and accuracy of fractional anisotropy (FA), mean diffusivity (MD), and the orientation of the principal eigenvector (PEV) as a function of the signal to noise ratio (SNR) in vivo.
Materials and Methods
A healthy male volunteer was scanned in three separate scanning sessions, yielding a total of 45 DTI scans. To provide FA, MD, and PEV as a function of SNR, sequential scans from a scan session were grouped into non-intersecting sets. Analysis of the accuracy and precision of the DTI-derived contrasts was done in both a voxel-wise and ROI-based manner.
An upward bias of FA and no significant bias in MD were present as SNR decreased, confirming results from simulation-based studies. Notably, while the precision of the PEV became worse at low SNR, no bias in the PEV orientation was observed. Overall, an accurate and precise quantification of FA values in GM requires substantially more SNR than the quantification of WM FA values
This study provides guidance for FA, MD, and PEV quantification and a means to investigate the minimal detectable differences within and across scan sessions as a function of SNR.
DTI; reproducibility; precision; fractional anisotropy; mean diffusivity; principal eigenvector
A 20-channel phased-array coil for Magnetic Resonance Imaging (MRI) of mice has been designed, constructed and validated with bench measurements and high resolution accelerated imaging. The technical challenges of designing a small, high density array have been overcome using individual small-diameter coil elements arranged on a cylinder in a hexagonal overlapping design with adjacent low impedance preamplifiers to further decouple the array elements. Signal-to-noise ratio (SNR) and noise amplification in accelerated imaging were simulated and quantitatively evaluated in phantoms and in vivo mouse images. Comparison between the 20-channel mouse array and a length-matched quadrature driven small animal birdcage coil showed an SNR increase at the periphery and in the center of the phantom of 3-fold and 1.3-fold, respectively. Comparison to a shorter but SNR-optimized birdcage coil (aspect ratio 1:1 and only half mouse coverage) showed an SNR gain of 2-fold at the edge of the phantom and similar SNR in the center. G-factor measurements indicate that the coil is well suited to acquire highly accelerated images.
magnetic resonance imaging; phased-array coil; small animal imaging; parallel imaging
In this work, a multi-echo parallel echo planar imaging (EPI) acquisition strategy is introduced as a way to improve the acquisition efficiency in parallel diffusion tensor imaging (DTI). With the use of an appropriate echo combination strategy, the sequence can provide a signal-to-noise ratio (SNR) enhancement while maintaining the advantages of parallel EPI. Simulations and in vivo experiments demonstrate that a weighted summation of multi-echo images provides a significant gain in SNR over the first echo image. It is experimentally demonstrated that this SNR gain can be utilized to reduce the number of measurements often required to ensure adequate SNR for accurate DTI measures. Furthermore, the multiple echoes can be used to derive a T2 map, providing additional information that might be useful in some applications.
Diffusion tensor imaging; diffusion weighted imaging; parallel imaging; GRAPPA; artifact reduction; SNR improvement
The relative signal-to-noise ratio (SNR) provided by 2D sensitivity encoding (SENSE) when applied to 3D contrast-enhanced MR angiography (CE-MRA) is studied. If an elliptical centric phase-encoding order is used to map the waning magnetization of the contrast bolus to k-space, the application of SENSE will reduce the degree of k-space signal modulation, providing a signal amplification A over corresponding nonaccelerated acquisitions. This offsets the SNR loss in R-accelerated SENSE due to
R and the geometry (g) factor. The theoretical bound on A is R and is reduced from this depending on the properties of the bolus profile and the duration over which it is imaged. In this work a signal amplification of 1.14–1.23 times that of nonvascular background tissue is demonstrated in a study of 20 volunteers using R = 4 2D SENSE whole-brain MR venography (MRV). The effects of a nonuniform g-factor and inhomogeneity of background tissue are accounted for. The observed amplification compares favorably with the value of 1.31 predicted numerically from a measured bolus curve.
2D SENSE contrast-enhanced MRA; elliptical centric view order; signal modulation and enhancement
In-vivo high resolution diffusion tensor imaging (DTI) of the mouse brain is often limited by the low signal to noise ratio (SNR) resulting from the required small voxel sizes. Recently, cryogenically cooled resonators (CCR) have demonstrated significant increase of the effective SNR. It is the objective of this study to enable fast DTI of the mouse brain. In this context, CCRs appear attractive for SNR improvement.
Three mice underwent a DTI examination at 1562×250 µm3 spatial resolution with a CCR at ultrahigh field (11.7T). Diffusion images were acquired along 30 gradient directions plus 5 references without diffusion encoding, resulting in a total acquisition time of 35 minutes. For comparison, mice additionally underwent a standardized 110 minutes acquisition protocol published earlier. Fractional anisotropy (FA) and fiber tracking (FT) results including quantitative tractwise fractional anisotropy statistics (TFAS) were qualitatively and quantitatively compared.
Qualitative and quantitative assessment of the calculated fractional anisotropy maps and fibre tracking results showed coinciding outcome comparing 35 minute scans to the standardized 110 minute scan. Coefficients of variation for ROI-based FA-comparison as well as for TFAS revealed comparable results for the different scanning protocols.
Mouse DTI at 11.7 T was performed with an acquisition time of approximately 30 minutes, which is considered feasible for cohort studies. The rapid acquisition protocol reveals reliable and reproducible FA-values and FT reconstructions, thus allowing an experimental setup for in-vivo large scale whole brain murine DTI cohort studies.
To improve spatial and temporal resolution and signal-to-noise ratio (SNR) in 3D radial contrast-enhanced (CE) time-resolved MR angiography by means of a novel hybrid phase contrast (PC) and CE MRA acquisition and HYPR reconstruction (PC HYPR Flow).
Materials and Methods
PC HYPR Flow consists of a CE exam immediately followed by a PC scan used to constrain the HYPR reconstruction of the time series. Temporal resolution of the new method was studied in computer simulations. The feasibility of the new technique was studied in healthy subjects and patients with brain arterio-venous malformations and in a canine model of aneurysms.
Simulations demonstrated preservation of contrast agent dynamics in proximal vessels, showing better performance than peer methods for acceleration up to 20 in 2D. In vivo, PC HYPR Flow yielded 3D time series with frame rate of 0.5 s and significantly outperformed two peer methods via a major increase in spatial resolution (0.8×0.8×0.8 mm3) and arterial/venous ratio, while maintaining necessary temporal waveform fidelity and high SNR.
This initial study indicates that PC HYPR Flow simultaneously provides 3D isotropic sub-millimeter spatial resolution, sub-second temporal reconstruction windows and high SNR level, which may benefit a wide range of CE MRA applications.
Constrained Reconstruction; Constrast Enhanced MRA; Phase Contrast MRA; HYPR; Angiography; Radial Trajectory
As optical coherence tomography (OCT) is widely used for diagnosis and monitoring of ocular pathology, especially in the elderly people, the influence of cataract on image quality and macular retinal thickness was studied.
In 29 patients scheduled for cataract surgery, preoperative and postoperative OCT scans were obtained. Cataracts were categorised as nuclear, posterior or cortical. Parameters for image quality (signal‐to‐noise ratio (SNR)) and signal strength and macular thickness were compared. A three‐level expert grading scale was used to evaluate the discriminative abilities of SNR and signal strength.
Nuclear cataracts (n = 12) provided better preoperative scans (higher SNR/signal strength) than posterior (n = 7) and cortical (n = 10) cataracts (p<0.004). Postoperatively SNR and signal strength increased significantly in all patients (p<0.001). The SNR was better at discriminating poor from acceptable and good scans than signal strength (area under the receiver operating curve: 0.879 and 0.810, respectively). Postoperative macular thickness overall showed a significant increase (p = 0.005), most evident in patients with posterior cataracts (p = 0.028).
OCT imaging is influenced by cataract; image quality is reduced preoperatively and macular thickness measurements are slightly increased postoperatively. In individual patients, OCT scans remain reliable for gross clinical interpretation, even in the presence of cataract.
Magnetic resonance spectroscopy (MRS) can benefit from increased signal-to-noise ratio (SNR) of high magnetic fields. In this work, the SNR gain of dynamic 31P MRS at 7 T was invested in temporal and spatial resolution. Using conventional slice selective excitation combined with localization by adiabatic selective refocusing (semi-LASER) with short echo time (TE = 23 ms), phosphocreatine quantification in a 38 mL voxel inside a single exercising muscle becomes possible from single acquisitions, with SNR = 42 ± 4 in resting human medial gastrocnemius.
The method was used to quantify the phosphocreatine time course during 5 min of plantar flexion exercise and recovery with a temporal resolution of 6 s (the chosen repetition time for moderate T1 saturation). Quantification of inorganic phosphate and pH required accumulation of consecutively acquired spectra when (resting) Pi concentrations were low. The localization performance was excellent while keeping the chemical shift displacement acceptably small. The SNR and spectral line widths with and without localization were compared between 3 T and 7 T systems in phantoms and in vivo.
The results demonstrate that increased sensitivity of ultra-high field can be used to dynamically acquire metabolic information from a clearly defined region in a single exercising muscle while reaching a temporal resolution previously available with MRS in non-localizing studies only. The method may improve the interpretation of dynamic muscle MRS data.
dynamic MRS; exercising muscle; phosphorous; localized MRS; high field