Time‐resolved angiography: Past,present, and future

The introduction of digital subtraction angiography (DSA) in 1980 provided a method for real time 2D subtraction imaging. Later, 4D magnetic resonance (MR) angiography emerged beginning with techniques like Keyhole and time‐resolved imaging of contrast kinetics (TRICKS) that provided frame rates of one every 5 seconds with limited spatial resolution. Undersampled radial acquisition was subsequently developed. The 3D vastly undersampled isotropic projection (VIPR) technique allowed undersampling factors of 30–40. Its combination with phase contrast displays time‐resolved flow dynamics within the cardiac cycle and has enabled the measurement of pressure gradients in small vessels. Meanwhile similar accelerations were achieved using Cartesian acquisition with projection reconstruction (CAPR), a Cartesian acquisition with 2D parallel imaging. Further acceleration is provided by constrained reconstruction techniques such as highly constrained back‐projection reconstruction (HYPR) and its derivatives, which permit acceleration factors approaching 1000. Hybrid MRA combines a separate phase contrast, time‐of flight, or contrast‐enhanced acquisition to constrain the reconstruction of contrast‐enhanced time frames providing exceptional spatial and temporal resolution and signal‐to‐noise ratio (SNR). This can be extended to x‐ray imaging where a 3D DSA examination can be used to constrain the reconstruction of time‐resolved 3D volumes. Each 4D DSA (time‐resolved 3D DSA) frame provides spatial resolution and SNR comparable to 3D DSA, thus removing a major limitation of intravenous DSA. Similar techniques have provided the ability to do 4D fluoroscopy. J. Magn. Reson. Imaging 2012; 36:1273–1286. © 2012 Wiley Periodicals, Inc.     

PC HYPR flow: A technique for rapid imaging of contrast dynamics

Purpose:

To improve spatial and temporal resolution and signal‐to‐noise ratio (SNR) in three‐dimensional (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 arteriovenous malformations and in a canine model of aneurysms.

Results:

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 by means of 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.

Conclusion:

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. J. Magn. Reson. Imaging 2010; 31: 447–456. © 2010 Wiley‐Liss, Inc.     

Time resolved contrast enhanced intracranial MRA using a single dose delivered as sequential injections and highly constrained projection reconstruction (HYPR CE)

Time‐resolved contrast‐enhanced magnetic resonance angiography of the brain is challenging due to the need for rapid imaging and high spatial resolution. Moreover, the significant dispersion of the intravenous contrast bolus as it passes through the heart and lungs increases the overlap between arterial and venous structures, regardless of the acquisition speed and reconstruction window. An innovative technique is presented that divides a single dose contrast into two injections. Initially a small volume of contrast material (2–3 mL) is used to acquiring time‐resolved weighting images with a high frame rate (2 frames/s) during the first pass of the contrast agent. The remaining contrast material is used to obtain a high resolution whole brain contrast‐enhanced (CE) magnetic resonance angiography (0.57 × 0.57 × 1 mm³) that is used as the spatial constraint for Local Highly Constrained Projection Reconstruction (HYPR LR) reconstruction. After HYPR reconstruction, the final dynamic images (HYPR CE) have both high temporal and spatial resolution. Furthermore, studies of contrast kinetics demonstrate that the shorter bolus length from the reduced contrast volume used for the first injection significantly improves the arterial and venous separation. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

CE‐MRA of the lower extremities using HYPR stack‐of‐stars

Purpose

To investigate the properties of HYPR (HighlY constrained back PRojection) processing—the temporal fidelity and the improvements of spatial/temporal resolution—for contrast‐enhanced MR angiography in a pilot study of the lower extremities in healthy volunteers.

Materials and Methods

HYPR processing with a radial three‐dimensional (3D) stack‐of‐stars acquisition was investigated for contrast‐enhanced MR angiography of the lower extremities in 15 healthy volunteers. HYPR images were compared with control images acquired using a fast, multiphase, 2D Cartesian method to verify the temporal fidelity of HYPR. HYPR protocols were developed for achieving either a high frame update rate or a minimal slice thickness by adjusting the acquisition parameters. HYPR images were compared with images obtained using 3D TRICKS, a widely used protocol in dynamic 3D MRA.

Results

HYPR images showed good temporal agreement with 2D control images. In comparison with TRICKS, HYPR stack‐of‐stars demonstrated higher spatial and temporal resolution. High radial undersampling factors for each time frame were permitted, typically approximately 50 to 100 compared with fully sampled radial imaging.

Conclusion

In this feasibility study, HYPR processing has been demonstrated to improve the spatial or temporal resolution in peripheral CE‐MRA. J. Magn. Reson. Imaging 2009;29:917–923. © 2009 Wiley‐Liss, Inc.     

Undersampled radial MR acquisition and highly constrained back projection (HYPR) reconstruction: Potential medical imaging applications in the post‐Nyquist era

During the past several years there has been extensive study of alternative MR acquisition strategies such as spiral and radial. Vastly undersampled imaging with projections (VIPR) is a three‐dimensional (3D) radial acquisition that provides acceptable images while violating the Nyquist theorem by factors of up to several hundred. For applications like magnetic resonance angiography (MRA), VIPR provides sparse data sets with incoherent artifacts that satisfy the requirements of emerging reconstruction approaches like iterative image norm minimization (compressed sensing) and highly constrained back projection (HYPR). All of these tools can be used in combination with parallel imaging to provide extremely high acceleration factors in MRI. In this review we do not attempt to do justice to the many exciting developments in the general field of constrained reconstruction but focus on preliminary results using VIPR and HYPR for non‐Cartesian, Nyquist‐violating MRI and the extension of HYPR processing to a broad range of medical imaging applications in which the acquisitions satisfy the Nyquist theorem but lack sufficient signal‐to‐noise ratio (SNR), leading to the possibility of radiation reduction, increased ultrasound resolution and field‐of‐view, and improved dynamic display of radiotracers. J. Magn. Reson. Imaging 2009;29:501–516. © 2009 Wiley‐Liss, Inc.     

Contrast-enhanced peripheral magnetic resonance angiography using time-resolved vastly undersampled isotropic projection reconstruction

PURPOSE: To investigate the application of time-resolved vastly undersampled isotropic projection reconstruction (VIPR) in contrast-enhanced magnetic resonance angiography of the distal extremity (single station), and peripheral run-off vasculature in the abdomen, thigh, and calf (three stations). MATERIALS AND METHODS: Time-resolved distal extremity imaging was performed using VIPR sequence through the comparison of two acquisition matrix sizes: 256 with TR/TE=3.7/1.4 msec and 320 with TR/TE=4.5/1.8 msec under the same scan time of two minutes. VIPR acquisition was combined with a bolus-chase technique to image the peripheral run-off vasculature. The time-resolved images were reconstructed using a revised sliding window reconstruction filter whose temporal aperture remained narrow for low spatial frequencies and increased quadratically to include all the projection data for high spatial frequencies. RESULTS: The new temporal filter significantly suppressed the undersampling streak artifacts and venous contamination, while maintaining a high temporal resolution. Both high spatial resolution (ranging from 1.56 x 1.56 x 1.56 mm to 1.25 x 1.25 x 1.25 mm) and high temporal resolution (three seconds per frame) distal extremity images and peripheral run-off images were generated using time-resolved VIPR acquisition, which provides isotropic spatial resolution and isotropic coverage. CONCLUSION: Time-resolved VIPR acquisition was demonstrated to be well suited for distal extremity imaging by providing isotropic spatial resolution, isotropic coverage, and high temporal resolution. The combination of time-resolved VIPR and bolus chase technique provided a novel approach for peripheral run-off examinations.     

Radial sliding‐window magnetic resonance angiography (MRA) with highly‐constrained projection reconstruction (HYPR)

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. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Whole‐heart coronary magnetic resonance angiography at 3.0T using short‐TR steady‐state free precession,vastly undersampled isotropic projection reconstruction

Purpose

To evaluate the feasibility of improving 3.0T steady‐state free precession (SSFP) whole‐heart coronary magnetic resonance angiography (MRA) using short‐TR (repetition time) VIPR (vastly undersampled isotropic projection reconstruction).

Materials and Methods

SSFP is highly sensitive to field inhomogeneity. VIPR imaging uses nonselective radiofrequency pulses, allowing short TR and reduced banding artifacts, while achieving isotropic 3D resolution. Coronary artery imaging was performed in nine healthy volunteers using SSFP VIPR. TR was reduced to 3.0 msec with an isotropic spatial resolution of 1.3 × 1.3 × 1.3 mm³. Image quality, vessel sharpness, and lengths of major coronary arteries were measured. Comparison between SSFP using Cartesian trajectory and SSFP using VIPR trajectory was performed in all volunteers.

Results

Short‐TR SSFP VIPR resulted in whole‐heart images without any banding artifacts, leading to excellent coronary artery visualization. The average image quality score for VIPR‐SSFP was 3.12 ± 0.42 out of four while that for Cartesian SSFP was 0.92 ± 0.61. A significant improvement (P < 0.05) in image quality was shown by Wilcoxon comparison. The visualized coronary artery lengths for VIPR‐SSFP were: 10.13 ± 0.79 cm for the left anterior descending artery (LAD), 7.90 ± 0.91 cm for the left circumflex artery (LCX), 7.50 ± 1.65 cm for the right coronary artery (RCA), and 1.84 ± 0.23 cm for the left main artery (LM). The lengths statistics for Cartesian SSFP were 1.57 ± 2.02 cm, 1.54 ± 1.93 cm, 0.94 ± 1.17 cm, 0.46 ± 0.53 cm, respectively. The image sharpness was also increased from 0.61 ± 0.13 (mm^?1) in Cartesian‐SSFP to 0.81 ± 0.11 (mm^?1) in VIPR‐SSFP.

Conclusion

With VIPR trajectory the TR is substantially decreased, reducing the sensitivity of SSFP to field inhomogeneity and resulting in whole‐heart images without banding artifacts at 3.0T. Image quality improved significantly over Cartesian sampling. J. Magn. Reson. Imaging 2010; 31:1230–1235. © 2010 Wiley‐Liss, Inc.

     

Ultrashort TE spectroscopic imaging (UTESI) using complex highly‐constrained backprojection with local reconstruction (HYPR LR)

Recently, the highly‐constrained backprojection (HYPR) and HYPR with local reconstruction (HYPR LR) methods have been introduced to reconstruct magnitude images from a series of highly undersampled data while preserving high spatial and temporal resolution and high signal‐to‐noise ratio (SNR) in applications with spatiotemporal correlations. However, these conventional HYPR algorithms are limited to the generation of magnitude images and, therefore, have limitations in their potential applications. In this work, the HYPR LR algorithm has been modified to extend the use of algorithms in the HYPR family to applications that require processing of complex data, such as MR chemical shift imaging (CSI) or spectroscopic imaging. The proposed method processes the magnitude information the same way as in original HYPR LR processing. In addition, it improves the phase images by subtracting the phase map of a synthesized composite image. The feasibility and efficiency of this algorithm has been demonstrated on CSI of cortical bone, Achilles tendon, and a healthy volunteer on a clinical 3T scanner. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Cartilage morphology at 3.0T: Assessment of three‐dimensional magnetic resonance imaging techniques

Purpose:

To compare six new three‐dimensional (3D) magnetic resonance (MR) methods for evaluating knee cartilage at 3.0T.

Materials and Methods:

We compared: fast‐spin‐echo cube (FSE‐Cube), vastly undersampled isotropic projection reconstruction balanced steady‐state free precession (VIPR‐bSSFP), iterative decomposition of water and fat with echo asymmetry and least‐squares estimation combined with spoiled gradient echo (IDEAL‐SPGR) and gradient echo (IDEAL‐GRASS), multiecho in steady‐state acquisition (MENSA), and coherent oscillatory state acquisition for manipulation of image contrast (COSMIC). Five‐minute sequences were performed twice on 10 healthy volunteers and once on five osteoarthritis (OA) patients. Signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) were measured from the volunteers. Images of the five volunteers and the five OA patients were ranked on tissue contrast, articular surface clarity, reformat quality, and lesion conspicuity. FSE‐Cube and VIPR‐bSSFP were compared to IDEAL‐SPGR for cartilage volume measurements.

Results:

FSE‐Cube had top rankings for lesion conspicuity, overall SNR, and CNR (P < 0.02). VIPR‐bSSFP had top rankings in tissue contrast and articular surface clarity. VIPR and FSE‐Cube tied for best in reformatting ability. FSE‐Cube and VIPR‐bSSFP compared favorably to IDEAL‐SPGR in accuracy and precision of cartilage volume measurements.

Conclusion:

FSE‐Cube and VIPR‐bSSFP produce high image quality with accurate volume measurement of knee cartilage. J. Magn. Reson. Imaging 2010;32:173–183. © 2010 Wiley‐Liss, Inc.     

Max CAPR: High‐resolution 3D contrast‐enhanced MR angiography with acquisition times under 5 seconds

High temporal and spatial resolution is desired in imaging of vascular abnormalities having short arterial‐to‐venous transit times. Methods that exploit temporal correlation to reduce the observed frame time demonstrate temporal blurring, obfuscating bolus dynamics. Previously, a Cartesian acquisition with projection reconstruction‐like (CAPR) sampling method has been demonstrated for three‐dimensional contrast‐enhanced angiographic imaging of the lower legs using two‐dimensional sensitivity‐encoding acceleration and partial Fourier acceleration, providing 1mm isotropic resolution of the calves, with 4.9‐sec frame time and 17.6‐sec temporal footprint. In this work, the CAPR acquisition is further undersampled to provide a net acceleration approaching 40 by eliminating all view sharing. The tradeoff of frame time and temporal footprint in view sharing is presented and characterized in phantom experiments. It is shown that the resultant 4.9‐sec acquisition time, three‐dimensional images sets have sufficient spatial and temporal resolution to clearly portray arterial and venous phases of contrast passage. It is further hypothesized that these short temporal footprint sequences provide diagnostic quality images. This is tested and shown in a series of nine contrast‐enhanced MR angiography patient studies performed with the new method. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Myocardial perfusion MRI with sliding‐window conjugate‐gradient HYPR

First‐pass perfusion MRI is a promising technique for detecting ischemic heart disease. However, the diagnostic value of the method is limited by the low spatial coverage, resolution, signal‐to‐noise ratio (SNR), and cardiac motion‐related image artifacts. In this study we investigated the feasibility of using a method that combines sliding window and CG‐HYPR methods (SW‐CG‐HYPR) to reduce the acquisition window for each slice while maintaining the temporal resolution of one frame per heartbeat in myocardial perfusion MRI. This method allows an increased number of slices, reduced motion artifacts, and preserves the relatively high SNR and spatial resolution of the “composite images.” Results from eight volunteers demonstrate the feasibility of SW‐CG‐HYPR for accelerated myocardial perfusion imaging with accurate signal intensity changes of left ventricle blood pool and myocardium. Using this method the acquisition time per cardiac cycle was reduced by a factor of 4 and the number of slices was increased from 3 to 8 as compared to the conventional technique. The SNR of the myocardium at peak enhancement with SW‐CG‐HYPR (13.83 ± 2.60) was significantly higher (P < 0.05) than the conventional turbo‐FLASH protocol (8.40 ± 1.62). Also, the spatial resolution of the myocardial perfection images was significantly improved. SW‐CG‐HYPR is a promising technique for myocardial perfusion MRI. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reconstruction of 3D dynamic contrast-enhanced magnetic resonance imaging using nonlocal means

Purpose

To develop and test a nonlocal means‐based reconstruction algorithm for undersampled 3D dynamic contrast‐enhanced (DCE) magnetic resonance imaging (MRI) of tumors.

Materials and Methods

We propose a reconstruction technique that is based on the recently proposed nonlocal means (NLM) filter which can relax trade‐offs in spatial and temporal resolutions in dynamic imaging. Unlike the original application of NLM for image denoising, the MR reconstruction framework here can offer high‐quality images from undersampled k‐space data. The method is based on enforcing similarity constraints in terms of neighborhoods of pixels rather than individual pixels. The method was applied on undersampled 3D DCE imaging of breast and brain tumor datasets and the results were compared to sliding window reconstructions and to a compressed sensing method using total variation constraints on the images.

Results

Undersampling factors of up to five were obtained with the proposed approach while preserving the spatial and temporal characteristics. The NLM reconstruction method offered improved performance over the sliding window and the total variation constrained reconstruction techniques.

Conclusion

The reconstruction framework here can give high‐quality images from undersampled DCE MRI data and has the potential to improve the quality of DCE tumor imaging. J. Magn. Reson. Imaging 2010;32:1217–1227. © 2010 Wiley‐Liss, Inc.     

Free‐breathing myocardial perfusion MRI using SW‐CG‐HYPR and motion correction

First‐pass perfusion MRI is a promising technique to detect ischemic heart disease. Sliding window (SW) conjugate‐gradient (CG) highly constrained back‐projection reconstruction (HYPR) (SW‐CG‐HYPR) has been proposed to increase spatial coverage, spatial resolution, and SNR. However, this method is sensitive to respiratory motion and thus requires breath‐hold. This work presents a non‐model‐based motion correction method combined with SW‐CG‐HYPR to perform free‐breathing myocardial MR imaging. Simulation studies were first performed to show the effectiveness of the proposed motion correction method and its independence from the pattern of the respiratory motion. After that, in vivo studies were performed in six healthy volunteers. From all of the volunteer studies, the image quality score of free breathing perfusion images with motion correction (3.11 ± 0.34) is improved compared with that of images without motion correction (2.27 ± 0.32), and is comparable with that of successful breath‐hold images (3.12 ± 0.38). This result was further validated by a quantitative sharpness analysis. The left ventricle and myocardium signal changes in motion corrected free‐breathing perfusion images were closely correlated to those observed in breath‐hold images. The correlation coefficient is 0.9764 for myocardial signals. Bland–Altman analysis confirmed the agreement between the free‐breathing SW‐CG‐HYPR method with motion correction and the breath‐hold SW‐CG‐HYPR. This technique may allow myocardial perfusion MRI during free breathing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Simulation of relative temporal resolution of time‐resolved MRA sequences

Time‐resolved MRA sequences are typically characterized by the display frame rate. However, true temporal resolution should be defined in a manner analogous to spatial resolution; it is not the ability of a sequence to update rapidly but rather the ability to discern changes that occur within a small time that should characterize temporal resolution. For view‐shared methods like Keyhole and time‐resolved imaging of contrast kinetics (TRICKS), regions of k‐space from multiple time frames are combined to form a single reconstructed time frame. This often causes the time needed to acquire all k‐space data points to be significantly longer than the displayed frame time, resulting in a poor frequency response. Simulated here are the temporal impulse response and temporal frequency response (TFR) curves of three time‐resolved MRA methods, including the recently introduced highly‐constrained backprojection local reconstruction (HYPR LR) method. It is found that the HYPR LR reconstruction method exhibits a better TFR for a larger spectrum of temporal and spatial frequencies than the Keyhole and TRICKS methods. Magn Reson Med 60:398–404, 2008. © 2008 Wiley‐Liss, Inc.     

3D undersampled golden‐radial phase encoding for DCE‐MRA using inherently regularized iterative SENSE

One of the current limitations of dynamic contrast‐enhanced MR angiography is the requirement of both high spatial and high temporal resolution. Several undersampling techniques have been proposed to overcome this problem. However, in most of these methods the tradeoff between spatial and temporal resolution is constant for all the time frames and needs to be specified prior to data collection. This is not optimal for dynamic contrast‐enhanced MR angiography where the dynamics of the process are difficult to predict and the image quality requirements are changing during the bolus passage. Here, we propose a new highly undersampled approach that allows the retrospective adaptation of the spatial and temporal resolution. The method combines a three‐dimensional radial phase encoding trajectory with the golden angle profile order and non‐Cartesian Sensitivity Encoding (SENSE) reconstruction. Different regularization images, obtained from the same acquired data, are used to stabilize the non‐Cartesian SENSE reconstruction for the different phases of the bolus passage. The feasibility of the proposed method was demonstrated on a numerical phantom and in three‐dimensional intracranial dynamic contrast‐enhanced MR angiography of healthy volunteers. The acquired data were reconstructed retrospectively with temporal resolutions from 1.2 sec to 8.1 sec, providing a good depiction of small vessels, as well as distinction of different temporal phases. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

A framework for generalized reference image reconstruction methods including HYPR-LR, PR-FOCUSS, and k-t FOCUSS

Purpose:

To investigate the relationships among highly constrained back projection (HYPR)‐LR, projection reconstruction focal underdetermined system solver (PR‐FOCUSS), and k‐t FOCUSS by showing how each method relates to a generalized reference image reconstruction method. That is, the generalized series model employs a fixed reference image and multiplicative corrections—that model is extended here to consider reference images more broadly, both in image space and in transform spaces (x‐t and x‐f spaces), and that can evolve with iteration.

Materials and Methods:

Theoretical relationships between the methods were derived. Computer simulations were done to compare HYPR‐LR to one iteration of PR‐FOCUSS. The generalized reference approaches applied in the x‐t or x‐f domain were compared using computer simulation, five cardiac cine imaging datasets, and six myocardial perfusion datasets.

Results:

PR‐FOCUSS and HYPR‐LR gave comparable errors, with PR‐FOCUSS slightly outperforming HYPR‐LR. The baseline image is important to the performance of k‐t FOCUSS and x‐t FOCUSS, as demonstrated by results from cardiac cine imaging. For cardiac perfusion reconstructions with the use of a temporal average image as the baseline image, k‐t FOCUSS gave lower errors than x‐t FOCUSS.

Conclusion:

HYPR‐LR and PR‐FOCUSS are closely related: both work for radial sampling and use reference images in the x‐t domain; HYPR‐LR is an approximate implementation of the generalized reference framework, while PR‐FOCUSS is a conjugate gradient implementation of the generalized reference framework. The superiority of generalized reference approaches applied in the x‐t or x‐f domain was sensitive to the characteristics of the acquired data and to the baseline image used. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Optimized density-weighted imaging for dynamic contrast-enhanced 3D-MR mammography

Purpose

To increase the spatial coverage and to reduce slice crosstalk combined with an optimal signal‐to‐noise ratio (SNR) in 3D dynamic contrast‐enhanced (DCE) magnetic resonance (MR) mammography.

Materials and Methods

Asymmetric sampling schemes and a new reconstruction strategy based on virtual coils are presented for density‐weighted (DW) 3D imaging. Additionally, for MR mammography an alternating DW (ADW) sampling along the k_y direction shifts the undersampling artifacts out of the signal reception region. Virtual coils for effective DW (VIDED) imaging suppresses the aliasing in undersampled DW imaging. VIDED and ADW were compared to the conventional Cartesian imaging in phantom and in vivo MR mammography studies.

Results

The slice crosstalk was significantly reduced by VIDED and compared to Cartesian imaging the SNR increased by 16%. Additionally, VIDED and ADW provided a substantially increased field of view (FOV) in the slice direction and allowed the spatial resolution to be improved (up to 60% for ADW and 30% for VIDED) without lengthening the scan time.

Conclusion

VIDED and ADW improve the image quality in 3D DCE MR mammography by enhancing the spatial resolution, reducing the slice crosstalk at nearly optimal SNR, and increasing the FOV in the slice direction. For VIDED no lengthening of the scan time or usage of multichannel receiver coils is necessary. J. Magn. Reson. Imaging 2011;33:328–339. © 2011 Wiley‐Liss, Inc.     

Time‐resolved bolus‐chase MR angiography with real‐time triggering of table motion

Time‐resolved bolus‐chase contrast‐enhanced MR angiography with real‐time station switching is demonstrated. The Cartesian acquisition with projection reconstruction‐like sampling (CAPR) technique and high 2D sensitivity encoding (SENSE) (6× or 8×) and 2D homodyne (1.8×) accelerations were used to acquire 3D volumes with 1.0‐mm isotropic spatial resolution and frame times as low as 2.5 sec in two imaging stations covering the thighs and calves. A custom real‐time system was developed to reconstruct and display CAPR frames for visually guided triggering of table motion upon passage of contrast through the proximal station. The method was evaluated in seven volunteers. High‐spatial‐resolution arteriograms with minimal venous contamination were consistently acquired in both stations. Real‐time stepping table contrast‐enhanced MR angiography is a method for providing time‐resolved images with high spatial resolution over an extended field‐of‐view. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Iterative projection reconstruction of time-resolved images using highly-constrained back-projection (HYPR).

Highly-constrained back-projection (HYPR) is a technique for the reconstruction of sparse, highly-undersampled time-resolved image data. A novel iterative HYPR (I-HYPR) algorithm is presented and validated in computer simulations. The reconstruction method is then applied to cerebral perfusion MRI simulated as a radial acquisition and contrast-enhanced angiography of the head to assess feasibility in accelerating acquisitions requiring high temporal resolution and accurate representation of contrast kinetics. The I-HYPR algorithm is shown to be more robust than standard HYPR in these applications in which the sparsity condition is not met or in which quantitative information is required. Specifically, iterative reconstruction of undersampled perfusion and contrast-enhanced angiography data improved accuracy of the representation of contrast kinetics and increased the temporal separation of arterial and venous contrast kinetics. The I-HYPR reconstruction may have important diagnostic applications in settings requiring high temporal resolution and quantitative signal dynamics. Because I-HYPR allows relaxation of the sparsity requirements for the composite frame, the iterative reconstruction can enable novel acquisition strategies that independently optimize the quality of the composite and temporal resolution of the dynamic frames.