An echo-shifted gradient-echo MRI method for efficient diffusion weighting

Three-dimensional echo-shifted EPI with simultaneous blip-up and blip-down acquisitions for correcting geometric distortion

PURPOSE

EPI with blip-up/down acquisition (BUDA) can provide high-quality images with minimal distortions by using two readout trains with opposing phase-encoding gradients. Because of the need for two separate acquisitions, BUDA doubles the scan time and degrades the temporal resolution when compared to single-shot EPI, presenting a major challenge for many applications, particularly fMRI. This study aims at overcoming this challenge by developing an echo-shifted EPI BUDA (esEPI-BUDA) technique to acquire both blip-up and blip-down datasets in a single shot.

METHODS

A 3D esEPI-BUDA pulse sequence was designed by using an echo-shifting strategy to produce two EPI readout trains. These readout trains produced a pair of k-space datasets whose k-space trajectories were interleaved with opposite phase-encoding gradient directions. The two k-space datasets were separately reconstructed using a 3D SENSE algorithm, from which time-resolved B0 -field maps were derived using TOPUP in FSL and then input into a forward model of joint parallel imaging reconstruction to correct for geometric distortion. In addition, Hankel structured low-rank constraint was incorporated into the reconstruction framework to improve image quality by mitigating the phase errors between the two interleaved k-space datasets.

RESULTS

The 3D esEPI-BUDA technique was demonstrated in a phantom and an fMRI study on healthy human subjects. Geometric distortions were effectively corrected in both phantom and human brain images. In the fMRI study, the visual activation volumes and their BOLD responses were comparable to those from conventional 3D echo-planar images.

CONCLUSION

The improved imaging efficiency and dynamic distortion correction capability afforded by 3D esEPI-BUDA are expected to benefit many EPI applications.

Pseudo multishot echo-planar imaging for geometric distortion improvement

Conventional echo-planar imaging (EPI) uses a radiofrequency pulse for excitation and a prolonged echo train to sample k space, while off-resonance and T₂ * decay effects caused by magnetic susceptibility variation accumulate within each echo, leading to geometric distortion. Multishot EPI methods, which divide k space into segments, can shorten the effective echo spacing and reduce the distortion on EPI images. But multiple shots cost longer scan time and render susceptibility to motion. In this study, we propose a new "multishot" EPI method termed pseudo multishot EPI (pmsEPI), in which phase-encoding lines are segmented as in multishot EPI but are collected within a single shot. With the magnetization divided into different pathways via interleaved excitation instead of refocusing in a single long echo train, the total phase error accumulation is reduced in each segmented acquisition, thereby improving distortion of the resultant EPI image. The performance of the pmsEPI method is demonstrated by phantom and in vivo brain experiments on a 3-T scanner. The experimental results show that the distortion displacements of pmsEPI acquisition compared with conventional EPI decrease by 50% with two pseudo shots and 66% with three pseudo shots, validating the ability of the method to obtain images with reduced distortion in a single shot, although magnetization splitting may induce more than 40% SNR loss and minor artifacts. Specifically, the ability of pmsEPI in diffusion-weighted imaging with different trajectory options is highlighted, and the flexibility is demonstrated in a single-shot blip up and down acquisition.

Renal Diffusion-Weighted Imaging (DWI) for Apparent Diffusion Coefficient (ADC), Intravoxel Incoherent Motion (IVIM), and Diffusion Tensor Imaging (DTI): Basic Concepts

The specialized function of the kidney is reflected in its unique structure, characterized by juxtaposition of disorganized and ordered elements, including renal glomerula, capillaries, and tubules. The key role of the kidney in blood filtration, and changes in filtration rate and blood flow associated with pathological conditions, make it possible to investigate kidney function using the motion of water molecules in renal tissue. Diffusion-weighted imaging (DWI) is a versatile modality that sensitizes observable signal to water motion, and can inform on the complexity of the tissue microstructure. Several DWI acquisition strategies are available, as are different analysis strategies, and models that attempt to capture not only simple diffusion effects, but also perfusion, compartmentalization, and anisotropy. This chapter introduces the basic concepts of DWI alongside common acquisition schemes and models, and gives an overview of specific DWI applications for animal models of renal disease.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Diffusion effect on T2 relaxometry in triple-echo steady state free precession sequence

PURPOSE

The purpose of this study is to evaluate the effect of diffusion on SSFP (Steady-state Free Precession) signals in triple-echo steady state (TESS) sequence and ultimately on the accuracy of T2 relaxometry.

METHODS

The extended phase graph (EPG) algorithm was used to study the effect of diffusion on SSFP signals and T2 relaxometry. The simulation results were verified by a commercial phantom and in vivo studies. Based on the simulation results, a correction scheme was proposed to correct the estimated T2 values.

RESULTS

T2 underestimation in TESS was evident in case of small flip angle and large unbalanced gradient moment on objects with large T2 and D values. The T2 underestimation mainly originated from the diffusion sensitivity of SSFP-echo. It was also observed that SSFP-FID (Free Induction Decay) signals increased with increasing diffusion weighting under some specific conditions. The proposed correction scheme corrected the T2 underestimation, which verified that the underestimation was due to the neglect of diffusion effect. For clinical practice of TESS in tissues with short T2 such as cartilage and muscle, the diffusion effect of TESS is negligible.

CONCLUSION

The effect of diffusion cannot be neglected during TESS T2 quantification as it is the main source of T2 underestimation when small flip angle and large unbalanced gradient moment is used, especially for objects with large T2 and D values.

An exploration of task based fMRI in neonates using echo-shifting to allow acquisition at longer TE without loss of temporal efficiency

Optimal contrast to noise ratio of the BOLD signal in neonatal and foetal fMRI has been hard to achieve because of the much longer T2(⁎) values in developing brain tissue in comparison to those in the mature adult brain. The conventional approach of optimizing fMRI sequences would suggest matching the echo time (TE) and the T2(⁎) of the neonatal and foetal brain. However, the use of a long echo time would typically increase the minimum repetition time (TR) resulting in inefficient sampling. Here we apply the concept of echo shifting to task based neonatal fMRI in order to achieve an improved contrast to noise ratio and efficient data sampling at the same time. Echo shifted EPI (es-EPI) is a modification of a standard 2D-EPI sequence which enables echo times longer than the time between consecutive excitations (TE>TS=TRNS, where NS is the number of acquired slices and TS the inter-slice repetition time). The proposed method was tested on neonatal subjects using a passive sensori-motor task paradigm. Dual echo EPI datasets with an identical readout structure to es-EPI were also acquired and used as control data to assess BOLD activation. From the results of the latter analysis, an average increase of 78±41% in contrast to noise ratio was observable when comparing late to short echoes. Furthermore, es-EPI allowed the acquisition of data with an identical contrast to the late echo, but more efficiently since a higher number of slices could be acquired in the same amount of time.

Preoperative evaluation of the petrosal vein with contrast-enhanced PRESTO imaging in petroclival meningiomas to establish surgical strategy

The present detailed radiological study investigated the relationship between petroclival meningiomas and petrosal veins with contrast-enhanced (CE) principles of echo-shifting with a train of observations (PRESTO) imaging to assess the potential contribution of the findings to the surgical strategy. Fourteen patients (13 women and 1 man) with unilateral petroclival meningiomas underwent microsurgical resection at Osaka City University Hospital between April 2009 and February 2011. Preoperatively, patients were examined using computed tomography (CT) and magnetic resonance (MR) imaging, including CE PRESTO imaging, focusing on the relationship between the tumor and the petrosal vein, and compared to the sensitivity of three-dimensional CT (3D-CT) venography or conventional MR imaging, including CE MR venography and constructive interference in steady-state (CISS) MR imaging. In 11 of 14 cases, we could identify the petrosal veins with intraoperative findings. In 10 of these 14 cases, the anatomical relationship between the tumor and the petrosal vein was detected preoperatively with CE PRESTO imaging, compared to 5 of 14 cases with 3D-CT venography, 5 of 14 cases with CE MR venography, and only 4 of 14 cases using CISS MR imaging. CE PRESTO imaging compares favorably to other approaches. There was no venous complication related to the surgery in any of the cases. CE PRESTO imaging is a non-invasive and useful method to assess the status of the petrosal vein in patients with petroclival meningiomas.

Multiplexed echo planar imaging for sub-second whole brain FMRI and fast diffusion imaging

Echo planar imaging (EPI) is an MRI technique of particular value to neuroscience, with its use for virtually all functional MRI (fMRI) and diffusion imaging of fiber connections in the human brain. EPI generates a single 2D image in a fraction of a second; however, it requires 2–3 seconds to acquire multi-slice whole brain coverage for fMRI and even longer for diffusion imaging. Here we report on a large reduction in EPI whole brain scan time at 3 and 7 Tesla, without significantly sacrificing spatial resolution, and while gaining functional sensitivity. The multiplexed-EPI (M-EPI) pulse sequence combines two forms of multiplexing: temporal multiplexing (m) utilizing simultaneous echo refocused (SIR) EPI and spatial multiplexing (n) with multibanded RF pulses (MB) to achieve m×n images in an EPI echo train instead of the normal single image. This resulted in an unprecedented reduction in EPI scan time for whole brain fMRI performed at 3 Tesla, permitting TRs of 400 ms and 800 ms compared to a more conventional 2.5 sec TR, and 2–4 times reductions in scan time for HARDI imaging of neuronal fibertracks. The simultaneous SE refocusing of SIR imaging at 7 Tesla advantageously reduced SAR by using fewer RF refocusing pulses and by shifting fat signal out of the image plane so that fat suppression pulses were not required. In preliminary studies of resting state functional networks identified through independent component analysis, the 6-fold higher sampling rate increased the peak functional sensitivity by 60%. The novel M-EPI pulse sequence resulted in a significantly increased temporal resolution for whole brain fMRI, and as such, this new methodology can be used for studying non-stationarity in networks and generally for expanding and enriching the functional information.

Steady-state diffusion-weighted imaging: theory, acquisition and analysis

Steady-state diffusion-weighted imaging (DWI) has long been recognized to offer potential benefits over conventional spin-echo methods. This family of pulse sequences is highly efficient and compatible with three-dimensional acquisitions, which could enable high-resolution, low-distortion images. However, the same properties that lead to its efficiency make steady-state imaging highly susceptible to motion and create a complicated signal with dependence on T(1), T(2) and flip angle. Recent developments in gradient hardware, motion-mitigation techniques and signal analysis offer potential solutions to these problems, reviving interest in steady-state DWI. This review offers a description of steady-state DWI signal formation and provides an overview of the current methods for steady-state DWI acquisition and analysis.

Furosemide increases water content in renal tissue

The present study was designed to evaluate the short-term effects of intravenous administration of furosemide on key functions in the kidney cortex and the outer and inner medulla of rats by using magnetic resonance imaging (MRI). Renal tissue water content, renal tissue oxygenation (in relation to the magnetic resonance spin-spin relaxation rate), the apparent diffusion coefficient (ADC) of water, and volume of renal blood flow were measured. Furosemide administration resulted in an increased water content in all regions of the kidney. In parallel with this, we found a significant reduction in ADC in the cortex (2.7 +/- 0.1 x 10(-3) to 2.3 +/- 0.1 x 10(-3) mm(2)/s; P < 0.01) and in the outer medulla (2.3 +/- 0.1 x 10(-3) to 2.0 +/- 0.1 x 10(-3) mm(2)/s; P < 0.01), indicating that the intra- to extracellular volume fraction of water increased in response to furosemide administration. Furosemide also decreased the blood oxygenation in the cortex (49.1 +/- 2.9 to 40.9 +/- 2.0 s(-1); P < 0.01), outer medulla (41.9 +/- 2.8 to 33.2 +/- 1.6 s(-1); P < 0.01) and in the inner medulla (37.1 +/- 2.9 to 26.7 +/- 1.8 s(-1); P < 0.01), indicating an increased amount of oxygenated Hb in the renal tissue. Moreover, renal blood flow decreased in response to furosemide (6.9 +/- 0.2 to 4.4 +/- 0.2 ml/min; P < 0.001). In conclusion, furosemide administration was associated with increased renal water content, an increase in the intra- to extracellular volume fraction of water, an increased oxygen tension, and a decrease in the renal blood flow. Thus MRI provides an integrated evaluation of changes in renal function, leading to decreased renal water and solute reabsorption in response to furosemide, and, in addition, MRI provides an alternative tool to monitor noninvasively changes at the cellular level.

Perfusion-weighted magnetic resonance imaging of the spinal cord in cervical spondylotic myelopathy

Principles of echo shifting with a train of observations was used to perform magnetic susceptibility-weighted magnetic resonance imaging with bolus-tracking in 14 patients with spondylotic myelopathy to assess changes in perfusion parameters of the spinal cord before and after decompression surgery for cervical spondylotic myelopathy. The mean transit time (MTT), bolus arrival time (T0), and time to peak (TTP) were obtained from regions of interest (ROIs) and assessed as the ratio between the spinal cord and the pons (MTT index = MTT(ROI)/MTT(pons), T0 index = T0(ROI)/T0(pons), TTP index = TTP(ROI)/TTP(pons)). The patients were divided into two groups according to percentage improvement on the Neurosurgical Cervical Spine Scale. The MTT index in patients with good recovery (> or =50%) was significantly reduced. The T0 index and TTP index showed no significant change in both groups. Reduction of MTT index may indicate improved perfusion of the spinal cord following surgery for cervical spondylotic myelopathy.

Steady state of echo–shifted sequences with radiofrequency phase cycling

Due to a delayed echo-formation, echo-shifted gradient echo sequences (ES-GRE) allow for an enhanced T(2)*-weighting at short repetition times. While they are in use with and without RF spoiling, analytical solutions are only known for the latter. The signal formation in the former could only be assessed in approximative form, so far. In this article an exact analytical solution is presented for TR-periodic ES-GRE sequences with RF phase cycling. Besides providing a better numerical performance, it should be useful for systematic sequence development. The relation to recent approximative solutions is discussed.

On the calculation and interpretation of signal intensity in echo-shifted sequences

Echo-shifted sequences have been shown to be useful in applications where strong T*2-weighting and short repetition times are wanted, such as BOLD-contrast fMRI, MR thermometry, and perfusion studies. However, a full understanding of signal formation with such methods, which is mandatory to optimize sequence parameters for particular applications, has still not been achieved. Here, two methods are proposed to calculate the steady-state signal intensity in coherent TR-periodic and TR-shifted gradient-echo sequences. The integration method, which consists of averaging the steady-state magnetization over all isochromats in a voxel, is shown to be a particularly efficient way of obtaining the analytical expression of the measurable signal. The partition method, based on a physical decomposition of the steady-state magnetization into a sum of contributions from past excitation pulses, reveals that the net transverse magnetization results from a destructive interference between the wanted component and a series of stimulated echoes. The analysis includes off-resonance effects and is illustrated by phantom measurements. Relationships with previous publications on this subject are discussed.

Abdomen: diffusion-weighted MR imaging with pulse-triggered single-shot sequences

Magnetic resonance (MR) diffusion measurements of the abdomen were performed in 12 healthy volunteers by using a diffusion-weighted single-shot sequence both without and with pulse triggering for different trigger delays. Pulse triggering to the diastolic heart phase led to reduced motion artifacts on the diffusion-weighted MR images and to significantly improved accuracy and reproducibility of measurements of the apparent diffusion coefficients, or ADCs, of abdominal organs.