[Feasibilities and limitations of high field parallel MRI]

In medical magnetic resonance imaging (MRI) it is standard to use MR scanners with a field strength of 1.5 Tesla. Recently, an ongoing development to higher field strength can be observed and a new clinical standard at 3.0 Tesla seems to be established. High field MRI with its intrinsic higher signal to noise ratio (SNR) can enable new applications of MRI in medical diagnosis, or can serve to improve existing methods. It is important to note, that the use of high field MRI is not without its limitations. Besides the SNR, other unwanted effects increase with a higher field strength. Without correction, these high field problems cause a serious loss in image quality. An elegant way to address these problems is the use of parallel imaging. In many clinical applications, parallel MRI (pMRI) is part of the standard protocol, because pMRI can enhance virtually every MRI application, without necessarily affecting the contrast behavior of the underlying imaging sequence. In high field MRI, besides the speed advantage of pMRI, the positive influence on high field specific problems and therefore on the image quality will be of major importance.

Imaging lung function using rapid dynamic acquisition of T 1-maps during oxygen enhancement

This paper describes imaging of lung function with oxygen-enhanced MRI using dynamically acquired T1 parameter maps, which allows an accurate, quantitative assessment of time constants of T1-enhancement and therefore lung function. Eight healthy volunteers were examined on a 1.5-T whole-body scanner. Lung T1-maps based on an IR Snapshot FLASH technique (TE = 1.4 ms, TR = 3.5 ms, FA = 7 (composite function )) were dynamically acquired from each subject. Without waiting for full relaxation between subsequent acquisition of T1-maps, one T1-map was acquired every 6.7 s. For comparison, all subjects underwent a standard pulmonary function test (PFT). Oxygen wash-in and wash-out time course curves of T1 relaxation rate (R1)-enhancement were obtained and time constants of oxygen wash-in (w(in)) and wash-out (w(out)) were calculated. Averaged over the whole right lung, the mean w(out) was 43.90 +/- 10.47 s and the mean (w(in)) was 51.20 +/- 15.53 s, thus about 17% higher in magnitude. Wash-in time constants correlated strongly with forced expired volume in one second in percentage of the vital capacity (FEV1 % VC) and with maximum expiratory flow at 25% vital capacity (MEF25), whereas wash-out time constants showed only weak correlation. Using oxygen-enhanced rapid dynamic acquisition of T1-maps, time course curves of R1-enhancement can be obtained. With w(in) and w(out) two new parameters for assessing lung function are available. Therefore, the proposed method has the potential to provide regional information of pulmonary function in various lung diseases.

Resolution enhancement in lung 1H imaging using parallel imaging methods

Resolution in (1)H lung imaging is limited mainly by the acquisition time. Today, half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences, with short echo time (TE) and short interecho spacing (T(inter)) have found increased use in lung imaging. In this study, a HASTE sequence was used in combination with a partially parallel acquisition (PPA) strategy to increase the spatial resolution in single-shot (1)H lung imaging. To investigate the benefits of using a combination of single-shot sequences and PPA, five healthy volunteers were examined. Compared to conventional imaging methods, substantially increased resolution is obtained using the PPA approach. Representative in vivo (1)H lung images acquired with a HASTE sequence in combination with the generalized autocalibrating partially parallel acquisition (GRAPPA) method, up to an acceleration factor of three, are presented.

Abdominal imaging with a modular combination of spin and gradient echoes

MR-CAT (combined acquisition technique), a modular hybrid imaging concept, was introduced recently. In this article it is demonstrated that the CAT principles can be applied to form a versatile combination of spin and gradient echoes for abdominal imaging. This CAT approach, which essentially integrates RARE and EPI modules in a sequential fashion, was used to implement a set of segmented and single-shot RARE/EPI-CAT imaging techniques. CAT was used in in vivo studies to perform high-resolution abdominal imaging in five healthy subjects. The results demonstrate the feasibility of abdominal imaging using the proposed CAT approach and the potential of this technique to reduce imaging time while preserving image quality.

Rapid quantitative lung (1)H T(1) mapping

In this contribution, a rapid and robust technique for quantitative T(1) mapping of the human lung is presented. Based on a series of Snapshot FLASH tomograms acquired after a single inversion pulse, high quality and quantitative T(1) parameter maps acquired in under five seconds were obtained from six healthy volunteers. The measured T(1) values are in good agreement with previously reported literature values. T(1) maps were also acquired with the volunteers breathing room air or 100% O(2). The T(1) difference between breathing room air and 100% O(2) is statistically significant at P < 0.0001.

Accelerated cardiac imaging using the SMASH technique

SMASH (SiMultaneous Acquisition of Spatial Harmonics) was recently introduced as a novel rapid-imaging technique. The SMASH technique uses a partially parallel acquisition strategy, using spatial information from a radiofrequency coil array to accelerate imaging. This study constitutes the first application of SMASH to cardiac magnetic resonance imaging. The increased imaging speed provided by SMASH was used to obtain images with reduced breathhold duration, enhanced spatial resolution, and increased temporal resolution in healthy volunteers. The results obtained demonstrate the feasibility and potential clinical utility of cardiac magnetic resonance imaging using the SMASH technique.

VD-AUTO-SMASH imaging

Recently a self-calibrating SMASH technique, AUTO-SMASH, was described. This technique is based on PPA with RF coil arrays using auto-calibration signals. In AUTO-SMASH, important coil sensitivity information required for successful SMASH reconstruction is obtained during the actual scan using the correlation between undersampled SMASH signal data and additionally sampled calibration signals with appropriate offsets in k-space. However, AUTO-SMASH is susceptible to noise in the acquired data and to imperfect spatial harmonic generation in the underlying coil array. In this work, a new modified type of internal sensitivity calibration, VD-AUTO-SMASH, is proposed. This method uses a VD k-space sampling approach and shows the ability to improve the image quality without significantly increasing the total scan time. This new k-space adapted calibration approach is based on a k-space-dependent density function. In this scheme, fully sampled low-spatial frequency data are acquired up to a given cutoff-spatial frequency. Above this frequency, only sparse SMASH-type sampling is performed. On top of the VD approach, advanced fitting routines, which allow an improved extraction of coil-weighting factors in the presence of noise, are proposed. It is shown in simulations and in vivo cardiac images that the VD approach significantly increases the potential and flexibility of rapid imaging with AUTO-SMASH.

Partially parallel imaging with localized sensitivities (PILS)

In this study a novel partially parallel acquisition method is presented, which can be used to accelerate image acquisition using an RF coil array for spatial encoding. In this technique, Parallel Imaging with Localized Sensitivities (PILS), it is assumed that the individual coils in the array have localized sensitivity patterns, in that their sensitivity is restricted to a finite region of space. Within the PILS model, a detailed, highly accurate RF field map is not needed prior to reconstruction. In PILS, each coil in the array is fully characterized by only two parameters: the center of coil's sensitive region in the FOV and the width of the sensitive region around this center. In this study, it is demonstrated that the incorporation of these coil parameters into a localized Fourier transform allows reconstruction of full FOV images in each of the component coils from data sets acquired with a reduced number of phase encoding steps compared to conventional imaging techniques. After the introduction of the PILS technique, primary focus is given to issues related to the practical implementation of PILS, including coil parameter determination and the SNR and artifact power in the resulting images. Finally, in vivo PILS images are shown which demonstrate the utility of the technique.

MR CAT scan: a modular approach for hybrid imaging

In this study, a modular concept for NMR hybrid imaging is presented. This concept essentially integrates different imaging modules in a sequential fashion and is therefore called CAT (combined acquisition technique). CAT is not a single specific measurement sequence, but rather a sequence design concept whereby distinct acquisition techniques with varying imaging parameters are employed in rapid succession in order to cover k-space. The power of the CAT approach is that it provides a high flexibility toward the acquisition optimization with respect to the available imaging time and the desired image quality. Important CAT sequence optimization steps include the appropriate choice of the k-space coverage ratio and the application of mixed bandwidth technology. Details of both the CAT methodology and possible CAT acquisition strategies, such as FLASH/EPI-, RARE/EPI- and FLASH/BURST-CAT are provided. Examples from imaging experiments in phantoms and healthy volunteers including mixed bandwidth acquisitions are provided to demonstrate the feasibility of the proposed CAT concept.

Hybrid cardiac imaging with MR-CAT scan: a feasibility study

We demonstrate the feasibility of a new versatile hybrid imaging concept, the combined acquisition technique (CAT), for cardiac imaging. The cardiac CAT approach, which combines new methodology with existing technology, essentially integrates fast low-angle shot (FLASH) and echoplanar imaging (EPI) modules in a sequential fashion, whereby each acquisition module is employed with independently optimized imaging parameters. One important CAT sequence optimization feature is the ability to use different bandwidths for different acquisition modules. Twelve healthy subjects were imaged using three cardiac CAT acquisition strategies: a) CAT was used to reduce breath-hold duration times while maintaining constant spatial resolution; b) CAT was used to increase spatial resolution in a given breath-hold time; and c) single-heart beat CAT imaging was performed. The results obtained demonstrate the feasibility of cardiac imaging using the CAT approach and the potential of this technique to accelerate the imaging process with almost conserved image quality.

High-resolution diffusion imaging using a radial turbo-spin-echo sequence: implementation, eddy current compensation, and self-navigation

This work describes a segmented radial turbo-spin-echo technique (DW-rTSE) for high-resolution multislice diffusion-weighted imaging and quantitative ADC mapping. Diffusion-weighted images with an in-plane resolution of 700 microm and almost free of bulk motion can be obtained in vivo without cardiac gating. However, eddy currents and pulsatile brain motion cause severe artifacts when strong diffusion weighting is applied. This work explains in detail the artifacts in projection reconstruction (PR) imaging arising from eddy currents and describes an effective eddy current compensation based on the adjustment of gradient timing. Application of the diffusion gradients in all three orthogonal directions is possible without degradation of the images due to eddy current artifacts, allowing studies of the diffusional anisotropy. Finally, a self-navigation approach is proposed to reduce residual nonrigid body motion artifacts. Five healthy volunteers were examined to show the feasibility of this method.

A multicoil array designed for cardiac SMASH imaging

Recently, several partially parallel acquisition (PPA) techniques have been presented which use spatial information inherent in an RF coil array to reconstruct an image from a reduced set of phase encoding steps. PPAs represent a change in paradigm for the RF coil designer since the focus for arrays to be used with PPAs is to optimize the spatial encoding that is provided by the array. One of the first practical implementations of PPA imaging was demonstrated using the SMASH technique. In this study, we present our results from the construction of the first array designed specifically for cardiac SMASH imaging. Additional design criteria are presented for SMASH arrays that are not considered in conventional array design. Using these design criteria, a four-element array was constructed and then tested in SMASH imaging experiments in the heart. This array has been used in all of our initial cardiac and head SMASH studies with good results.

Silent functional magnetic resonance imaging demonstrates focal activation in rapid eye movement sleep

Functional imaging of human sleep has been performed with nuclear medicine methods, but MRI has been difficult to implement, in part because of the noise associated with echo-planar imaging as well as the difficulty in reading physiologic signals in the MRI environment. We describe a silent MR sequence that can record brain activation over many hours with simultaneous acquisition of an EEG. This shows activation of occipital cortex and deactivation of frontal cortex during REM sleep, in agreement with previous studies using other techniques. MRI-Sleep-REM sleep.

Pulmonary disorders: ventilation-perfusion MR imaging with animal models

PURPOSE

To demonstrate the capability of magnetic resonance (MR) imaging to assess alteration in regional pulmonary ventilation and perfusion with animal models of airway obstruction and pulmonary embolism.

MATERIALS AND METHODS

Airway obstruction was created by inflating a 5-F balloon catheter into a secondary bronchus. Pulmonary emboli were created by injecting thrombi into the inferior vena cava. Regional pulmonary ventilation was assessed with 100% oxygen as a T1 contrast agent. Regional pulmonary perfusion was assessed with a two-dimensional fast low-angle shot, or FLASH, sequence with short repetition and echo times after intravenous administration of gadopentetate dimeglumine.

RESULTS

Matched ventilation and perfusion abnormalities were identified in all animals with airway obstruction. MR perfusion defects without ventilation abnormalities were seen in all animals with pulmonary emboli.

CONCLUSION

Ventilation and perfusion MR imaging are able to provide regional pulmonary functional information with high spatial and temporal resolution. The ability of MR imaging to assess both the magnitude and regional distribution of pulmonary functional impairment could have an important effect on the evaluation of lung disease.

Resolution enhancement in single-shot imaging using simultaneous acquisition of spatial harmonics (SMASH)

Spatial resolution in single-shot imaging is limited by signal attenuation due to relaxation of transverse magnetization. This effect can be reduced by minimizing acquisition times through the use of short interecho spacings. However, the minimum interecho spacing is constrained by limits on gradient switching rates, radiofrequency (RF) power deposition and RF pulse length. Recently, simultaneous acquisition of spatial harmonics (SMASH) has been introduced as a method to acquire magnetic resonance images at increased speeds using a reduced number of phase-encoding gradient steps by extracting spatial information contained in an RF coil array. In this study, it is shown that SMASH can be used to reduce the effects of relaxation, resulting in single-shot images with increased spatial resolution without increasing imaging time. After a brief theoretical discussion, two strategies to reduce signal attenuation and increase spatial resolution in single-shot imaging are introduced and their performance is evaluated in phantom studies. In vivo single-shot echoplanar imaging (EPI), BURST, and half-Fourier single-shot turbo spin-echo (HASTE) images are then presented demonstrating the practical implementation of these resolution enhancement strategies. Images acquired with SMASH show increased spatial resolution and improved image quality when compared with images obtained with the conventional acquisitions. The general principles presented for imaging with SMASH can also be applied to other partially parallel imaging techniques.

SMASH imaging

SMASH imaging is a new MR imaging technique that can be used to multiply the speed of existing imaging sequences. It operates by using an array of radiofrequency (RF) detection coils to perform some of the spatial encoding normally accomplished with magnetic field gradients. The speed of the SMASH technique results from appropriate combinations of coil array RF signals in which multiple lines of image data are gathered simultaneously, rather than one after another. SMASH can be used in conjunction with most rapid imaging sequences, including EPI, resulting in multiplicative gains in imaging speed. This article reviews the basic principles of SMASH imaging, outlines requirements for practical implementation, and presents a variety of in vivo results, highlighting ways in which SMASH may be used to increase imaging speed and to improve image quality for clinical MR imaging applications.

Signal-to-noise ratio and signal-to-noise efficiency in SMASH imaging

A general theory of signal-to-noise ratio (SNR) in simultaneous acquisition of spatial harmonics (SMASH) imaging is presented, and the predictions of the theory are verified in imaging experiments and in numerical simulations. In a SMASH image, multiple lines of k-space are generated simultaneously through combinations of magnetic resonance signals in a radiofrequency coil array. Here, effects of noise correlations between array elements as well as new correlations introduced by the SMASH reconstruction procedure are assessed. SNR and SNR efficiency in SMASH images are compared with results using traditional array combination strategies. Under optimized conditions, SMASH achieves the same average SNR efficiency as ideal pixel-by-pixel array combinations, while allowing imaging to proceed at otherwise unattainable speeds. The k-space nature of SMASH reconstructions can lead to oscillatory spatial variations in noise standard deviation, which can produce local enhancements of SNR in particular regions.

Oxygen enhanced MR ventilation imaging of the lung

The current work is a continuation of a new MRI technique that was proposed for the non-invasive assessment of regional lung ventilation using inhaled molecular oxygen as a T1 contrast agent. Several improvements of this technique are described in this work. The signal-to-noise ratio in the ventilation-scan images was optimized using a centrically reordered single-shot RARE sequence with a short effective echo time and short inter-echo spacing. The contrast-to-noise ratio was improved using an optimized inversion delay time. The optimized MR-ventilation-scan was successfully performed in healthy volunteers and in an animal model with airway obstruction. The experimental results demonstrate the feasibility and clinical potential of the MR ventilation imaging technique for assessment of regional pulmonary function.

AUTO-SMASH: a self-calibrating technique for SMASH imaging. SiMultaneous Acquisition of Spatial Harmonics

Recently a new fast magnetic resonance imaging strategy, SMASH, has been described, which is based on partially parallel imaging with radiofrequency coil arrays. In this paper, an internal sensitivity calibration technique for the SMASH imaging method using self-calibration signals is described. Coil sensitivity information required for SMASH imaging is obtained during the actual scan using correlations between undersampled SMASH signal data and additionally sampled calibration signals with appropriate offsets in k-space. The advantages of this sensitivity reference method are that no extra coil array sensitivity maps have to be acquired and that it provides coil sensitivity information in areas of highly non-uniform spin-density. This auto-calibrating approach can be easily implemented with only a small sacrifice of the overall time savings afforded by SMASH imaging. The results obtained from phantom imaging experiments and from cardiac studies in nine volunteers indicate that the self-calibrating approach is an effective method to increase the potential and the flexibility of rapid imaging with SMASH.

A phantom for diffusion-weighted imaging of acute stroke

A tissue phantom for diffusion-weighted imaging was developed, basing its contrast between two compartments on different apparent diffusion coefficients, without contrast due to T2 relaxation and proton density. These contrast properties of the phantom simulate the situation found in normal gray matter and areas of acute ischemia. A possible application of the phantom was demonstrated for the investigation of the accuracy of volume measurements based on diffusion-weighted images.

Functional burst imaging

A quiet magnetic resonance (MR) imaging technique for detecting changes in cerebral activity functions is presented. This single-shot method, functional Burst imaging (FBI), combines elements of Burst imaging with an offset technique known as asymmetric spin echo (ASE). The FBI sequence has the unique feature of being nearly silent, because of the low number of gradient switching steps involved. Furthermore, this approach has the key advantage that it can be implemented on conventional MR systems. Established auditory and visual paradigms were used to evaluate whether FBI can detect changes in cerebral activity using a 1.5 Tesla MR system. In a second set of experiments, the FBI technique was used to evaluate cerebral activity changes during different sleep stages in humans. The results obtained demonstrate that the FBI sequence provides an alternative approach for functional imaging of brain activity in primary and secondary sensory areas of the human brain. Furthermore, in using this quiet MR technique, it was possible to scan continuously during different stages of human sleep without acoustic noise perturbation.

Basic pulse sequences for fast cardiac MR imaging

Technical challenges of cardiac MRI include minimizing the effects of cardiac and respiratory motion and developing techniques that allow for both high spatial resolution and high SNR given the small size of small structures such as the coronary vessels. Fast imaging techniques provide considerable time savings and increased flexibility which allow to further optimize image quality.

[Diffusion-weighted imaging in acute stroke]

Magnetic resonance imaging represents today the most important tool in neuroradiology for both clinical practice and research. MRI allows imaging of the human body in 2 or 3 dimensions with variable tissue contrast. The natural diffusion of tissue protons can now be used as a supplementary contrast mechanism. Different MRI techniques can be used to obtain clinically useful diffusion-weighted images. These techniques all require the use of strong gradient pulses in order to obtain the diffusion contrast. In the current article, the most important physical principles of diffusion measurement are presented. After a short introduction into the basic physical principles, we will present the prerequisites and limitations of clinically relevant applications today. Finally a few select examples of clinical use of these techniques in the acute diagnosis of stroke will be presented.

Turbo spin-echo diffusion-weighted MR of ischemic stroke

PURPOSE

Our objective was to determine whether a multisection technique, diffusion-weighted half-Fourier single-shot turbo spin-echo (HASTE) imaging, can compensate for the drawbacks common to other diffusion-weighted techniques; specifically, the need for echo-planar technology and the presence of susceptibility artifacts in areas close to the skull base.

METHODS

Forty subjects who were referred to the stroke service with signs of acute (less than 24 hour) neurologic dysfunction were included in this prospective study. MR imaging of the brain was performed with diffusion-weighted echo-planar and diffusion-weighted HASTE sequences. The images obtained with both sequences were analyzed for the presence of hyperintensities corresponding to ischemic lesions as well as for the presence of image artifacts and distortions.

RESULTS

Diffusion-weighted HASTE images showed areas of hyperintensity corresponding to the infarcts present on diffusion-weighted echo-planar imaging studies without distortion or susceptibility artifacts in all the patients who had a stroke. Twelve patients had no acute ischemic lesions; of these, five had other findings, six had normal findings, and in one patient, a hyperintensity seen on diffusion-weighted echo-planar images proved to be an artifact on diffusion-weighted HASTE images.

CONCLUSIONS

Diffusion-weighted HASTE is equal to diffusion-weighted echo-planar imaging in the detection of early ischemia. Because of the absence of significant image distortions and other artifacts, diffusion-weighted HASTE permits fast multiplanar imaging in artifact-prone regions, such as the posterior fossa and the inferior frontal and temporal lobes. Diffusion imaging can be performed on conventional systems with strengths of 1.5 T that do not have echo-planar imaging capabilities.