Biomedical subjects

M A Griswold

Publications and source records attributed to M A Griswold.

13 recordsLinked to original sources

PubMed2001

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.

Artifacts↗

PubMed2000

Dynamic three-dimensional magnetic resonance abdominal angiography and perfusion: implementation and preliminary experience.

Implementation of and preliminary experience with an ultra-fast partial-Fourier radiofrequency (RF) spoiled gradient-echo sequence for gadolinium-enhanced imaging are presented. Three-dimensional angiograms can be acquired in less than 6 seconds. Repetition of the acquisition allows the three-dimensional visualization of several distinct vascular phases. Feasibility is demonstrated in three healthy volunteers. The trade-offs among spatial resolution, temporal resolution, and spatial coverage as well as the technical aspects of gadolinium-enhanced pulse sequences are discussed.

Abdomen↗

PubMed2000

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.

Algorithms↗

PubMed2000

SMASH imaging with an eight element multiplexed RF coil array.

SMASH (SiMultaneous Acquisition of Spatial Harmonics) is a technique which can be used to acquire multiple lines of k-space in parallel, by using spatial information from a radiofrequency coil array to perform some of the encoding normally produced by gradients. Using SMASH, imaging speed can be increased up to a maximum acceleration factor equal to the number of coil array elements. This work is a feasibility study which examines the use of SMASH with specialized coil array and data reception hardware to achieve previously unattainable accelerations. An eight element linear SMASH array was designed to operate in conjunction with a time domain multiplexing system to examine the effectiveness of SMASH imaging with as much as eightfold acceleration factors. Time domain multiplexing allowed the multiple independent array elements to be sampled through a standard single-channel receiver. SMASH-reconstructed images using this system were compared with reference images, and signal to noise ratio and reconstruction artifact power were measured as a function of acceleration factor. Results of the imaging experiments showed an almost constant SNR for SMASH acceleration factors of up to eight. Artifact power remained low within this range of acceleration factors. This study demonstrates that efficient SMASH imaging at high acceleration factors is feasible using appropriate hardware, and that time domain multiplexing is a convenient strategy to provide the multiple channels required for rapid imaging with large arrays.

Biophysical Phenomena↗

PubMed2000

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.

Biophysical Phenomena↗

PubMed1999

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.

Abdomen↗

PubMed1999

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.

Computer Simulation↗

PubMed1999

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.

Airway Obstruction↗

PubMed1999

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.

Feasibility Studies↗

PubMed1999

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.

Humans↗

PubMed1998

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.

Airway Obstruction↗

PubMed1998

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.

Adult↗

PubMed1997

Half-Fourier BURST imaging on a clinical scanner.

A modified BURST imaging technique with a nearly twofold reduction in acquisition time, improved signal-to-noise ratio and clear improvement of the image quality is introduced. The method applies half-Fourier reconstruction and can be implemented on a standard clinical scanner. This approach allows the collection of single-shot magnetization prepared images of the human brain with a matrix size of 112 x 128 within 86-250 ms. The signal-to-noise properties of the pulse sequence are analyzed and compared with a standard BURST experiment.

Brain↗