M. K. Stehling

Improvements in snap-shot nuclear magnetic resonance imaging

New variants of the ultra-high-speed echo-planar imaging technique have been used to obtain snap-shot images of adult patients and volunteers at 0.1 T. Modified pulsed-gradient sequences together with non-linear signal sampling and activity screened gradients have greatly improved the image quality obtainable by single-shot methods. A particular variant, modulus blipped echo-planar single-pulse technique (MBEST), although slightly slower than the blipped echo-planar single-pulse technique (BEST), is experimentally more robust and incorporates intrinsic T2 weighting. An account of these improvements together with some experimental results is presented.

ECHO PLANAR IMAGING OF THE HUMAN FETUS IN UTERO AT 0.5 T

The snap-shot capability of the echo-planar imaging technique is used to freeze motion effectively in human fetal studies in utero. These first results obtained at 0.5 T demonstrate diagnostic quality images without the need for averaging. Although averaging improves the image signal to noise ratio, it is shown that significant image blurring is produced even when only eight separate images are averaged over a period of a few seconds. Results are presented showing anatomical detail of the internal organs of the fetus. Some pathology is also demonstrated. These results were obtained using the modulus blipped echo-planar single-pulse technique (MBEST). Running at 10 frames/second, the modulus version of the fast low-angle excitation echo-planar technique (FLEET) is used to produce ungated fetal cardiac movies.

Non-invasive temperature mapping using MRI: comparison of two methods based on chemical shift and T1-relaxation

PURPOSE To implement and evaluate the accuracy of non-invasive temperature mapping using MRI methods based on the chemical shift (CS) and T1 relaxation in media of various heterogeneity during focal (laser) and external thermal energy deposition. MATERIALS AND METHODS All measurements were performed on a 1.5 T superconducting clinical scanner using the temperature dependence of the water proton chemical shift and the T1 relaxation time. Homogeneous gel and heterogeneous muscle phantoms were heated focally with a fiberoptic laser probe and externally of varying degree ex vivo by water circulating in a temperature range of 20-50 degrees C. Magnetic resonance imaging data were compared to simultaneously recorded fiberoptic temperature readings. RESULTS Both methods provided accurate results in homogeneous media (turkey) with better accuracy for the chemical shift method (CS:+/-1.5 degrees C, T1:+/-2.0 degrees C). In gel, the accuracy with the CS method was +/-0.6 degrees C. The accuracy decreased in heterogeneous media containing fat (T1:+/-3.5 degrees C, CS: +5 degrees C). In focal heating of turkey muscle, the accuracy was within 1.5 degrees C with the T1 method. CONCLUSION Temperature monitoring with the chemical shift provides better results in homogeneous media containing no fat. In fat tissue, the temperature calculation proved to be difficult.

Study of internal structure of the human fetus in utero by echo-planar magnetic resonance imaging

The ultrafast echo-planar magnetic resonance imaging technology, developed and built in Nottingham, has been used to produce the first snapshot images of the human fetus in utero. The imager, operating at a proton resonance frequency of 22 MHz, produces transaxial views in 64 or 128 milliseconds. These images comprise either 64 x 128 or 128 x 128 pixels with an in-plane resolution of 3 x 3 mm2. The slice thickness is 10 mm. Fetal scans of up to 32 contiguous slices are produced in a few minutes. These have been used to study the internal structure of the uterus and the fetus in a range of cases with gestations ranging from 26 weeks to term. Echo-planar imaging seems particularly suitable as an imaging modality since its high speed obviates image blurring arising from fetal motion.

Improved signal in “snapshot” FLASH by variable flip angles

Conventional fast gradient-echo techniques such as “snapshot” or turboFLASH1V2 employ small (8-12”) flip angles of constant amplitude. Data is usually acquired after a few dummy excitations while the longitudinal magnetization M,(n) is still evolving toward equilibrium; n denotes the nth phase-encoding step. Since all phase-encoding steps conventionally are acquired in linear order ( N < n s + IV), the maximum of the transverse magnetization [M,,,(n) = M,(n) sin (w] at the beginning of the sequence does not coincide with the centre of k space, resulting in submaximal signal utilization. Moreover, longitudinal magnetization remains unutilized at the end of the sequence.

Echo-Planar Imaging

Single shot echo-planar imaging is the fastest magnetic resonance imaging method existing on every human MRI scanner. This is one of the reasons why it is so heavily used in functional magnetic resonance imaging as it allows acquiring images of an entire brain in a matter of a few seconds. Another reason is that echo-planar imaging easily allows susceptibility weighting, the key to measure the blood oxygenation level-dependent signal, the essence for performing functional magnetic resonance imaging. MR perfusion imaging, in brain tumors for example, is another method benefitting enormously from echo-planar imaging. Quantitative blood flow measurement can be exploited also with echo-planar imaging. Due to its fast acquisition scheme EPI is also the key to measure diffusion of spins, which is essential for early stroke and tumor detection, as well as neuronal fiber tracking in the brain and spinal cord. In the last 5–10 years methods to further accelerate Echo-planar imaging acquisition time have been developed allowing scanning the entire brain in a time shorter than a second. This is especially helpful for resting state functional magnetic resonance imaging and high resolution fiber tracking using diffusion imaging. Echo-planar imaging therefore provides functional and anatomical information beyond pixel brightness of classical magnetic resonance imaging.

Ultrafast Magnetic-Resonance Scanning of the Liver with Echo- Planar Imaging

Echo-planar imaging (EPI) is a magnetic resonance imaging (MRI) technique which provides MR images in, typically, 50-100 ms. The potential of EPI as an imaging modality for the liver has been investigated in volunteers and patients with liver disease. Images with improved quality are presented. Obtained at a field strength of 0.52 Tesla, these true unaveraged snap-shot images have larger data arrays, comprising 128 X 128 pixels.

Echo-volumar imaging

Echo-volumar imaging is a hyperfast technique capable of producing volumetric magnetic resonance images in times of the order of 100 ms. By increasing the gradient strengths and introducing real-time processing and display, we have been able to produce the first 16×16×16 voxel snapshot head images on volunteers.

Observation of cerebrospinal fluid flow with echo-planar magnetic resonance imaging

Using echo-planar (EP) magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) flow patterns have been demonstrated in the normal subject and patients with pathological conditions including communicating hydrocephalus, aqueduct stenosis and syringohydromyelia. Snap-shot imaging times of 128 ms allow detailed demonstration of transient intraventricular CSF flow patterns, which is not possible with conventional MRI. The potential of EPI as a method for qualitative and quantitative assessment of CSF dynamics is illustrated.

Nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo planar method

In nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo-planar method, only a part of the k-space is scanned in the phase-coding direction per data acquisition, i.e., per radio-frequency excitation pulse. A phase-coding gradient is used such that regions of the k-space which are interleaved relative to each other are scanned in successive data acquisitions in the phase-coding direction. The number of echoes employed for the raw data matrix, and thus the resolution in phase-coding direction, or the length of the individual pulses of the read-out gradient, and thus the resolution in read-out direction, can thereby be enhanced.