R. Coxon

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.

Echo-volumar Imaging (evi) of the Brain at 3.0 T: First Normal Volunteer and Functional Imaging Results

Objective Our goal was to present the first echo-volumar brain images obtained at 3.0 T, together with the first functional imaging results using echo volumar imaging. Materials and Methods The results presented were obtained on volunteers using an in-house designed and constructed 3.0 T echo-planar/volumar imager. Results The results demonstrate the feasibility of obtaining snapshot volumar images comprising up to 64 ± 64 ± 8 voxels corresponding to a spatial resolution of 3.0 ± 3.0 ± 2.5 mm3. Results are also presented showing local cortical changes in signal in response to an external visual stimulus for both the left and the right brain hemispheres. Conclusion The snapshot acquisition of a whole-volume data set has a number of advantages when considering motional effects or functional image changes that may involve time delays or phase effects within different cortical regions. Instantaneous acquisition of the whole data set means accurate phase information may be straightforwardly obtained.

Echo-planar imaging of the brain at 3.0 T: first normal volunteer results.

Objective To present the first echo-planar brain images of diagnostic quality obtained at 3.0 T and to point out some of the problems experienced in performing it. Materials and Methods The results presented were obtained on volunteers using an in-house designed and constructed 3.0 T EPI imager. Results The results demonstrate the feasibility of obtaining snapshot imaging comprising up to 256 × 256 pixels and corresponding to a spatial resolution of 0.75 × 0.75 mm2 with a slice thickness of 2.5 mm. Conclusion Potentially augmented diagnostic information can be obtained with high field EPI of the brain. Some susceptibility artifact is apparent at the bone-air interfaces as expected at 3.0 T.

Active acoustic control in gradient coils for MRI.

The new principles of active acoustic control in gradient coil design recently introduced by Mansfield and Haywood (MAGMA 1999;8(Suppl 1):55) are further developed theoretically for the far‐field acoustic output for a single sector of a coil system comprising four or more flat rectangular coil sectors. Each sector consists of a split plate arrangement in which are embedded two windings, an outer primary winding and a narrow inner re‐entrant loop control winding immediately adjacent to and surrounding the split or air gap. The wire spacing of the control winding is made small so as not to affect substantially the magnetic field created by the primary winding. Experimental results are presented for two sectors each made of a different readily available plastic material and tested over a range of frequencies. They both show substantial average reductions in acoustic output over the full output when the control winding is appropriately driven. New theoretical expressions are derived for particular frequencies based on normal mode expansions for the plate. This new approach is better able to explain the acoustic output difference between the full and reduced output modes. Empirical expressions are also developed which include longitudinal as well as transverse plate characteristics and used to fit the experimental acoustic output data as a function of frequency and indicate good agreement with regard to both the form and amplitude of the acoustic output response. Magn Reson Med 46:807–818, 2001.

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.

Zonally magnified EPI in real time by NMR

To overcome the problems of gradient strength, large receiver bandwidth and computing time in nuclear magnetic resonance real-time echo-planar imaging (EPI) with large pixel arrays, the concept of zonal image magnification or zoomed EPI (ZEPI) is introduced. The image zone is defined by two selective RF pulses and the receiver bandwidth. For a fixed image array size it is shown that zooming requires less gradient strength to achieve a given spatial resolution than in an equivalent unzoomed image. Two different zoom techniques are described, ZEPI-1 and ZEPI-2, which employ a single and a double pulse sequence respectively. Experimental results obtained with these sequences on a phantom and a live piglet are presented.

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.

Transectional echo planar imaging of the heart in cyanotic congenital heart disease

Echo planar imaging is that form of magnetic resonance imaging which gives very short image acquisition times. The method has been used to produce images of the infant heart which are free of cardiorespiratory motion artefact, despite tachypnoea and tachycardia. EPI transections of the normal heart are compared with transections in truncus arteriosus, tetralogy of Fallot, right heart hypoplasia and transposition of the great arteries. The diagnosis of the cause of cyanosis in these infants was established by the noninvasive EPI method and validation of the findings may be found in transectional postmortem analyses reported in the literature.

Echo planar imaging of normal and abnormal connections of the heart and great arteries

Echo planar imaging (EPI) is that form of magnetic resonance imaging which provides very short image acquisition times. EPI also provides very rapid sequential imaging. The EPI method is ideal for imaging the heart and thoracic content because images are devoid of cardio-respiratory motion artefact. Previously an analysis of transectional images has been presented [1]. This paper is concerned with the study of the heart by the use of EPI constructions in the sagittal and coronal planes. Defining connections between ventricle and great artery is of cardinal importance in paediatric cardiology. EPI constructions in the normal heart, transposition, truncus arteriosus and right heart hypoplasia are presented and discussed.