S. K. Hilal

Magnetic field measurement by NMR imaging

Measurement of magnetic field distributions is performed using a method based on the nuclear magnetic resonance (NMR) technique of Fourier imaging. By selecting objects which limit the spatial distribution of the NMR-observable nuclei, images can be obtained which allow easy interpretation of the magnetic field distribution and facilitate rapid shimming of magnets. Results are presented showing the effect of a shim coil set of a superconducting solenoid magnet of the type use for NMR imaging. The application of this type of measurement to the rapid shimming of similar magnet designs is analysed. The effects of steel plates close to the magnet are also observed.

Maximum Likelihood PET with Real Data

We compare the maximum likelihood and convolution-backprojection reconstructions of a particular real phantom with count data obtained from the Columbia University Positron Emission Tomographic Scanner, Dichotom II. We find the maximum likelihood reconstruction reduces noise and streak artifacts confirming earlier work with simulated count data.

True Three-Dimensional Reconstruction (TTR) Application of Algorithm toward Full Utilization of Oblique Rays

The true three-dimensional reconstruction (TTR) algorithm previously proposed by the authors is extended to an algorithm with which full utilization of all the oblique rays is possible. Through this extended TTR (ETTR) algorithm, it is now possible not only to reconstruct an image of a larger object but also possible to obtain images which have substantially better signal-to-noise ratio. The basic TTR algorithm, as well as its extended version, will be discussed together with computer simulation results. In the appendixes, a new two-dimensional Fourier domain weighting function necessary for the implementation of the TTR and ETTR algorithms as well as the generality of the proposed TTR algorithm are discussed.

Radio frequency field intensity mapping using a composite spin-echo sequence

A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90 degrees y-180 degrees x-90 degrees y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented.

Spherical positron emission tomograph (S-PET) I - performance analysis

Abstract A spherical positron emission tomograph (S-PET) is proposed and its performance analyzed. Computer modelling and simulation studies indicate that a high resolution and high sensitivity positron emission tomograph having a spatial resolution of 2–3 mm fwhm can be realized with acceptable imaging time and sensitivity. Multilayer S-PET having 16–24 layers is specifically analyzed and compared with its corresponding cylindrical counterpart. The result shows that the sensitivity gain of the spherical system is a factor 3 over the cylindrical system with identical true to scatter and random ratios.

High-resolution circular ring positron tomograph with dichotomic sampling: Dichotom-I

The circular ring transaxial positron camera developed earlier (see S.E. Derenzo et al., IEEE Trans. Nucl. Sci., vol. NS-24, p.554-8, 1977) was refitted with a new dichotomic sampling scheme and aperture collimators on the detector array to improve the sampling and the overall system resolution. The z-axis slice thickness collimators were also limited to 1 cm, which corresponds to a slice thickness of 0.5 cm FWHM. Two different types of aperture collimators were adopted for high resolution (HR) and very high resolution (VHR) imaging, respectively. In HR mode a resolution of 6.5 mm FWHM was obtained without appreciable degradation of overall sensitivity, which represents a threefold improvement in resolution over the original system. In phantom studies with HR mode a sensitivity of 4500 counts s-1 mu Ci-1 cm-3 was obtained for a 20 cm diameter uniform phantom filled with water. A VHR mode experiment was also conducted to observe the ultimate resolution capability of the Dichotom-I system, and a resolution of 4.2 mm FWHM was obtained at the expense of sensitivity which was reduced by a factor of four from the HR mode experiment. The experience gained with Dichotom-I suggests a relatively simple and inexpensive modification of the existing NaI(Tl) ring positron cameras, most of which suffer from low resolution due to poor sampling and poor intrinsic detector resolution.

An optimized multislice acquisition sequence for the inversion-recovery MR imaging.

An optimized multislice data acquisition scheme for inversion-recovery MR imaging is proposed and experimental results are presented. In this new scheme, instead of forming a set of multislice inversion-recovery sequences in series for a given phase encoding step, 180 degrees inversion pulses corresponding to different slices are interwoven with the spin echo data acquisition sequence in an optimal way depending on the desired inversion-recovery time. For example, between the 180 degrees inversion RF pulse and the spin-echo imaging sequence, a number of imaging and inversion sequences are inserted with different slice combinations, i.e., long inversion-recovery time is effectively utilized for the other slice pre-inversion and data acquisition. With the optimized sequence, imaging time has been reduced by as much as a factor of four compared with the existing methods.

Detector Identification in a 4×4 BGO Crystal Array Coupled to Two Dual PMTS for High Resolution Positron Emission Tomography

A detector identification technique for a detector assembly in which sixteen BGO crystals are packed in a 4×4 array and coupled to two dual PMTs has been developed for high resolution and high sensitivity multilayer positron emission tomography. This allows dense packing of small BGO crystals which is essential to the achievement of high resolution not only in the transaxial plane but also in the axial direction. Experimental results on the timing and detector identification are presented.

True three-dimensional cone-beam reconstruction (TTCR) algorithm

A true three-dimensional cone-beam reconstruction (TTCR) algorithm for direct volume image reconstruction from 2-D cone-beam projections is developed for the complete sphere geometry. The algorithm is derived from the parallel-beam true three-dimensional reconstruction (TTR) algorithm and is based on the modified filtered backprojection technique, which uses a set of 2-D space-invariant filters. The proposed algorithm proved to be superior in spatial resolution to the parallel-beam TTR algorithm and to offer better computational efficiency.

Electronics and Instrumentation for NMR Imaging

The development of Nuclear Magnetic Resonance (NMR) imaging has brought together the experimental techniques of NMR spectroscopy and image processing with the consequence that new experimental requirements and apparatus are currently being developed. Examples of these requirements include the large amounts of data which can be generated and the effect on data processing and storage requirements. Also, the increased size of objects under investigation necessitate some major changes in the magnet and r.f. system. A discussion of these specific requirements is presented together with an overview of all components of the NMR imaging system.