Z.H. Cho

A true three-dimensional reconstruction algorithm for the spherical positron emission tomograph

A spherical detector configuration in truncated form, consisting of multiple layers of ring detector arrays, can substantially improve the reconstructed image quality in positron emission tomography since all inclined annihilation photon rays can now be used for reconstruction of the image. A new true three-dimensional reconstruction (TTR) algorithm suitable for spherical and quasi-spherical forms of detector arrays is presented. In this algorithm each set of two-dimensional projection data is filtered with a 2-D filter function appropriate to the angular orientations, then back-projected onto the object space in every direction. An example of statistical image quality improvement for this algorithm over the conventional two-dimensional algorithm using only intraplane rays is presented using computer simulation.

Line-Integral Projection Reconstruction (LPR) with Slice Encoding Techniques: Multislice Regional Imaging in NMR Tomography

Line-integral projection reconstruction (LPR) in NMR imaging was found to be useful and has several advantages such as the imaging capability of objects having short T2 and compensation of phase fluctuations arising from the system instability. Although single slice LPR is found to be inefficient and poor in signal-to-noise ratio (SNR), the multislice encoded LPR method is of interest since it has a high SNR and also the capability of selected regional volume or multislice imaging. The latter, i.e., regional volume imaging capability, is a unique property of NMR imaging and offers a variety of imaging capabilities such as simultaneous multislice imaging of sagittal, transaxial, or coronal views. In this paper, we have investigated two basic forms of the multislice encoded imaging methods using LPR, i.e., Fourier and Hadamard-like encoding matrices. Applications of the methods to the experimented NMR imaging show good agreement with predicted behavior.

A New Sampling Scheme for the Ring Positron Camera: Dichotomic Ring Sampling

A new sampling scheme applicable to the circular ring geometry positron camera is described. The new scheme employs Dichotomic-Ring (two half rings) with linearmotion which allows parallel or fan mode sampling with any desired degree of sampling interval. The parallel and fan data sets obtained by dichotomic motion provide d/4 and d/2 samplings where d is the detector to detector spacing. Computer simulations were made which confirmed the expected performances and results are compared with other conventional schemes such as wobbling and d/2 angular motion sampling schemes for a ring geometry positron camera.

A multinuclear magnetic resonance imaging technique-simultaneous proton and sodium imaging☆

Simultaneous imaging of proton and sodium is achieved using a new two coil system and a time-multiplexing technique with a 1.5 T NMR imaging system. Distinctly different NMR parameters of protons and sodium, such as resonant frequencies and T1 relaxation rates, are incorporated in the imaging with a new dual coil arrangement. Since the T1 relaxation time of sodium is substantially shorter than that of protons, a set of fast repeating sodium pulse sequences is inserted between the proton pulse repetition intervals. The different resonant frequencies of the nuclei provide good isolation between the two RF coils. In this paper, the system configuration and the pulse sequence employed for the proposed simultaneous multinuclear imaging of protons and sodium are presented along with some preliminary results.

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.

Study of R.f., Gradient Pulse and Magnet Instability Effect in NMR Tomography

Newly emerged NMR imaging requires careful studies on r. f. pulse shapes and sequencing for the selection of the region, gradient pulsing for the 2- or 3-D spatial coding, and suitable signal handling technique for the compensation of the inherent instability of the system, especially fluctuation of the static magnetic field. Above subjects are discussed in detail and a new method which would be useful for the line integral projection reconstruction is proposed. The method could equally be applied to other 3-D NMR imaging techniques such as KWE (Kumar-Welti-Ernst) direct Fourier reconstruction or planar integral projection type reconstruction.