Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

MicroPET is a high resolution positron emission tomography (PET) scanner designed for imaging small laboratory animals. It consists of a ring of 30 position-sensitive scintillation detectors, each with an 8/spl times/8 array of small lutetium oxyorthosilicate (LSO) crystals coupled via optical fibers to a multi-channel photomultiplier tube. The detectors have an intrinsic resolution averaging 1.68 mm, an energy resolution between 15 and 25% and 2.4 ns timing resolution at 511 keV. The detector ring diameter of microPET is 17.2 cm with an imaging field of view of 112 mm transaxially by 18 mm axially. The scanner has no septa and operates exclusively in 3D mode. Reconstructed image resolution 1 cm from the center of the scanner is 2.0 mm and virtually isotropic, yielding a volume resolution of 8 mm/sup 3/. For comparison, the volume resolution of state-of-the-art clinical PET systems is in the range of 50-75 mm/sup 3/. Initial images of phantoms have been acquired and are reported. A computer controlled bed is under construction and will incorporate a small wobble motion to improve spatial sampling. This is projected to further enhance spatial resolution. MicroPET is the first PET scanner to incorporate the new scintillator LSO and to our knowledge is the highest resolution multi-ring PET scanner currently in existence.

MicroPET: a high resolution PET scanner for imaging small animals

PET is widely considered the most sensitive technique available for noninvasively studying physiology, metabolism, and molecular pathways in the living human being. However, the utility of PET, being a photon-deficient modality, remains constrained by factors including low signal-to-noise ratio, long imaging times, and concerns about radiation dose. Two developments offer the potential to dramatically increase the effective sensitivity of PET. First by increasing the geometric coverage to encompass the entire body, sensitivity can be increased by a factor of about 40 for total-body imaging or a factor of about 4-5 for imaging a single organ such as the brain or heart. The world's first total-body PET/CT scanner is currently under construction to demonstrate how this step change in sensitivity affects the way PET is used both in clinical research and in patient care. Second, there is the future prospect of significant improvements in timing resolution that could lead to further effective sensitivity gains. When combined with total-body PET, this could produce overall sensitivity gains of more than 2 orders of magnitude compared with existing state-of-the-art systems. In this article, we discuss the benefits of increasing body coverage, describe our efforts to develop a first-generation total-body PET/CT scanner, discuss selected application areas for total-body PET, and project the impact of further improvements in time-of-flight PET.

https://jnm.snmjournals.org/content/jnumed/early/2017/09/21/jnumed.116.184028.full.pdf

Total-Body PET: Maximizing Sensitivity to Create New Opportunities for Clinical Research and Patient Care.

Studies in terahertz (THz) imaging have revealed a significant difference between skin cancer (basal cell carcinoma) and healthy tissue. Since water has strong absorptions at THz frequencies and tumours tend to have different water content from normal tissue, a likely contrast mechanism is variation in water content. Thus, we have previously devised a finite difference time-domain (FDTD) model which is able to closely simulate the interaction of THz radiation with water. In this work we investigate the interaction of THz radiation with normal human skin on the forearm and palm of the hand in vivo. We conduct the first ever systematic in vivo study of the response of THz radiation to normal skin. We take in vivo reflection measurements of normal skin on the forearm and palm of the hand of 20 volunteers. We compare individual examples of THz responses with the mean response for the areas of skin under investigation. Using the in vivo data, we demonstrate that the FDTD model can be applied to biological tissue. In particular, we successfully simulate the interaction of THz radiation with the volar forearm. Understanding the interaction of THz radiation with normal skin will form a step towards developing improved imaging algorithms for diagnostic detection of skin cancer and other tissue disorders using THz radiation.

https://iopscience.iop.org/article/10.1088/0031-9155/49/9/001/pdf

In vivo study of human skin using pulsed terahertz radiation

OBJECTIVE: A positron emission tomography (PET) study has suggested that dopamine transporter density of the caudate/putamen is reduced in methamphetamine users. The authors measured nucleus accumbens and prefrontal cortex density, in addition to caudate/putamen density, in methamphetamine users and assessed the relation of these measures to the subjects’ clinical characteristics. METHOD: PET and 2-β-carbomethoxy-3β-(4-[11C] fluorophenyl)tropane, a dopamine transporter ligand, were used to measure dopamine transporter density in 11 male methamphetamine users and nine male comparison subjects who did not use methamphetamine. Psychiatric symptoms in methamphetamine users were evaluated by using the Brief Psychiatric Rating Scale and applying a craving score. RESULTS: The dopamine transporter density in all three of the regions observed was significantly lower in the methamphetamine users than the comparison subjects. The severity of psychiatric symptoms was significantly correlated with the duration of meth...

https://ajp.psychiatryonline.org/doi/pdfplus/10.1176/appi.ajp.158.8.1206

Methamphetamine-related psychiatric symptoms and reduced brain dopamine transporters studied with PET.

In the PET system a block detector, which is a position-sensitive photomultiplier (PMT) tube coupled to four arrays of 1.7-mm-wide BGO crystals, is utilized. Fifteen block detectors are positioned to form a 35-cm-diameter ring, with four BGO arrays coupled to each PMT. This provides four detector rings, giving the system a 7-slice imaging capability. The gantry head tilts up to +or-90 degrees , and is capable of moving up and down by 30 cm. These positioning capabilities allow flexibility and great ease of use in subject positioning. A gantry entrance size of 22 cm in diameter allows studies ranging from rats and mice to primates such as Rhesus and squirrel monkeys. The physical performance of the system has been evaluated. >

A high resolution PET for animal studies

This paper presents a row-action maximum likelihood algorithm (RAMLA), in which the relaxation parameter is controlled in such a way that the noise propagation from projection data to the reconstructed image is substantially independent of the access order of the input data (subsets) in each cycle of the sub-iterations. The 'subset-dependent' relaxation parameter lambda(k) (q) is expressed as lambda(k)(q) = beta0/(beta0 + q + gamma k M), where M is the number of angular views, q (0 < or = q < or = M - 1) is the access order of the angular view, k is the iteration number and beta0 and gamma are constants. The constant beta0 deals with the balance of the noise propagation and the constant gamma controls the convergence of iterations. The value of beta0 is determined from the geometrical correlation coefficients among lines of coincidence response. The proposed RAMLA using the subset-dependent (dynamic) relaxation 'dynamic RAMLA (DRAMA)' provides a reasonable signal-to-noise ratio with a satisfactory spatial resolution by a few iterations in the two-dimensional image reconstruction for PET. Dynamic OS-EM (DOSEM) has also been developed, which allows the use of a larger number of subsets (OS level) Msub without loss of signal-to-noise ratio as compared to the conventional OS-EM. DRAMA is a special case of DOSEM, where Msub = M, and it is no more profitable to use DOSEM with a smaller Msub (< M), because DRAMA provides similar performance with the fastest convergence and smallest computer burden. This paper describes the theory, algorithm and the results of the simulation studies on the performance of DRAMA and DOSEM.

https://iopscience.iop.org/article/10.1088/0031-9155/48/10/312/pdf

Subset-dependent relaxation in block-iterative algorithms for image reconstruction in emission tomography

We develop an OpenPET system which can provide an accessible open space to the patient during PET scanning. Our first-generation OpenPET geometry which we called dual-ring OpenPET consisted of two separated detector rings and it could extend its axial field of view (FOV) therefore enabling imaging the gap region in addition to the in-ring region. However, applications such as dose verification by in-beam PET measurement during particle therapy and real-time tumor tracking by PET require sensitivity focused onto the gap rather than on the wide FOV. In this paper, we propose a second-generation OpenPET geometry, single-ring OpenPET, which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the earlier one. The proposed geometry has a cylinder shape cut at a slant angle, in which the shape of each cut end becomes an ellipse. We provided a theoretical analysis for sensitivity of the proposed geometry, compared with the dual-ring OpenPET and a geometry where the conventional PET was positioned at a slant angle against the patient bed to form an accessible open space, which we called a slant PET. The central sensitivity depends on the solid angle of these geometries. As a result, we found that the single-ring OpenPET has a sensitivity 1.2 times higher than the dual-ring OpenPET and 1.3 times higher than the slant PET when designed for a 600 mm bed width with 300 mm accessible open space and about 200 detector blocks, each with a front area of 2500 mm². In addition, numerical simulation was carried out to show the imaging property of the proposed geometry realized with the ellipsoidal rings and these results indicate that the depth-of-interaction detector can provide uniform resolution even when the detectors are arranged in an ellipsoidal ring.

http://iopscience.iop.org/article/10.1088/0031-9155/57/14/4705/pdf

A single-ring OpenPET enabling PET imaging during radiotherapy

The image-forming performance of multilayer positron tomographs for extended sources is evaluated analytically. The analysis is simplified by "rotation transform," by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and a single- and double-scattered coincidence are formulated for a uniform cylinder phantom as functions of various design parameters. Angle factors for Compton scattering and other parameters used in the evaluation are presented. Scatter components in projections and their effect on the reconstructed images are also evaluated. The scatter component in the reconstructed image depends critically on the detector ring radius, phantom radius, method of attenuation correction, etc. When the director radius is relatively small (40 or approximately 45 cm in diameter), the scatter/true ratio at the image center of a 20 cm diameter phantom may be larger than the scatter/true ratio in the event rates. Comparison with experimental data obtained with a head positron tomograph, POSITOLOGICA, showed reasonable agreement both in the total coincidence rates and in the scatter components in the images for a cylindrical phantom of 20 cm in diameter.

Analytical study of the performance of a multilayer positron computed tomography scanner.

A new positron emission tomography (PET) scanner for whole-body studies has been developed. The scanner has 720 block detectors, each of which consists of a flat panel position-sensitive photomultiplier and a 16/spl times/8 BGO crystal array. The detector system is composed of 12 layers of block detector rings stacked axially, and each ring consists of a circular array of 60 block detectors. Since each block detector ring contains eight crystal rings, the total number of detector rings is 96 and the axial field of view (FOV) is 685 mm, which is sufficient to measure a whole-body with two steps of scan. The detector ring diameter is 1 020 mm and the transaxial FOV is 600 mm in diameter. Coarse slice septa are placed between the block detector rings, which are effective to reduce scattered coincidence events while keeping a high detection sensitivity. Parallel 16 personal computers are used for the data acquisition and processing to deal with a large amount of event data. The acquired data in 3-D manner are converted to 2-D data set with Fourier rebinning algorithm and reconstructed with a fast iterative algorithm "DRAMA". The reconstructed images for the whole body are obtained within 5 min after the scan. By using the coarse septa, the scatter fraction measured with NEMA NU2-2001 standard was 31.4% whose value was significantly lower than that of 3-D PET without septa. The system sensitivity was 9.72 cps/kBq, and the peak counts of the noise equivalent count rate used with direct random-subtraction was 113.6 kcps at 10.5 kBq/ml.

Eiichi Tanaka

Papers

A high resolution PET for animal studies

Subset-dependent relaxation in block-iterative algorithms for image reconstruction in emission tomography

A single-ring OpenPET enabling PET imaging during radiotherapy

Analytical study of the performance of a multilayer positron computed tomography scanner.

A high-throughput whole-body PET scanner using flat panel PS-PMTs