PET instrumentation: what are the limits?

The detection instrumentation and geometric design of clinical PET scanner: towards better performance and broader clinical applications

Positron emission tomography (PET) is a powerful medical imaging modality used in nuclear medicine to diagnose and monitor various clinical diseases in patients. It is more sensitive and produces a highly quantitative mapping of the three-dimensional biodistribution of positron-emitting radiotracers inside the human body. The underlying technology is constantly evolving, and recent advances in detection instrumentation and PET scanner design have significantly improved the medical diagnosis capabilities of this imaging modality, making it more efficient and opening the way to broader, innovative, and promising clinical applications. Some significant achievements related to detection instrumentation include introducing new scintillators and photodetectors as well as developing innovative detector designs and coupling configurations. Other advances in scanner design include moving towards a cylindrical geometry, 3D acquisition mode, and the trend towards a wider axial field of view and a shorter diameter. Further research on PET camera instrumentation and design will be required to advance this technology by improving its performance and extending its clinical applications while optimising radiation dose, image acquisition time, and manufacturing cost. This article comprehensively reviews the various parameters of detection instrumentation and PET system design. Firstly, an overview of the historical innovation of the PET system has been presented, focusing on instrumental technology. Secondly, we have characterised the main performance parameters of current clinical PET and detailed recent instrumental innovations and trends that affect these performances and clinical practice. Finally, prospects for this medical imaging modality are presented and discussed. This overview of the PET system's instrumental parameters enables us to draw solid conclusions on achieving the best possible performance for the different needs of different clinical applications.

Objective Image Quality Comparison Between Brain-Dedicated PET and PET/CT Scanners

As part of a clinical validation of a new brain-dedicated PET system (CMB), image quality of this scanner has been compared to that of a whole-body PET/CT scanner. To that goal, Hoffman phantom and patient data were obtined with both devices. Since CMB does not use a CT for attenuation correction (AC) which is crucial for PET images quality, this study includes the evaluation of CMB PET images using emission-based or CT-based attenuation maps. PET images were compared using 34 image quality metrics. Moreover, a neural network was used to evaluate the degree of agreement between both devices on the patients diagnosis prediction. Overall, results showed that CMB images have higher contrast and recovery coefficient but higher noise than PET/CT images. Although SUVr values presented statistically significant differences in many brain regions, relative differences were low. An asymmetry between left and right hemispheres, however, was identified. Even so, the variations between the two devices were minor. Finally, there is a greater similarity between PET/CT and CMB CT-based AC PET images than between PET/CT and the CMB emission-based AC PET images.

Long-axial field-of-view PET/CT: perspectives and review of a revolutionary development in nuclear medicine based on clinical experience in over 7000 patients

Recently introduced long-axial field-of-view (LAFOV) PET/CT systems represent one of the most significant advancements in nuclear medicine since the advent of multi-modality PET/CT imaging. The higher sensitivity exhibited by such systems allow for reductions in applied activity and short duration scans. However, we consider this to be just one small part of the story: Instead, the ability to image the body in its entirety in a single FOV affords insights which standard FOV systems cannot provide. For example, we now have the ability to capture a wider dynamic range of a tracer by imaging it over multiple half-lives without detrimental image noise, to leverage lower radiopharmaceutical doses by using dual-tracer techniques and with improved quantification. The potential for quantitative dynamic whole-body imaging using abbreviated protocols potentially makes these techniques viable for routine clinical use, transforming PET-reporting from a subjective analysis of semi-quantitative maps of radiopharmaceutical uptake at a single time-point to an accurate and quantitative, non-invasive tool to determine human function and physiology and to explore organ interactions and to perform whole-body systems analysis. This article will share the insights obtained from 2 years' of clinical operation of the first Biograph Vision Quadra (Siemens Healthineers) LAFOV system. It will also survey the current state-of-the-art in PET technology. Several technologies are poised to furnish systems with even greater sensitivity and resolution than current systems, potentially with orders of magnitude higher sensitivity. Current barriers which remain to be surmounted, such as data pipelines, patient throughput and the hindrances to implementing kinetic analysis for routine patient care will also be discussed.

Silicon photomultiplier signal readout and multiplexing techniques for positron emission tomography: a review

In recent years, silicon photomultiplier (SiPM) is replacing the photomultiplier tube (PMT) in positron emission tomography (PET) systems due to its superior properties, such as fast single-photon timing response, small gap between adjacent photosensitive pixels in the array, and insensitivity to magnetic fields. One of the technical challenges when developing SiPM-based PET systems or other position-sensitive radiation detectors is the large number of output channels coming from the SiPM array. Therefore, various signal multiplexing methods have been proposed to reduce the number of output channels and the load on the subsequent data acquisition (DAQ) system. However, the large PN-junction capacitance and quenching resistance of the SiPM yield undesirable resistance–capacitance delay when multiple SiPMs are combined, which subsequently causes the accumulation of dark counts and signal fluctuation of SiPMs. Therefore, without proper SiPM signal handling and processing, the SiPMs may yield worse timing characteristics than the PMTs. This article reviews the evolution of signal readout and multiplexing methods for the SiPM. In this review, we focus primarily on analog electronics for SiPM signal multiplexing, which allows for the reduction of DAQ channels required for the SiPM-based position-sensitive detectors used in PET and other radiation detector systems. Although the applications of most technologies described in the article are not limited to PET systems, the review highlights efforts to improve the physical performance (e.g. spatial, energy, and timing resolutions) of PET detectors and systems.

Physics and technology of time-of-flight PET detectors

The imaging performance of clinical positron emission tomography (PET) systems has evolved impressively during the last ∼15 years. A main driver of these improvements has been the introduction of time-of-flight (TOF) detectors with high spatial resolution and detection efficiency, initially based on photomultiplier tubes, later silicon photomultipliers. This review aims to offer insight into the challenges encountered, solutions developed, and lessons learned during this period. Detectors based on fast, bright, inorganic scintillators form the scope of this work, as these are used in essentially all clinical TOF-PET systems today. The improvement of the coincidence resolving time (CRT) requires the optimization of the entire detection chain and a sound understanding of the physics involved facilitates this effort greatly. Therefore, the theory of scintillation detector timing is reviewed first. Once the fundamentals have been set forth, the principal detector components are discussed: the scintillator and the photosensor. The parameters that influence the CRT are examined and the history, state-of-the-art, and ongoing developments are reviewed. Finally, the interplay between these components and the optimization of the overall detector design are considered. Based on the knowledge gained to date, it appears feasible to improve the CRT from the values of 200-400 ps achieved by current state-of-the-art TOF-PET systems to about 100 ps or less, even though this may require the implementation of advanced methods such as time resolution recovery. At the same time, it appears unlikely that a system-level CRT in the order of ∼10 ps can be reached with conventional scintillation detectors. Such a CRT could eliminate the need for conventional tomographic image reconstruction and a search for new approaches to timestamp annihilation photons with ultra-high precision is therefore warranted. While the focus of this review is on timing performance, it attempts to approach the topic from a clinically driven perspective, i.e. bearing in mind that the ultimate goal is to optimize the value of PET in research and (personalized) medicine.

Gold nanoparticle detection and quantification in therapeutic MV beams via pair production

PURPOSE

We propose a new detection method of gold nanoparticles (AuNP) in therapeutic megavoltage (MV) x-ray beams by means of coincidence counting of annihilation photons following pair production in gold.

METHODS

The proposed MV x-ray induced positron emission (MVIPE) imaging technique is studied by radiation transport computations using MCNP6 (3D) and CEPXS/ONEDANT (1D) codes for two water phantoms: a 35 cm slab and a similarly sized cylinder, both having a 5 cm AuNP filled region in the center. MVIPE is compared to the standard x-ray fluorescence computed tomography (XFCT). MVIPE adopts MV x-ray sources (Co-60, 2 MV, 6 MV, 6 MV with closed MLC and 15 MV) and relies on the detection of 511 keV photon-pairs. XFCT uses kilovoltage sources (100 kVp, 120 kVp and 150 kVp) and imaging is characterized by analysis of k _α1,2 Au characteristic lines. Three levels of AuNP concentration were studied: 0.1%, 1% and 10% by weight.

RESULTS

Annihilation photons in the MVIPE technique originate both in the AuNP and in water along the x-ray beam path with significantly larger production in the AuNP-loaded region. MVIPE signal from AuNP is linearly increasing with AuNP concentration up to 10%wt, while XFCT signal reaches saturation due to self-absorption within AuNP. The production of annihilation photons is proportional to the MV source energy. MVIPE technique using a 15 MV pencil beam and 10 wt% AuNP detects about 4.5 × 10³ 511 keV-photons cm^-2 at 90° w/r to the incident beam per 10⁹ source photons cm^-2; 500 of these come from AuNP. In contrast, the XFCT technique using 150 kVp detects only about 100 k _α1-photons cm^-2 per 10⁹ source photons cm^-2.

CONCLUSIONS

In MVIPE, the number of annihilation photons produced for different MV-beam energies and AuNP concentrations is significantly greater than the k _α1 photons generated in XFCT. Coincidence counting in MVIPE allows to avoid collimation, which is a major limiting factor in XFCT. MVIPE challenges include the filtering of Compton scatter and annihilation photons originating in water.

Total Body PET: Why, How, What for?

PET instruments are now available with a long axial field-of-view (LAFOV) to enable imaging the total-body, or at least head and torso, simultaneously and without bed translation. This has two major benefits, a dramatic increase in system sensitivity and the ability to measure kinetics with wider axial coverage so as to include multiple organs. This manuscript presents a review of the technology leading up to the introduction of these new instruments, and explains the benefits of a LAFOV PET-CT instrument. To date there are two platforms developed for TB-PET, an outcome of the EXPLORER Consortium of the University of California at Davis (UC Davis) and the University of Pennsylvania (Penn). The uEXPLORER at UC Davis has an AFOV of 194 cm and was developed by United Imaging Healthcare. The PennPET EXPLORER was developed at Penn and is based on the digital detector from Philips Healthcare. This multi-ring system is scalable and has been tested with 3 rings but is now being expanded to 6 rings for 140 cm. Initial human studies with both EXPLORER systems have demonstrated the successful implementation and benefits of LAFOV scanners for both clinical and research applications. Examples of such studies are described in this manuscript.

Development of Dedicated Brain PET Imaging Devices: Recent Advances and Future Perspectives

Whole-body PET scanners are not optimized for imaging small structures in the human brain. Several PET devices specifically designed for this task have been proposed either for stand-alone operation or as MR-compatible inserts. The main distinctive features of some of the most recent concepts and their performance characteristics, with a focus on spatial resolution and sensitivity, are reviewed. The trade-offs between the various performance characteristics, desired capabilities, and cost that need to be considered when designing a dedicated brain scanner are presented. Finally, the aspirational goals for future-generation scanners, some of the factors that have contributed to the current status, and how recent advances may affect future developments in dedicated brain PET instrumentation are briefly discussed.

Polarized x-ray excitation for scatter reduction in x-ray fluorescence computed tomography

PURPOSE

X-ray fluorescence computer tomography (XFCT) is a new molecular imaging modality which uses x-ray excitation to stimulate the emission of fluorescent photons in high atomic number contrast agents. Scatter contamination is one of the main challenges in XFCT imaging which limits the molecular sensitivity. When polarized x rays are used, it is possible to reduce the scatter contamination significantly by placing detectors perpendicular to the polarization direction. This study quantifies scatter contamination for polarized and unpolarized x-ray excitation and determines the advantages of scatter reduction.

METHODS

The amount of scatter in preclinical XFCT is quantified in Monte Carlo simulations. The fluorescent x rays are emitted isotropically, while scattered x rays propagate in polarization direction. The magnitude of scatter contamination is studied in XFCT simulations of a mouse phantom. In this study, the contrast agent gold is examined as an example, but a scatter reduction from polarized excitation is also expected for other elements. The scatter reduction capability is examined for different polarization intensities with a monoenergetic x-ray excitation energy of 82 keV. The study evaluates two different geometrical shapes of CZT detectors which are modeled with an energy resolution of 1 keV FWHM at an x-ray energy of 80 keV. Benefits of a detector placement perpendicular to the polarization direction are shown in iterative and analytic image reconstruction including scatter correction. The contrast to noise ratio (CNR) and the normalized mean square error (NMSE) are analyzed and compared for the reconstructed images.

RESULTS

A substantial scatter reduction for common detector sizes was achieved for 100% and 80% linear polarization while lower polarization intensities provide a decreased scatter reduction. By placing the detector perpendicular to the polarization direction, a scatter reduction by factor up to 5.5 can be achieved for common detector sizes. The image reconstruction showed that for a scatter magnitude decrease by a factor of 2.4, the molecular sensitivity could almost be doubled.

CONCLUSION

Scatter reduction lowers the amount of noise in the projection datasets and reconstructed images which enhance molecular sensitivity at equal dose. The results support the use of linear polarized x rays to reduce scatter in XFCT imaging.

Three-dimensional reconstruction of blood vessels in the rabbit eye by X-ray phase contrast imaging

Background

A clear understanding of the blood vessels in the eye is helpful in the diagnosis and treatment of ophthalmic diseases, such as glaucoma. Conventional techniques such as micro-CT imaging and histology are not sufficiently accurate to identify the vessels in the eye, because their diameter is just a few microns. The newly developed medical imaging technology, X-ray phase-contrast imaging (XPCI), is able to distinguish the structure of the vessels in the eye. In this study, XPCI was used to identify the internal structure of the blood vessels in the eye.

Methods

After injection with barium sulfate via the ear border artery, an anesthetized rabbit was killed and its eye was fixed in vitro in 10% formalin solution. We acquired images using XPCI at the Shanghai Synchrotron Radiation Facility. The datasets were converted into slices by filtered back-projection (FBP). An angiographic score was obtained as a parameter to quantify the density of the blood vessels. A three-dimensional (3D) model of the blood vessels was then established using Amira 5.2 software.

Results

With XPCI, blood vessels in the rabbit eye as small as 18 μm in diameter and a sixth of the long posterior ciliary artery could be clearly distinguished. In the 3D model, we obtained the level 4 branch structure of vessels in the fundus. The diameters of the arteria centralis retinae and its branches are about 200 μm, 110 μm, 95 μm, 80 μm and 40 μm. The diameters of the circulus arteriosus iridis major and its branches are about 210 μm, 70 μm and 30 μm. Analysis of vessel density using the angiographic score showed that the blood vessels had maximum density in the fundus and minimum density in the area anterior to the equator (scores 0.27 ± 0.029 and 0.16 ± 0.032, respectively). We performed quantitative angiographic analysis of the blood vessels to further investigate the density of the vessels.

Conclusions

XPCI provided a feasible means to determine the structure of the blood vessels in the eye. We were able to determine the diameters and morphological characteristics of the vessels from both 2D images and the 3D model. By analyzing the images, we obtained measurements of the density distribution of the microvasculature, and this approach may provide valuable reference information prior to glaucoma filtration surgery.

Fundamentals of Positron Emission Tomography and Applications in Preclinical Drug Development

Positron emission tomography (PET) is a nuclear imaging technique that can dynamically image trace amounts of positron-labeled radiopharmaceuticals in vivo. Tracer concentrations can be determined quantitatively, and by application of appropriate tracer kinetic models, the rates of a wide range of different biological processes can be measured noninvasively in humans. PET has been used as a research tool for more than 25 years and has also found clinical applications, particularly in oncology, neurological disorders, and cardiovascular disease. Recently, there has been tremendous interest in applying PET technology to in vivo small-animal imaging. Significant improvements in the imaging technology now permit a wide range of PET studies in mice and rats, using compact, relatively low-cost, dedicated small-animal PET scanners. This article reviews the fundamental basis of PET imaging and discusses the development of small-animal PET scanners and their possible application in preclinical drug development.

[Neuroimaging: technical aspects and practice]

Neuroimaging using both functional and structural examinations like positron emission tomography (PET), single photon emission tomography (SPECT), computed tomography (CT) and magnetic nuclear imaging (MRI) provide supportive information of great importance for the diagnosis and treatment of patients with central nervous system disorders. Therefore, they have become commonplace in clinical practice and basic biomedical research. In recent years we have seen the development of multimodality equipment that enables PET or SPECT to be combined with a CT structural image. Moreover, experimental equipment combining PET and MRI has now been developed. Additionally, methodological features that provide a higher image quality, and analysis tools for objective quantification and interpretation have been refined. This article reviews the technical aspects of those imaging methods, highlighting the most significant and recent advances in the development of neuroimaging.

Rapid three-dimensional quantification of VEGF-induced scaffold neovascularisation by microcomputed tomography

Microcomputed tomography (micro-CT) is increasingly being used to analyze the three-dimensional structure and architecture of microvascular networks. Therefore we have evaluated a micro-CT analysis of VEGF-induced vessel ingrowth into a porous polyurethane scaffold through comparison with analyses by CD31 immunohistochemistry, vascular perfusion by intravital Lycopersicon esculentum lectin perfusion and vascular corrosion casting. Micro-CT scanning found a similar level of vascularisation within the VEGF treated scaffolds to that determined by the other analytical methods. However, although the relative increase in vascularisation (17 fold above PBS controls p<0.05) induced by VEGF determined by micro-CT was similar to the perfusion based analyses (20.1 and 10.4 fold for lectin perfusion and vascular corrosion respectively p<0.05), it differed substantially from that determined by CD31 immunohistochemistry (3.2 fold p<0.05). This difference was due to a large proportion of unperfused vessels in the PBS control that were not present in the VEGF group. The increase in perfusion probably resulted in part from an increase in average vessel diameter. Though this increase was detected by micro-CT, the actual diameters were overestimated by 60-90% most likely as a consequence of a merging effect for juxtaposed vessels. Thus whilst micro-CT gives an accurate three-dimensional quantification of the VEGF-induced increase in perfused vessels, resolution needs to be maximized for accurate sizing of a microvascular network's components.

Recent developments in PET detector technology

Positron emission tomography (PET) is a tool for metabolic imaging that has been utilized since the earliest days of nuclear medicine. A key component of such imaging systems is the detector modules--an area of research and development with a long, rich history. Development of detectors for PET has often seen the migration of technologies, originally developed for high energy physics experiments, into prototype PET detectors. Of the many areas explored, some detector designs go on to be incorporated into prototype scanner systems and a few of these may go on to be seen in commercial scanners. There has been a steady, often very diverse development of prototype detectors, and the pace has accelerated with the increased use of PET in clinical studies (currently driven by PET/CT scanners) and the rapid proliferation of pre-clinical PET scanners for academic and commercial research applications. Most of these efforts are focused on scintillator-based detectors, although various alternatives continue to be considered. For example, wire chambers have been investigated many times over the years and more recently various solid-state devices have appeared in PET detector designs for very high spatial resolution applications. But even with scintillators, there have been a wide variety of designs and solutions investigated as developers search for solutions that offer very high spatial resolution, fast timing, high sensitivity and are yet cost effective. In this review, we will explore some of the recent developments in the quest for better PET detector technology.

Annihilation photon acollinearity in PET: volunteer and phantom FDG studies

Annihilation photon acollinearity is a fundamental but little investigated problem in positron emission tomography (PET). In this paper, the cause of the angular deviation from 180.00 degrees is described as well as how to evaluate it under conditions of a spatially distributed radiation source and a limited acquisition time for the human body. A relationship between the shape of the photopeak spectrum and the angular distribution is formulated using conservation laws of momentum and energy over the pair annihilation. Then the formula is used to evaluate the acollinearity for a pool phantom and the human body with FDG injected. The angular distribution for the pool phantom agrees well with that for pure water which had been directly measured by Colombino et al in 1965 (Nuovo Cimento 38 707-23), and also with that for the human body determined in this study. Pure water can be considered as a good approximation of the human body regarding the angular deviation. The blurring coefficient to be multiplied by the ring diameter in calculations of the PET spatial resolution is experimentally determined for the first time as 0.00243 +/- 0.00014; this is 10% larger than the value widely used by investigators.

Advances in the production, processing and microPET image quality of technetium-94m

This work involves the production, processing and imaging of the short-lived, rarely used positron emission tomography (PET) radionuclide technetium-94m (94mTc). Our procedures are an extension of methods reported in the literature and are detailed within. A key modification was the development of a single step that combines purification and concentration of an aqueous 94mTc-pertechnetate solution, which both reduces processing time and increases the final concentration of the solution. Additionally, a convenient method for the direct recovery of 94mTc into an organic solvent was developed, eliminating the solvent transfer step needed for organic syntheses using 94mTc. Each of these advances potentially extends the scope of syntheses possible with this short-lived radionuclide. To explore the imaging potential of 94mTc, we carried out phantom imaging studies on small-scale high-resolution PET scanners to estimate the limitations of detection associated with 94mTc and PET. Preliminary studies demonstrate that useful images can be obtained with modern image reconstruction algorithms when using a correction for the cascade gamma ray contamination.

High-Resolution Fluorodeoxyglucose Positron Emission Tomography with Compression ("Positron Emission Mammography") is Highly Accurate in Depicting Primary Breast Cancer

We sought to prospectively assess the diagnostic performance of a high-resolution positron emission tomography (PET) scanner using mild breast compression (positron emission mammography [PEM]). Data were collected on concomitant medical conditions to assess potential confounding factors. At four centers, 94 consecutive women with known breast cancer or suspicious breast lesions received 18F-fluorodeoxyglucose (FDG) intravenously, followed by PEM scans. Readers were provided clinical histories and x-ray mammograms (when available). After excluding inevaluable cases and two cases of lymphoma, PEM readings were correlated with histopathology for 92 lesions in 77 women: 77 index lesions (42 malignant), 3 ipsilateral lesions (3 malignant), and 12 contralateral lesions (3 malignant). Of 48 cancers, 16 (33%) were clinically evident; 11 (23%) were ductal carcinoma in situ (DCIS), and 37 (77%) were invasive (30 ductal, 4 lobular, and 3 mixed; median size 21 mm). PEM depicted 10 of 11 (91%) DCIS and 33 of 37 (89%) invasive cancers. PEM was positive in 1 of 2 T1a tumors, 4 of 6 T1b tumors, 7 of 7 T1c tumors, and 4 of 4 cases where tumor size was not available (e.g., no surgical follow-up). PEM sensitivity for detecting cancer was 90%, specificity 86%, positive predictive value (PPV) 88%, negative predictive value (NPV) 88%, accuracy 88%, and area under the receiver-operating characteristic curve (Az) 0.918. In three patients, cancer foci were identified only on PEM, significantly changing patient management. Excluding eight diabetic subjects and eight subjects whose lesions were characterized as clearly benign with conventional imaging, PEM sensitivity was 91%, specificity 93%, PPV 95%, NPV 88%, accuracy 92%, and Az 0.949 when interpreted with mammographic and clinical findings. FDG PEM has high diagnostic accuracy for breast lesions, including DCIS.

The influence of photon depth of interaction and non-collinear spread of annihilation photons on PET image spatial resolution

PURPOSE

The quality of PET imaging is impaired by parallax errors. These errors produce misalignment between the projected location of the true origin of the annihilation event and the line of response determined by the coincidence detection system. Parallax errors are due to the varying depths of photon interaction (DOI) within the scintillator and the non-collinear (NC) emission of the annihilation photons. The aim of this work was to address the problems associated with the DOI and the NC spread of annihilation photons and to develop a quantitative model to assess their impact on image spatial resolution losses for various commonly used scintillators and PET geometries.

METHODS

A theoretical model based on Monte Carlo simulations was developed to assess the relative influence of DOI and the NC spread of annihilation photons on PET spatial resolution for various scintillator materials (BGO, LSO, LuAP, GSO, NaI) and PET geometries.

RESULTS

The results demonstrate good agreement between simulated, experimental and published overall spatial resolution for some commercial systems, with maximum differences around 1 mm in both 2D and 3D mode. The DOI introduces an impairment of non-stationary spatial resolution along the radial direction, which can be very severe at peripheral positions. As an example, the radial spatial resolution loss due to DOI increased from 1.3 mm at the centre to 6.7 mm at 20 cm from the centre of a BGO camera with a 412-mm radius in 2D mode. Including the NC, the corresponding losses were 3.0 mm at the centre and 7.3 mm 20 cm from the centre.

CONCLUSION

Without a DOI detection technique, it seems difficult to improve PET spatial resolution and increase sensitivity by reducing the detector ring radius or by extending the detector in the axial direction. Much effort is expended on the design and configuration of smaller detector elements but more effort should be devoted to the DOI complexity.

Imaging Multidrug Resistance P-glycoprotein Transport Function Using MicroPET with Technetium-94m-Sestamibi

The best characterized mechanism of multidrug resistance (MDR) in cancer involves the MDR1 efflux transporter P-glycoprotein (Pgp). The positron-emitting radiotracer hexakis(2-methoxyisobutylisonitrile)-(94m)Tc ((94m)Tc-MIBI) was synthesized and validated in cell transport studies as a substrate for MDR1 Pgp. In vivo small-scale PET imaging and biodistribution studies of mdr1a/1b (-/-) gene deleted and wild-type mice demonstrated the use of (94m)Tc-MIBI to detect Pgp function. The reversal effect of a Pgp modulator was shown in tissue distribution studies of KB 3-1 (Pgp-) and KB 8-5 (Pgp+) tumor-bearing nude mice. The current (94m)Tc-MIBI experiments parallel previous studies employing (99m)Tc-MIBI, showing essentially identical performance of the two technetium radiotracers and providing biological validation of (94m)Tc-MIBI for PET imaging of multidrug resistance.

Positron emission tomography in the staging and management of breast cancer

Background

Breast cancer is the commonest cause of cancer death in women in the Western world, and imaging is essential in its diagnosis and staging. Metabolic imaging is a novel approach to improving the detection of cancers, as malignant transformation of cells is often associated with increased metabolic activity. This review assesses the possible role of positron emission tomography (PET) as a single non-invasive imaging modality to replace or complement current imaging and surgical practices in the diagnosis and staging of breast cancer.

Methods and results

A Medline search was performed and articles were cross-referenced with other relevant material. Evaluation of primary breast cancer with PET has shown a sensitivity of between 64 and 100 per cent and a specificity of 33–100 per cent; diagnostic accuracy appears to be related to tumour size. Difficulties arise in altered fluorodeoxyglucose uptake in lobular carcinoma, carcinoma in situ and benign inflammatory breast disease. In axillary staging, sensitivities of between 25 and 100 per cent have been reported, but with a false-negative of up to 20 per cent. In the assessment of distant metastasis and asymptomatic patients with raised levels of tumour markers, PET was superior to conventional imaging modalities.

Conclusion

PET is not a single diagnostic and staging tool that can replace current surgical, histological and radiological staging. Its main role in breast cancer lies in the investigation of metastatic disease and the evaluation of pathological response to various chemotherapeutic regimens.

Prospective multicenter study of axillary nodal staging by positron emission tomography in breast cancer: a report of the staging breast cancer with PET Study Group

PURPOSE

To determine the accuracy of positron emission tomography with fluorine-18-labeled 2-fluoro-2-deoxy-d-glucose (FDG-PET) in detecting axillary nodal metastases in women with primary breast cancer.

PATIENTS AND METHODS

In this prospective multicenter study, 360 women with newly diagnosed invasive breast cancer underwent FDG-PET. Images were blindly interpreted by three experienced readers for abnormally increased axillary FDG uptake. Imaging results from 308 assessable axillae were compared with axillary node pathology.

RESULTS

For detecting axillary nodal metastasis, the mean estimated area under the receiver operator curve for the three readers was 0.74 (range, 0.70 to 0.76). If at least one probably or definitely abnormal axillary focus was considered positive, the mean (and range) sensitivity, specificity, and positive and negative predictive values for PET were 61% (54% to 67%), 80% (79% to 81%), 62% (60% to 64%), and 79% (76% to 81%), respectively. False-negative axillae on PET had significantly smaller and fewer tumor-positive lymph nodes (2.7) than true-positive axillae (5.1; P <.005). Semiquantitative analysis of axillary FDG uptake showed that a nodal standardized uptake value (lean body mass) more than 1.8 had a positive predictive value of 90%, but a sensitivity of only 32%. Finding two or more intense foci of tracer uptake in the axilla was highly predictive of axillary metastasis (78% to 83% positive predictive value), albeit insensitive (27%).

CONCLUSION

FDG-PET has moderate accuracy for detecting axillary metastasis but often fails to detect axillae with small and few nodal metastases. Although highly predictive for nodal tumor involvement when multiple intense foci of tracer uptake are identified, FDG-PET is not routinely recommended for axillary staging of patients with newly diagnosed breast cancer.

Can the cerebral metabolic rate of oxygen be estimated with near-infrared spectroscopy?

We have measured the changes in oxy-haemoglobin and deoxy-haemoglobin in the adult human brain during a brief finger tapping exercise using near-infrared spectroscopy (NIRS). The cerebral metabolic rate of oxygen (CMRO2) can be estimated from these NIRS data provided certain model assumptions. The change in CMRO2 is related to changes in the total haemoglobin concentration, deoxy-haemoglobin concentration and blood flow. As NIRS does not provide a measure of dynamic changes in blood flow during brain activation, we relied on a Windkessel model that relates dynamic blood volume and flow changes, which has been used previously for estimating CMRO2 from functional magnetic resonance imaging (fMRI) data. Because of the partial volume effect we are unable to quantify the absolute changes in the local brain haemoglobin concentrations with NIRS and thus are unable to obtain an estimate of the absolute CMRO2 change. An absolute estimate is also confounded by uncertainty in the flow-volume relationship. However, the ratio of the flow change to the CMRO2 change is relatively insensitive to these uncertainties. For the linger tapping task, we estimate a most probable flow-consumption ratio ranging from 1.5 to 3 in agreement with previous findings presented in the literature, although we cannot exclude the possibility that there is no CMRO2 change. The large range in the ratio arises from the large number of model parameters that must be estimated from the data. A more precise estimate of the flow-consumption ratio will require better estimates of the model parameters or flow information, as can be provided by combining NIRS with fMRI.

Imaging of angiogenesis: from microscope to clinic

Advances in imaging are transforming our understanding of angiogenesis and the evaluation of drugs that stimulate or inhibit angiogenesis in preclinical models and human disease. Vascular imaging makes it possible to quantify the number and spacing of blood vessels, measure blood flow and vascular permeability, and analyze cellular and molecular abnormalities in blood vessel walls. Microscopic methods ranging from fluorescence, confocal and multiphoton microscopy to electron microscopic imaging are particularly useful for elucidating structural and functional abnormalities of angiogenic blood vessels. Magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), ultrasonography and optical imaging provide noninvasive, functionally relevant images of angiogenesis in animals and humans. An ongoing dilemma is, however, that microscopic methods provide their highest resolution on preserved tissue specimens, whereas clinical methods give images of living tissues deep within the body but at much lower resolution and specificity and generally cannot resolve vessels of the microcirculation. Future challenges include developing new imaging methods that can bridge this resolution gap and specifically identify angiogenic vessels. Another goal is to determine which microscopic techniques are the best benchmarks for interpreting clinical images. The importance of angiogenesis in cancer, chronic inflammatory diseases, age-related macular degeneration and reversal of ischemic heart and limb disease provides incentive for meeting these challenges.

In Vivo Noninvasive Imaging for Gene Therapy

Gene therapy is reaching a stage where some clinical benefits have been demonstrated on patients involved in phase I/II clinical trials. However, in many cases, the clinical benefit is hardly measurable and progress in the improvement of gene therapy formulations is hampered by the lack of objective clinical endpoints to measure transgene delivery and to quantitate transgene expression. However, these endpoints rely almost exclusively on the analysis of biopsies by molecular and histopathological methods. These methods provide only a limited picture of the situation. Therefore, there is a need for a technology that would allow precise, spacio-temporal measurement of gene expression on a whole body scale upon administration of the gene delivery vector. In the field of gene therapy, a considerable effort is being invested in the development of noninvasive imaging of gene expression and this review presents the various strategies currently being developed.

Adenovirus biodistribution and noninvasive imaging of gene expression in vivo by positron emission tomography using human sodium/iodide symporter as reporter gene

Amongst the various methods that can be developed for noninvasive monitoring of gene expression in vivo, the use of positron emission tomography (PET) appears to be the most promising both for preclinical and clinical studies. Various genes have been described as potential PET reporters, but there is a need to develop new approaches that exploit transgenes with both therapeutic and imaging potential. The Na/I symporter (NIS) gene is expressed mainly in the thyroid and is responsible for iodide accumulation in this organ. The NIS gene has been used in gene therapy experimentation. Ectopic expression of this gene in various type of malignant cells has led to radiosensitization and in some cases tumor regression in xenograft models in nude mice, highlighting the therapeutic potential of this approach. In the present study, we demonstrate the potential of the human NIS gene (hNIS) as a reporter gene. Expression of hNIS, after plasmid transfection or adenoviral gene delivery, can be monitored in vitro on incubation with (125)I. Iodide uptake in the transduced cells can be directly correlated with the levels of gene expression in vitro. Ectopic expression of the NIS gene in vivo can be monitored in biodistribution studies on intravenous injection of (125)I. Adenovirus delivery induces gene expression essentially in the liver, adrenal glands, lungs, pancreas, and spleen. Expression of hNIS in tumor xenograft models can also be detected when the virus is injected intratumorally. Finally, hNIS expression was monitored by PET after intravenous injection of (124)I, demonstrating the potential of this approach for noninvasive imaging.

Imaging transgenic animals

Transgenic and eugenic animals as small as 30 g can be studied non-invasively by radionuclides with resolutions of 1-2 mm, by MRI with resolution of 100 microns and by light fluorescence and bioluminescence with high sensitivities. The technologies of radionuclide emission, magnetic resonance imaging, magnetic resonance spectroscopy, optical tomography, optical fluorescence and optical bioluminescence are currently being applied to small-animal studies. These technologies and examples of their applications are reviewed in this chapter.

Detection of motion in hybrid PET/SPECT imaging based on the correlation of partial sinograms

This paper describes a motion detection method specific to hybrid positron emission tomography/single photon emission computed tomography systems. The method relies on temporal fractionation of the acquisition into three data sets followed by an algorithm based on the cross correlation (CC) of partial sinograms from successive sets at different rotations of the camera. Spatial inconsistencies due to motion are detected by decreases in the CC between two sets. This permits to separate data into premotion and postmotion sets of consistent data that are reconstructed independently then registered and summed. Rigid motions greater than 1-cm translation or 10 degrees rotation were detected with this method from experimental data obtained by manually moving phantoms made of radioactive spheres as well as from a patient lung study corrupted by artificial motion. The different motion studies showed that the image contrast does not seem to be a limiting factor and that the motion is best detected when the gantry is parallel to the direction of motion. The registration and fusion of the reconstructed premotion and postmotion sets lead in all cases to a reduction of the motion artifacts and an increase in signal-to-noise ratio.

The biological application of small animal PET imaging

The short history of small animal PET is reviewed in the context of its application in the laboratory. Early work has demonstrated a role for the technique in both drug development and in the in vivo monitoring of neuroreceptor function with time. As spatial resolution approaches 1 mm, challenges in quantification remain. However, the ability to carry out animal PET studies that are analogous to human PET will form an important bridge between laboratory and clinical sciences.

Axillary lymph node staging in breast cancer by 2-fluoro-2-deoxy-D-glucose-positron emission tomography: clinical evaluation and alternative management

BACKGROUND

Surgical removal of axillary lymph node and histologic examination for metastases are used to determine whether adjuvant treatment is necessary for patients with breast cancer. Axillary lymph node dissection (ALND) is a costly procedure associated with various side effects, and 80% or more of patients with tumors of 20 mm or less are lymph node negative and might avoid ALND. In this study, we evaluated whether an alternative, noninvasive method--i.e., positron emission tomography (PET) with 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG)-- could be used to determine axillary lymph node status in patients with breast cancer.

METHODS

One hundred sixty-seven consecutive patients with breast cancers of 50 mm or less (range = 5-50 mm; mean = 21 mm) scheduled for complete ALND were studied preoperatively with FDG-PET, and then PET and pathology results from ALND were compared. All statistical tests were two-sided.

RESULTS

The overall sensitivity, specificity, and accuracy of lymph node staging with PET were 94.4% (PET detected 68 of 72 patients with axillary involvement; 95% confidence interval [CI] = 86.0% to 98.2%), 86.3% (82 of 95 patients without axillary involvement; 95% CI = 77.8% to 91.9%), and 89.8% (150 of 167 patients with breast cancer; 95% CI = 84.2% to 93.6%), respectively. Positive- and negative-predictive values were 84.0% (68 patients with histologically positive lymph nodes of 81 patients with positive FDG-PET scan; 95% CI = 74.2% to 90.5%) and 95.3% (82 patients with histologically negative lymph nodes of 86 patients with negative FDG-PET scan; 95% CI = 88.2% to 98.5%), respectively. When PET results for axillary metastasis were analyzed by tumor size, the diagnostic accuracy was similar for all groups (86.0%-94.2%), with higher sensitivity for tumors of 21-50 mm (98.0%) and higher specificity for tumors of 10 mm or less (87.8%), and the range was 93.5%-97.3% for negative-predictive values and 54.5%-94.1% for positive-predictive values. Among the 72 patients with axillary involvement, PET detected three or fewer metastatic lymph nodes in 27 (37.5%) patients, about 80% of whom had no clinically palpable axillary lymph nodes.

CONCLUSIONS

Noninvasive FDG-PET appears to be an accurate technique to predict axillary status in patients with breast cancer and thus to identify patients who might avoid ALND. These results should be confirmed in large multicenter studies.

The role of positron emission tomography with 18F-fluoro-2-deoxy-D-glucose in respiratory oncology

In the past 5 yrs, positron emission tomography (PET) with 18F-fluoro-2-deoxy-D-glucose (FDG) has become an important imaging modality in lung cancer patients. At this time, the indication of FDG-PET as a complimentary tool to computed tomography in the diagnosis and staging of nonsmall cell lung cancer has gradually gained more widespread acceptance and also reimbursement in many European countries. This review focuses on the data of FDG-PET in the diagnosis of lung nodules and masses, and in locoregional and extrathoracic staging of nonsmall cell lung cancer. Emphasis is put on the potential clinical implementation of the currently available FDG-PET data. The use of FDG-PET in these indications now needs further validation in large-scale multicentre randomized studies, focusing mainly on treatment outcome parameters, survival and cost-efficacy. Interesting findings with 18F-fluoro-2-deoxy-D-glucose-positron emission tomography have also been reported for the evaluation of response to radio- or chemotherapy, in radiotherapy planning, recurrence detection and assessment of prognosis. Finally, a whole new field of application of positron emission tomography in molecular biology, using new radiopharmaceuticals, is under extensive investigation.

High-resolution functional imaging with ultrasound contrast agents based on RF processing in an in vivo kidney experiment

Knowledge of the relative tissue perfusion distribution is valuable in the diagnosis of numerous diseases. Techniques for the assessment of the relative perfusion distribution, based on ultrasound (US) contrast agents, have several advantages compared to established nuclear techniques. These are, among others, a better spatial and temporal resolution, the lack of exposure of the patient to ionizing radiation and the relatively low cost. In the present study, US radiofrequency (RF) image sequences are acquired, containing the signal intensity changes associated with the transit of a bolus contrast agent through the microvasculature of a dog kidney. The primary objective is to explore the feasibility of calculating functional images with high spatial resolution. The functional images characterize the transit of the contrast agent bolus and represent distributions of peak time, peak value, transit time, peak area, wash-in rate and wash-out decay constant. For the evaluation of the method, dog experiments were performed under optimized conditions where motion artefacts were minimized and an IA injection of the contrast agent Levovist was employed. It was demonstrated that processing of RF signals obtained with a 3.5-MHz echo system can provide functional images with a high spatial resolution of 2 mm in axial resolution, 2 to 5 mm in lateral resolution and a slice thickness of 2 mm. The functional images expose several known aspects of kidney perfusion, like perfusion heterogeneity of the kidney cortex and a different peripheral cortical perfusion compared to the inner cortex. Based on the findings of the present study, and given the results of complimentary studies, it is likely that the functional images reflect the relative perfusion distribution of the kidney.

Quantitative studies of bone with the use of 18F-fluoride and 99mTc-methylene diphosphonate

This article discusses methods for quantifying bone turnover based on tracer kinetic studies of the short-lived radiopharmaceuticals 99mTc-MDP and 18F-fluoride. Measurements of skeletal clearance obtained by using these tracers reflect the combined effects of skeletal blood flow and osteoblastic activity. The pharmacokinetics of each tracer is described, together with some of the quantitative tests of skeletal function that have been described in the literature. The physiologic interpretation of quantitative measurements of bone obtained with the use of short half-life radionuclides is discussed, and the advantages and limitations of 99mTc-MDP and 18F-fluoride are compared and contrasted. Currently, 18F-fluoride dynamic positron emission tomography (PET) is the technique of choice for physiologically precise quantitative studies of bone. However, comparable data could probably be obtained by using 99mTc-MDP if methods for single photon emission computed tomography (SPECT) quantitation were improved.

Radioligands for the study of brain 5-HT(1A) receptors in vivo--development of some new analogues of way

[Carbonyl-(11)C]WAY-100635 (WAY) has proved to be a very useful radioligand for the imaging of brain 5-HT(1A) receptors in human brain in vivo with positron emission tomography (PET). WAY is now being applied widely for clinical research and drug development. However, WAY is rapidly cleared from plasma and is also rapidly metabolised. A comparable radioligand, with a higher and more sustained delivery to brain, is desirable since these properties might lead to better biomathematical modelling of acquired PET data. There are also needs for other types of 5-HT(1A) receptor radioligands, for example, ligands sensitive to elevated serotonin levels, ligands labelled with longer-lived fluorine-18 for distribution to "satellite" PET centres, and ligands labelled with iodine-123 for single photon emission computerised tomography (SPECT) imaging. Here we describe our progress toward these aims through the exploration of WAY analogues, including the development of [carbonyl-(11)C]desmethyl-WAY (DWAY) as a promising, more brain-penetrant radioligand for PET imaging of human 5-HT(1A) receptors, and (pyridinyl-6-halo)-analogues as promising leads for the development of radiohalogenated ligands.