Clinical evaluation of 2D versus 3D whole-body PET image quality using a dedicated BGO PET scanner

Comparison between new-generation SiPM-based and conventional PMT-based TOF-PET/CT

PURPOSE

This study aimed to determine whether the SiPM-PET/CT, Discovery MI (DMI) performs better than the PMT-PET/CT system, Discovery 710 (D710).

METHODS

The physical performance of both systems was evaluated using NEMA NU 2 standards. Contrast (%), uniformity and image noise (%) are criteria proposed by the Japanese Society of Nuclear Medicine (JSNM) for phantom tests and were determined in images acquired from Hoffman and uniform phantoms using the DMI and D710. Brain and whole-body [¹⁸F]FDG images were also acquired from a healthy male using the DMI and D710.

RESULTS

The spatial resolution at 1.0cm off-center in the DMI and D710 was 3.91 and 4.52mm, respectively. The sensitivity of the DMI and D710 was 12.62 and 7.50cps/kBq, respectively. The observed peak noise-equivalent count rates were 185.6kcps at 22.5kBq/mL and 137.0kcps at 29.0kBq/mL, and the scatter fractions were 42.1% and 37.9% in the DMI and D710, respectively. The D710 had better contrast recovery and lower background variability. Contrast, uniformity and image noise in the DMI were 61.0%, 0.0225, and 7.85%, respectively. These outcomes were better than those derived from the D710 and satisfied the JSNM criteria. Brain images acquired by the DMI had better grey-to-white matter contrast and lower image noise at the edge of axial field of view.

CONCLUSIONS

The DMI offers better sensitivity, performance under conditions of high count rates and image quality than the conventional PMT-PET/CT system, D710.

Effects of a novel tungsten-impregnated rubber neck shield on the quality of cerebral images acquired using 15O-labeled gas

The present study aimed to validate the effects of a novel tungsten-impregnated rubber neck shield on the quality of phantom and clinical ¹⁵O-labeled gas positron emission tomography (PET) images. Images were acquired in the presence or absence of a neck shield from a cylindrical phantom containing [¹⁵O]H₂O (phantom study) and from three individuals using [¹⁵O]CO₂, [¹⁵O]O₂ and [¹⁵O]CO gas (clinical study). Data were acquired in three-dimensional (3D) mode using a Discovery PET/CT 710. Values for cerebral blood flow, cerebral blood volume, oxygen extraction fraction, and cerebral metabolic rate of oxygen with and without the neck shield were calculated from ¹⁵O-labeled gas images. Arterial radioactivity and count characteristics were evaluated in the phantom and clinical studies. The coefficient of variance (CV) for the phantom study and the standard deviation (SD) for functional images were also analyzed. The neck shield decreased the random count rates by 25-59% in the phantom and clinical studies. The noise equivalent count rate (NECR) increased by 44-66% in the phantom and clinical studies. Random count rates and NECR in [¹⁵O]CO₂ images significantly differed with and without the neck shield. The improvement in visual and physical image quality with the neck shield was not observed in the phantom and clinical studies. The novel neck shield reduced random count rate and improved NECR in a 3D PET study using ¹⁵O-labeled gas. The image quality with the neck shield was similar to that without the neck shield.

Classification and evaluation strategies of auto-segmentation approaches for PET: Report of AAPM task group No. 211

PURPOSE

The purpose of this educational report is to provide an overview of the present state-of-the-art PET auto-segmentation (PET-AS) algorithms and their respective validation, with an emphasis on providing the user with help in understanding the challenges and pitfalls associated with selecting and implementing a PET-AS algorithm for a particular application.

APPROACH

A brief description of the different types of PET-AS algorithms is provided using a classification based on method complexity and type. The advantages and the limitations of the current PET-AS algorithms are highlighted based on current publications and existing comparison studies. A review of the available image datasets and contour evaluation metrics in terms of their applicability for establishing a standardized evaluation of PET-AS algorithms is provided. The performance requirements for the algorithms and their dependence on the application, the radiotracer used and the evaluation criteria are described and discussed. Finally, a procedure for algorithm acceptance and implementation, as well as the complementary role of manual and auto-segmentation are addressed.

FINDINGS

A large number of PET-AS algorithms have been developed within the last 20 years. Many of the proposed algorithms are based on either fixed or adaptively selected thresholds. More recently, numerous papers have proposed the use of more advanced image analysis paradigms to perform semi-automated delineation of the PET images. However, the level of algorithm validation is variable and for most published algorithms is either insufficient or inconsistent which prevents recommending a single algorithm. This is compounded by the fact that realistic image configurations with low signal-to-noise ratios (SNR) and heterogeneous tracer distributions have rarely been used. Large variations in the evaluation methods used in the literature point to the need for a standardized evaluation protocol.

CONCLUSIONS

Available comparison studies suggest that PET-AS algorithms relying on advanced image analysis paradigms provide generally more accurate segmentation than approaches based on PET activity thresholds, particularly for realistic configurations. However, this may not be the case for simple shape lesions in situations with a narrower range of parameters, where simpler methods may also perform well. Recent algorithms which employ some type of consensus or automatic selection between several PET-AS methods have potential to overcome the limitations of the individual methods when appropriately trained. In either case, accuracy evaluation is required for each different PET scanner and scanning and image reconstruction protocol. For the simpler, less robust approaches, adaptation to scanning conditions, tumor type, and tumor location by optimization of parameters is necessary. The results from the method evaluation stage can be used to estimate the contouring uncertainty. All PET-AS contours should be critically verified by a physician. A standard test, i.e., a benchmark dedicated to evaluating both existing and future PET-AS algorithms needs to be designed, to aid clinicians in evaluating and selecting PET-AS algorithms and to establish performance limits for their acceptance for clinical use. The initial steps toward designing and building such a standard are undertaken by the task group members.

Influence of slice overlap on positron emission tomography image quality

PET scans use overlapping acquisition beds to correct for reduced sensitivity at bed edges. The optimum overlap size for the General Electric (GE) Discovery 690 has not been established. This study assesses how image quality is affected by slice overlap. Efficacy of 23% overlaps (recommended by GE) and 49% overlaps (maximum possible overlap) were specifically assessed. European Association of Nuclear Medicine (EANM) guidelines for calculating minimum injected activities based on overlap size were also reviewed. A uniform flood phantom was used to assess noise (coefficient of variation, (COV)) and voxel accuracy (activity concentrations, Bq ml(-1)). A NEMA (National Electrical Manufacturers Association) body phantom with hot/cold spheres in a background activity was used to assess contrast recovery coefficients (CRCs) and signal to noise ratios (SNR). Different overlap sizes and sphere-to-background ratios were assessed. COVs for 49% and 23% overlaps were 9% and 13% respectively. This increased noise was difficult to visualise on the 23% overlap images. Mean voxel activity concentrations were not affected by overlap size. No clinically significant differences in CRCs were observed. However, visibility and SNR of small, low contrast spheres (⩽13 mm diameter, 2:1 sphere to background ratio) may be affected by overlap size in low count studies if they are located in the overlap area. There was minimal detectable influence on image quality in terms of noise, mean activity concentrations or mean CRCs when comparing 23% overlap with 49% overlap. Detectability of small, low contrast lesions may be affected in low count studies-however, this is a worst-case scenario. The marginal benefits of increasing overlap from 23% to 49% are likely to be offset by increased patient scan times. A 23% overlap is therefore appropriate for clinical use. An amendment to EANM guidelines for calculating injected activities is also proposed which better reflects the effect overlap size has on image noise.

Evaluation of image reconstruction algorithms encompassing Time-Of-Flight and Point Spread Function modelling for quantitative cardiac PET: phantom studies

BACKGROUND

To perform kinetic modelling quantification, PET dynamic data must be acquired in short frames, where different critical conditions are met. The accuracy of reconstructed images influences quantification. The added value of Time-Of-Flight (TOF) and Point Spread Function (PSF) in cardiac image reconstruction was assessed.

METHODS

A static phantom was used to simulate two extreme conditions: (i) the bolus passage and (ii) the steady uptake. Various count statistics and independent noise realisations were considered. A moving phantom filled with two different radionuclides was used to simulate: (i) a great range of contrasts and (ii) the cardio/respiratory motion. Analytical and iterative reconstruction (IR) algorithms also encompassing TOF and PSF modelling were evaluated.

RESULTS

Both analytic and IR algorithms provided good results in all the evaluated conditions. The amount of bias introduced by IR was found to be limited. TOF allowed faster convergence and lower noise levels. PSF achieved near full myocardial activity recovery in static conditions. Motion degraded performances, but the addition of both TOF and PSF maintained the best overall behaviour.

CONCLUSIONS

IR accounting for TOF and PSF can be recommended for the quantification of dynamic cardiac PET studies as they improve the results compared to analytic and standard IR.

Benefits of point-spread function and time of flight for PET/CT image quality in relation to the body mass index and injected dose

The PET image quality of overweight patients and patients who receive low injected doses deteriorates because of increases in statistical noise. The purpose of this study was to investigate the benefits of the point-spread function (PSF) and time-of-flight (TOF) for PET/CT image quality in such patients.

METHODS

The PET images were reconstructed using the baseline ordered-subsets expectation-maximization algorithm (OSEM), OSEM + PSF, OSEM + TOF, and OSEM + PSF + TOF. In the phantom study, we used a National Electrical Manufacturers Association body phantom with different radioactivity concentrations and analyzed image quality using the coefficient of variance in the background (CVphantom). In the clinical study, we retrospectively studied 39 patients who underwent clinical F-FDG PET/CT. The patients were classified into groups based on body mass index and injected dose. Image quality was evaluated using the CV in the liver (CVliver).

RESULTS

In the phantom study, PSF and TOF improved the CVphantom, especially in low-activity models. Among all of the reconstructions, the best CVphantom was obtained with OSEM + PSF + TOF. In the clinical study, the CVliver of the low-dose group with OSEM + PSF + TOF was comparable to that of the high-dose group with conventional OSEM.

CONCLUSIONS

Point-spread function and TOF improved PET/CT image quality for overweight patients who received a lower injected dose. Therefore, the use of PSF and TOF is suggested to maintain the image quality of such patients without extending scanning times. It is greatly beneficial to obtain sufficient image quality for larger patients, especially in delivery institutions where the injection dose cannot be easily increased.

Small lesions detectability with the Biograph 16 Hi-Rez PET/CT scanner and fast imaging protocols: performance evaluation using an anthropomorphic thoracic phantom and ROC analyses

OBJECTIVES

The purpose of this study was to evaluate the impact on lesion detectability of fast imaging protocols using 18F-FDG and a 3-dimensional LSO-based PET/CT scanner.

METHODS

An anthropomorphic thoracic phantom was used simulating the anatomical structures of radioactivity distribution for the upper torso of an underweight patient. Irregularly shaped targets of small dimensions, the zeolites, were located inside the phantom in an unpredictable position for the observers. Target-to background ratios and target dimensions were selected in order to sample the range of detectability. Repeated imaging was performed to acquire PET images with varying emission scan duration (ESD) of 1, 2, 3 and 4 min/bed and background activity concentrations of 10, 5 and 3 kBq/mL in the torso cavity. Three observers ranked the targets and a receiver operating characteristic analysis was performed for each acquisition protocol.

RESULTS

Detection performances improved when passing from a short (ESD = 1 min) protocol to longer (ESD C 2 min) protocols. This improvement was established with adequate statistical significance.

CONCLUSIONS

Short image acquisition times of 1 min/bed using 18F-FDG and the specific scanner model considered in the study lead to reduced lesion detectability and should be avoided also in underweight patients.

Predictive factors of [(11)C]choline PET/CT in patients with biochemical failure after radical prostatectomy

PURPOSE

Detection of recurrence in prostate cancer patients with biochemical failure after radical prostatectomy by [(11)C]choline PET/CT depends on the prostate-specific antigen (PSA) level. The role of other clinical and pathological variables has not been explored.

METHODS

A total of 2,124 prostate cancer patients referred to our Institution for [(11)C]choline PET/CT from December 2004 to January 2007 for restaging of disease were retrospectively considered for this study. Inclusion criteria were: previous treatment by radical prostatectomy, and biochemical failure, defined as at least two consecutive PSA measurements of >0.2 ng/ml. These criteria were met for 358 patients. Binary logistic analysis was used to investigate the predictive factors of [(11)C]choline PET/CT. PET/CT findings were validated using criteria based on histological analysis, and follow-up clinical and imaging data. Receiver operating characteristic (ROC) analysis was used to assess the performance of [(11)C]choline PET/CT in relation to PSA levels.

RESULTS

The mean PSA level was 3.77 +/- 6.94 ng/ml (range 0.23-45 ng/ml; median 1.27 ng/ml). PET/CT was positive for recurrence in 161 of 358 patients (45%). On an anatomical region basis, [(11)C]choline pathological uptake was observed in lymph nodes (107/161 patients, 66%), prostatectomy bed (55/161 patients, 34%), and in the skeleton (46/161 patients, 29%). PET/CT findings were validated using histological criteria (46/358, 13%), and follow-up clinical and imaging criteria (312/358, 87%). Sensitivity, specificity, positive predictive value, negative predictive value, and overall accuracy were, respectively, 85%, 93%, 91%, 87%, and 89%. In multivariate analysis, high PSA levels, advanced pathological stage, previous biochemical failure and older age were significantly (P < 0.05) associated with an increased risk of positive PET/CT findings. The percentage of positive scans was 19% in those with a PSA level between 0.2 and 1 ng/ml, 46% in those with a PSA level between 1 and 3 ng/ml, and 82% in those with a PSA level higher than 3 ng/ml. ROC analysis showed that PET/CT-positive and PET/CT-negative patients could be best distinguished using a PSA cut-off value of 1.4 ng/ml.

CONCLUSIONS

In addition to PSA levels, pathological stage, previous biochemical failure and age should be considered by physicians when referring prostate cancer patients with biochemical failure after radical prostatectomy to [(11)C]choline PET/CT.

The effects of technological developments on nuclear medicine technologist productivity: a systematic review

OBJECTIVES

Nuclear medicine has changed rapidly as a result of technological developments. Very little is reported on the effects these developments may have on technologist productivity. This study aims to determine whether advances have created a workplace where more patient studies can be performed with fewer technologists. The level of change in automation or time taken to perform a routine task by the nuclear medicine technologist as a result of technological development over the past decade is reported.

METHODS

A systematic review was conducted using Embase.com, Medline, INSPEC, and Cinahl. Two authors reviewed each article for eligibility. Technological developments in routine areas over the past decade were reviewed. The resultant automation or time effects on data acquisition, data processing, and image processing were summarized.

RESULTS

Sixteen articles were included in the areas of myocardial perfusion, information technology, and positron emission tomography (PET). Gamma camera design has halved the acquisition time for myocardial perfusion studies, automated analysis requires little manual intervention and information technologies and filmless departments are more efficient. Developments in PET have reduced acquisition to almost one-fifth of the time.

CONCLUSIONS

Substantial efficiencies have occurred over the decade thereby increasing productivity, but whether staffing levels are appropriate for safe, high quality practice is unclear. Future staffing adequacy is of concern given the anticipated increasing service needs.

Estudo comparativo da qualidade de imagem dos modos de aquisição da PET: validação de um protocolo para reduzir a dose de radiação

OBJETIVO: O presente experimento visa a validar um protocolo de aquisição em 3D na tomografia por emissão de pósitrons, em substituição ao modo 2D, de forma a reduzir a dose de radiação nos pacientes, sem perda da qualidade de imagens. MATERIAIS E MÉTODOS: Foram realizadas 27 simulações em equipamento Discovery ST, nos modos 2D com quatro minutos de aquisição e 3D com dois e quatro minutos. Utilizou-se um simulador do protocolo da National Electrical Manufacturers Association. No interior deste simulador estão inseridas seis esferas com diferentes diâmetros para a determinação da qualidade de imagem. As aquisições foram comparadas por três médicos nucleares, sem que eles identificassem o modo de aquisição. Cada observador atribuiu o valor igual a 1 quando alguma esfera não foi identificada ou valor 2 para esferas visíveis. RESULTADOS: A análise qualitativa pelo kappa generalizado demonstrou que a frequência de esferas visíveis foi maior no modo 3D com quatro minutos (85%) e a porcentagem de concordância também foi maior (88,9%), com kappa generalizado = 0,725 [0,507;0,942]. CONCLUSÃO: O modo 3D com quatro minutos de aquisição e com menores atividades de FDG-18F pode ser utilizado em pacientes com biótipo equivalente ao simulador, sem perda de qualidade de imagem.

Objective and subjective comparison of standard 2-D and fully 3-D reconstructed data on a PET/CT system

OBJECTIVE

The relative advantage of fully 3-D versus 2-D mode for whole-body imaging is currently the focus of considerable expert debate. The nature of 3-D PET acquisition for FDG PET/CT theoretically allows a shorter scan time and improved efficiency of FDG use than in the standard 2-D acquisition. We therefore objectively and subjectively compared standard 2-D and fully 3-D reconstructed data for FDG PET/CT on a research PET/CT system.

MATERIALS AND METHODS

In a total of 36 patients (mean 58.9 years, range 17.3-78.9 years; 21 male, 15 female) referred for known or suspected malignancy, FDG PET/CT was performed using a research PET/CT system with advanced detector technology with improved sensitivity and spatial resolution. After 45 min uptake, a low-dose CT (40 mAs) from head to thigh was performed followed by 2-D PET (emission 3 min per field) and 3-D PET (emission 1.5 min per field) with both seven slices overlap to cover the identical anatomical region. Acquisition time was therefore 50% less (seven fields; 21 min vs. 10.5 min). PET data was acquired in a randomized fashion, so in 50% of the cases 2-D data was acquired first. CT data was used for attenuation correction. 2-D (OSEM) and 3-D PET images were iteratively reconstructed. Subjective analysis of 2-D and 3-D images was performed by two readers in a blinded, randomized fashion evaluating the following criteria: sharpness of organs (liver, chest wall/lung), overall image quality and detectability and dignity of each identified lesion. Objective analysis of PET data was investigated measuring maximum standard uptake value with lean body mass (SUV(max,LBM)) of identified lesions.

RESULTS

On average, per patient, the SUV(max) was 7.86 (SD 7.79) for 2-D and 6.96 (SD 5.19) for 3-D. On a lesion basis, the average SUV(max) was 7.65 (SD 7.79) for 2-D and 6.75 (SD 5.89) for 3-D. The absolute difference on a paired t-test of SUV 3-D-2-D based on each measured lesion was significant with an average of -0.956 (P=0.002) and an average of -0.884 on a patient base (P<0.05). With 3-D the SUV(max) decreased by an average of 5.2% for each lesion, and an average of 6.0% for each patient. Subjective analysis showed fair inter-observer agreement regarding detectability (kappa=0.24 for 3-D; 0.36 for 3-D) and dignity (kappa=0.44 for 3-D and 0.4 for 2-D) of the lesions. There was no significant diagnostic difference between 3-D and 2-D. Only in one patient, a satellite liver metastasis of a colon cancer was missed in 3-D and detected only in 2-D. On average, the overall image quality for 3-D images was equal (in 24%) or inferior (in 76%) compared to 2-D.

CONCLUSION

A possible major advantage of 3-D data acquisition is the faster patient throughput with a 50% reduction in scan time. The fully 3-D reconstruction technique has overcome the technical drawbacks of current 3-D imaging technique. In our limited number of patients there was no significant diagnostic difference between 2-D and fully 3-D.

Accuracy of 3D acquisition mode for myocardial FDG PET studies using a BGO-based scanner

PURPOSE

The aim of the present study was to evaluate the quantitative and qualitative accuracy of 3D PET acquisitions for myocardial FDG studies.

METHODS

Phantom studies were performed with both a homogeneous and an inhomogeneous phantom. Activity profiles were generated along the phantoms using 2D and several 3D reconstructions, varying the 3D scaling value to adjust the scatter correction algorithm. Furthermore, ten patients underwent a dynamic myocardial FDG PET scan, using an interleaved protocol consisting of frames with alternating 2D and 3D acquisition. For each myocardial study, 13 volumes of interest were defined, representing 13 myocardial segments. First, the optimal scaling value for the scatter correction algorithm was determined using data from the phantom and four patient studies. This scaling value was then applied to all ten patients. 2D and 3D acquisitions were compared for both static (i.e. activity concentrations in the last 2D and 3D frames) and dynamic imaging (calculation of the metabolic rate of glucose).

RESULTS

For both phantom and patient studies, suboptimal results were obtained when the default scaling value for the scatter correction algorithm was used. After adjusting the scaling value, for all ten myocardial FDG studies, a very good correlation (r2=0.99) was obtained between 2D and 3D data. With the present protocol no significant differences were observed in qualitative interpretation.

CONCLUSION

The 3D FDG acquisition mode is accurate and has clear advantages over the 2D mode for myocardial FDG studies. A prerequisite is, however, optimisation of the 3D scatter correction algorithm.

Recent developments and future trends in nuclear medicine instrumentation

Molecular imaging using high-resolution single-photon emission computed tomography (SPECT) and positron emission tomography (PET) has advanced elegantly and has steadily gained importance in the clinical and research arenas. Continuous efforts to integrate recent research findings for the design of different geometries and various detector technologies of SPECT and PET cameras have become the goal of both the academic comcameras have become the goal of both the academic community and nuclear medicine industry. As PET has recently become of more interest for clinical practice, several different design trends seem to have developed. Systems are being designed for "low cost" clinical applications, very high-resolution research applications (including small-animal imaging), and just about everywhere in-between. The development of dual-modality imaging systems has revolutionized the practice of nuclear medicine. The major advantage being that SPECT/PET data are intrinsically aligned to anatomical information from the X-ray computed tomography (CT), without the use of external markers or internal landmarks. On the other hand, combining PET with Magnetic Resonance Imaging (MRI) technology is scientifically more challenging owing to the strong magnetic fields. Nevertheless, significant progress has been made resulting in the design of a prototype small animal PET scanner coupled to three multichannel photomultipliers via optical fibers, so that the PET detector can be operated within a conventional MR system. Thus, many different design paths are being pursued--which ones are likely to be the main stream of future commercial systems? It will be interesting, indeed, to see which technologies become the most popular in the future. This paper briefly summarizes state-of-the art developments in nuclear medicine instrumentation. Future prospects will also be discussed.