A new flood phantom to measure homogeneity of gamma cameras and to correct SPECT studies

A new flood phantom is presented. It combines patient like emission spectra, homogeneous activity distribution, and mechanical stability. Its properties and easy handling makes it the flood phantom of choice for measuring the homogeneity of a gamma camera or for correcting SPECT studies.     

Interest of the ordered subsets expectation maximization (OS-EM) algorithm in pinhole single-photon emission tomography reconstruction: a phantom study

Pinhole single-photon emission tomography (SPET) has been proposed to improve the trade-off between sensitivity and resolution for small organs located in close proximity to the pinhole aperture. This technique is hampered by artefacts in the non-central slices. These artefacts are caused by truncation and by the fact that the pinhole SPET data collected in a circular orbit do not contain sufficient information for exact reconstruction. The ordered subsets expectation maximization (OS-EM) algorithm is a potential solution to these problems. In this study a three-dimensional OS-EM algorithm was implemented for data acquired on a single-head gamma camera equipped with a pinhole collimator (PH OS-EM). The aim of this study was to compare the PH OS-EM algorithm with the filtered back-projection algorithm of Feldkamp, Davis and Kress (FDK) and with the conventional parallel-hole geometry as a whole, using a line source phantom, Picker's thyroid phantom and a phantom mimicking the human cervical column. Correction for the angular dependency of the sensitivity in the pinhole geometry was based on a uniform flood acquisition. The projection data were shifted according to the measured centre of rotation. No correction was made for attenuation, scatter or distance-dependent camera resolution. The resolution measured with the line source phantom showed a significant improvement with PH OS-EM as compared with FDK, especially in the axial direction. Using Picker's thyroid phantom, one iteration with eight subsets was sufficient to obtain images with similar noise levels in uniform regions of interest to those obtained with the FDK algorithm. With these parameters the reconstruction time was 2.5 times longer than for the FDK method. Furthermore, there was a reduction in the artefacts caused by the circular orbit SPET acquisition. The images obtained from the phantom mimicking the human cervical column indicated that the improvement in image quality with PH OS-EM is relevant for future clinical use and that the improvements obtained using the OS-EM algorithm are more significant for the pinhole geometry than for the conventional parallel-hole geometry. We conclude that PH OS-EM is a practical and promising alternative for pinhole SPET reconstruction.     

SPECT性能的定量质量控制的新进展

目的：单光子发射计算讥断层仪（SPECT）的完整性性能测试项目，过去发表和使用的步骤很复杂．化费时间很长，并需要特殊的测试条件．美国核医学委员会最近推荐了简单、实用、需少量测试仪器的方法，几小时内就能用常用的核医学仪器完成，它能够测出均匀度和灵敏度．旋转的稳定性，断层的空间分辨率均匀度和对比度，并可进行准确的衰减校正，此步骤在3小时内即可全部完成、仅需要^57Co泛源、线源和断层圆柱形模型．此方法对仪器的购买验收及仪器的常规检测有实用价值。     

Interest of the ordered subsets expectation maximization (OS-EM) algorithm in pinhole single-photon emission tomography reconstruction: a phantom study

Pinhole single-photon emission tomography (SPET) has been proposed to improve the trade-off between sensitivity and resolution for small organs located in close proximity to the pinhole aperture. This technique is hampered by artefacts in the non-central slices. These artefacts are caused by truncation and by the fact that the pinhole SPET data collected in a circular orbit do not contain sufficient information for exact reconstruction. The ordered subsets expectation maximization (OS-EM) algorithm is a potential solution to these problems. In this study a three-dimensional OS-EM algorithm was implemented for data acquired on a single-head gamma camera equipped with a pinhole collimator (PH OS-EM). The aim of this study was to compare the PH OS-EM algorithm with the filtered back-projection algorithm of Feldkamp, Davis and Kress (FDK)and with the conventional parallel-hole geometry as a whole,using a line source phantom, Picker’s thyroid phantom and a phantom mimicking the human cervical column. Correction for the angular dependency of the sensitivity in the pinhole geometrywas based on a uniform flood acquisition. The projection data were shifted according to the measured centre of rotation. No correction was made for attenuation, scatter or distance-dependent camera resolution. The resolution measured with the line source phantom showed a significant improvement with PH OS-EM as compared with FDK, especially in the axial direction. Using Picker’s thyroid phantom, one iteration with eight subsets was sufficient to obtain images with similar noise levels in uniform regions of interest to those obtained with the FDK algorithm. With these parameters the reconstruction time was 2.5 times longer than for the FDK method. Furthermore, there was a reduction in the artefacts caused by the circular orbit SPET acquisition. The images obtained from the phantom mimicking the human cervical column indicated that the improvement in image quality with PH OS-EM is relevant for future clinical useand that the improvements obtained using the OS-EM algorithm are more significant for the pinhole geometry than for the conventional parallel-hole geometry. We conclude that PH OS-EM is a practical and promising alternative for pinhole SPET reconstruction. Received 2 June and in revised form 1 September 1999     

Quantitative SPECT by attenuation correction of the projection set using transmission data: evaluation of a method

A method for measuring attenuation coefficients in single-photon emission tomography (SPECT) is described and evaluated, together with a method for attenuation correction using these measured attenuation coefficients. Build-up, caused by scattered photons, is corrected for by a simple substitution in the algorithms. Transmission studies are performed with a 99mTc- or 57Co flood source, and emission phantom studies with 99mTc line sources. The method is evaluated with variable but well-defined phantoms. The result is accurate attenuation coefficients for different densities, dimensions and geometries, and an accuracy of corrected emission activities of better than +/- 10% in most cases. The present limitations of the method for attenuation correction are discussed.     

External quality control of gamma cameras--results of an inter-laboratory comparison study in Austria

Some 28 nuclear medicine departments out of a total of 40 participated in a nationwide Austrian interlaboratory comparison of the imaging quality of gamma cameras. The participation was voluntary, and confidentiality of the individual results was guaranteed. The survey was completed within 2 months, employing instructors to administrate the tests. An emission phantom simulating a flood field with non-uniformities was imaged by 43 cameras, and 54 images were evaluated. The test images were read by the participants using a graded rating scale to indicate the probability of the presence of a non-uniformity in the various parts of the image. The rating data were used to construct individual ROC curves for each image. The area under the ROC curve was used as the ranking parameter for image quality. The results show a spread of the ROC areas between 0.6 and 0.99, with a median of 0.81. A correlation was found between the year of installation and the ROC area obtained from the gamma camera, indicating improvements of performance in more recent cameras, which accounts for part of the variation of the ROC areas. The remaining variations are due to differences in the performance of the gamma cameras. Feedback was provided to the participants by describing individual performance with respect to the true structure of the phantom and by comparing this performance with that of the group. A questionnaire accompanying the test phantom yielded information about the practice of routine quality control and about details of the acquisition and analysis of images.(ABSTRACT TRUNCATED AT 250 WORDS)     

Application of phase contrast imaging to mammography

Phase contrast images were obtained experimentally by using a customized mammography unit with a nominal focal spot size of 100 microm and variable source-to-image distances of up to 1.5 m. The purpose of this study was to examine the applicability and potential usefulness of phase contrast imaging for mammography. A mammography phantom (ACR156 RMI phantom) was imaged, and its visibility was examined. The optical density of the phantom images was adjusted to approximately 1.3 for both the contact and phase contrast images. Forty-one observers (18 medical doctors and 23 radiological technologists) participated in visual evaluation of the images. Results showed that, in comparison with the images of contact mammography, the phantom images of phase contrast imaging demonstrated statistically significantly superior visibility for fibers, clustered micro-calcifications, and masses. Therefore, phase contrast imaging obtained by using the customized mammography unit would be useful for improving diagnostic accuracy in mammography.     

Low-cost 3D print–based phantom fabrication to facilitate interstitial prostate brachytherapy training program

PurposeThe purpose of this study was to manufacture a realistic and inexpensive prostate phantom to support training programs for ultrasound-based interstitial prostate brachytherapy.Methods and MaterialsFive phantom material combinations were tested and evaluated for material characteristics; Ecoflex 00–30 silicone, emulsion silicone with 20% or 50% mineral oil, and regular or supersoft polyvinyl chloride (PVC). A prostate phantom which includes an anatomic simulated prostate, urethra, seminal vesicles, rectum, and normal surrounding tissue was created with 3D-printed molds using 20% emulsion silicone and regular and supersoft PVC materials based on speed of sound testing. Needle artifact retention was evaluated at weekly intervals.ResultsSpeed of sound testing demonstrated PVC to have the closest ultrasound characteristics of the materials tested to that of soft tissue. Several molds were created with 3D-printed PLA directly or cast on 3D-printed PLA with high heat resistant silicone. The prostate phantom fabrication workflow was developed, including a method to produce dummy seeds for low-dose-rate brachytherapy practice. A complete phantom may be fabricated in 1.5–2 h, and the material cost for each phantom was approximated at $23.98.ConclusionsA low-cost and reusable phantom was developed based on 3D-printed molds for casting. The proposed educational prostate phantom is an ideal cost-effective platform to develop and build confidence in fundamental brachytherapy procedural skills in addition to actual patient caseloads.     

Optimal (57)Co flood source activity and acquisition time for lymphoscintigraphy localization images

We evaluated different (57)Co flood source activities and acquisition times to obtain an optimal localization image for breast lymphoscintigraphy that would adequately outline the body and allow detection of nodes seen on the emission scan while minimizing unnecessary radiation exposure to the patient. METHODS: An anthropomorphic thorax breast phantom representing an average-size patient was used to simulate nodes on a breast lymphoscintigraphy scan. The activities in the nodes at the time of acquisition ranged from 37 to 185 kBq (1-5 microCi). Four experiments were performed, consisting of 10-min emission and 3-min localization images. Anterior, posterior, and right and left lateral views of the thorax phantom were acquired, using each of 5 different (57)Co flood sources with activities ranging from 37 to 269 MBq (1.0-7.26 mCi). Ten 1-min localization images for each source were acquired and compared for quality. Three-minute localization images for 2 phantom thicknesses of 10 and 20 cm were acquired to determine the contrast-to-noise ratio for each (57)Co source. The total exposure was measured using an ion chamber survey meter. RESULTS: All sources allowed visualization of the lymphatic nodes in acquisitions as short as 3 min. Images using the 126-MBq (3.41-mCi) source demonstrated an adequate body outline along with visualization of all nodes seen on the emission image. The 37-MBq (1.0-mCi) source did not provide sufficient definition of the body outline, whereas the hotter sources decreased node visualization by increasing the background around the nodes at the same time that they increased the patient exposure. Node activity of 37 kBq (1 microCi) became undetectable on the anterior localization images yet was still visible on the lateral image because of greater attenuation of (57)Co photons. The estimated dose rate from the (57)Co sheet sources was 0.641 microSv/MBq/h. CONCLUSION: Acquiring a 3-min localization scan using a 126-MBq (3.41-mCi) source provided the best combination of clear-body outline and visualization of all nodes seen on the emission image. The estimated dose to the patient from the 126-MBq (3.41-mCi) sheet source was very low (8.7 microSv for unilateral and 13.1 microSv for bilateral). Node detectability decreased in localization images acquired using (57)Co sources of higher activity. This effect would be more pronounced in lymphoscintigrams of thin patients compared with those of patients of average thickness.     

External quality control of gamma cameras —results of an inter-laboratory comparison study in Austria

Some 28 nuclear medicine departments out of a total of 40 participated in a nationwide Austrian interlaboratory comparison of the imaging quality of gamma cameras. The participation was voluntary, and confidentiality of the individual results was guaranteed. The survey was completed within 2 months, employing instructors to administrate the tests. An emission phantom simulating a flood field with non-uniformities was imaged by 43 cameras, and 54 images were evaluated. The test images were read by the participants using a graded rating scale to indicate the probability of the presence of a non-uniformity in the various parts of the image. The rating data were used to construct individual ROC curves for each image. The area under the ROC curve was used as the ranking parameter for image quality. The results show a spread of the ROC areas between 0.6 and 0.99, with a median of 0.81. A correlation was found between the year of installation and the ROC area obtained from the gamma camera, indicating improvements of performance in more recent cameras, which accounts for part of the variation of the ROC areas. The remaining variations are due to differences in the performance of the gamma cameras. Feedback was provided to the participants by describing individual performance with respect to the true structure of the phantom and by comparing this performance with that of the group. A questionnaire accompanying the test phantom yielded information about the practice of routine quality control and about details of the acquisition and analysis of images. The survey demonstrated the feasibility and usefulness of interlaboratory comparisons of this type even for a small number of participants. The feedback was found by the participants to be helpful and stimulated awareness of the importance of quality control.     

Ability of calibration phantom to reduce the interscan variability in electron beam computed tomography

OBJECTIVE: To test the hypothesis that a calibration phantom would improve interpatient and interscan variability in coronary artery calcium (CAC) studies. METHODS: We scanned 144 patients twice with or without the calibration phantom and then scanned 93 patients with a single calcific lesion twice and, finally, scanned a cork heart with calcific foci. RESULTS: There were no linear correlations in computed tomography Hounsfield unit (CT HU) and CT HU interscan variation between blood pool and phantom plugs at any slice level in patient groups (p > 0.05). The CT HU interscan variation in phantom plugs (2.11 HU) was less than that of the blood pool (3.47 HU; p < 0.05) and CAC lesion (20.39; p < 0.001). Comparing images with and without a calibration phantom, there was a significant decrease in CT HU as well as an increase in noise and peak values in patient studies and the cork phantom study. CONCLUSION: The CT HU attenuation variations of the interpatient and interscan blood pool, calibration phantom plug, and cork coronary arteries were not parallel. Therefore, the ability to adjust the CT HU variation of calcific lesions by a calibration phantom is problematic and may worsen the problem.     

The application of Markov theory to contrast-detail analysis

RATIONALE AND OBJECTIVES: To improve the accuracy of contrast-detail (C-D) analysis, we have devised a new evaluation method of the detection performance in the C-D analysis by using the Markov process model. This article describes this new evaluation method and the results of applying it to the illustrative examples. MATERIALS AND METHODS: A commercially available C-D phantom was employed as a test object, and 20 phantom radiographs with an average background density of 0.8 were prepared. Thirty-five observers interpreted all phantom radiographs independently of each other with the room lights off. We assume the Markov model in which the selection of the smallest visible contrast for each row of disks with the same diameter depends only on the contrast selected as the smallest visible one for the same row in the just-before reading experiment. By using this Markov model, the contrast detectability for each disk diameter was calculated, and a C-D diagram was constructed based on these results. The conventional C-D diagram was also derived from the averages of contrast detectability obtained from the interpretations of all phantom radiographs. RESULTS: All C-D diagrams obtained from our devised method agreed well with each other and were in good accord with the conventional C-D diagram from the interpretations of all phantom radiographs. CONCLUSIONS: We have devised a new evaluation method of C-D analysis by using the Markov model and have shown this method will be consistent with the conventional evaluation method in the C-D analysis.     

The contribution of the internal scatter to radiation dose during CT scan of the head

Summary An acrylic head phantom was irradiated during a computed tomographic scan with four commercial scanners. Measurements of the spatial distribution of the radiation dose on the surface and internal to the phantom were performed for the scan plane and the scattered beam at various distances from the scan plane. The surface scatter dose was found to be considerably smaller than that for internal scatter. A significant increase in radiation exposure within the head phantom due to internal radiation seatter, and an asymmetrical primary beam profile for dual slice scanners were also noted.     

Measurement of background signals due to scattered and off-focal radiation on CT scanners

The magnitude of scattered and off-focal radiation in relation to the primary radiation has been measured for 4 commercial CT scanners of third generation (only rotation). The measurement has been done by placing a lead beam stop in front of a scattering phantom as well as without a phantom and registering the detector signal behind and outside the lead beam stop. The off-focal radiation has been found to be very similar in magnitude for 3 of the scanners compared, while the scattered radiation from the phantom was found to vary with parameters like distance between phantom and detector, grid in front of the detector and self collimation of the detector.     

Estimating the absorbed dose to critical organs during dual X-ray absorptiometry.

OBJECTIVE: The purpose of this study is to estimate a patient's organ dose (effective dose) during performance of dual X-ray absorptiometry by using the correlations derived from the surface dose and the depth doses in an anthropomorphic phantom. MATERIALS AND METHODS: An anthropomorphic phantom was designed and TLDs (Thermoluminescent Dosimeters) were placed at the surface and these were also inserted at different depths of the thyroid and uterus of the anthropomorphic phantom. The absorbed doses were measured on the phantom for the spine and femur scan modes. The correlation coefficients and regression functions between the absorbed surface dose and the depth dose were determined. The derived correlation was then applied for 40 women patients to estimate the depth doses to the thyroid and uterus. RESULTS: There was a correlation between the surface dose and depth dose of the thyroid and uterus in both scan modes. For the women's dosimetry, the average surface doses of the thyroid and uterus were 1.88 microGy and 1.81 microGy, respectively. Also, the scan center dose in the women was 5.70 microGy. There was correlation between the thyroid and uterus surface doses, and the scan center dose. CONCLUSION: We concluded that the effective dose to the patient's critical organs during dual X-ray absorptiometry can be estimated by the correlation derived from phantom dosimetry.     

Evaluation of variations in absorbed dose and image noise according to patient forms in X-ray computed tomography

Excessive radiation exposure in pediatric computed tomography (CT) scanning has become a serious problem, and it is difficult to select scan parameters for the scanning of small patients such as children. We investigated differences in absorbed dose and standard deviation (SD) in Hounsfield unit (HU) caused by differences in the form of the subject using a body-type phantom with removable body parts. Using four X-ray CT scanners, measurements were made with values from 50 mAs to 300 mAs, with slices of 50 mAs, using scan protocols that were assumed to perform thorough examinations. The results showed that the mAs values and absorbed doses were almost proportional, and the absorbed doses in the phantom without body parts were about 1.1-2.2-fold higher than those of the phantom with body parts at the same points. The SD values obtained indicated that the absorbed doses in the phantom with body parts were 0.3-0.6 times those of the phantom without body parts when the mAs values used were adjusted so that both SD values were the same. The absorbed doses in various patient forms can be estimated from these results, and they will become critical data for the selection of appropriate scan protocols.     

Design and construction of a brain phantom to simulate neonatal MR images

This paper presents the design and construction of a 3D digital neonatal neurocranial phantom and its application for the simulation of brain magnetic resonance (MR) images. Commonly used digital brain phantoms (e.g. BrainWeb) are based on the adult brain. With the growing interest in computer-aided methods for neonatal MR image processing, there is a growing demand a digital phantom and brain MR image simulator especially for the neonatal brains. This is due to the pronounced differences between adult and neonatal brains not only in terms of size but also, more importantly, in terms of geometrical proportions and the need to subdivide white matter into two different tissue types in neonates. Therefore the neonatal brain phantom created in the here presented work consists of 9 different tissue types: skin, fat, muscle, skull, dura mater, gray matter, myelinated white matter, nonmyelinated white matter and cerebrospinal fluid. Each voxel has a vector consisting of 9 components, one for each of these nine tissue types. This digital phantom can be used to map simulated magnetic resonance signal intensities resulting in simulated MR images of the newborns head. These images with controlled degradation of the image data present a representative, reproducible data set ideal for development and evaluation of neonatal MRI analysis methods, e.g. segmentation and registration algorithms.     

Development and validation of a bespoke phantom to test accuracy of Cobb angle measurements

IntroductionAdolescent idiopathic scoliosis (AIS) is a spinal deformity that causes the spine to bend laterally. Patients with AIS undergo frequent X-ray examinations to monitor the progression of the disorder by through the measurement of the Cobb angle. Frequent exposure of adolescents poses the risk of radiation-induced cancer. The aim of this research was to design and build a bespoke phantom representing a 10-year-old child with AIS to allow optimisation of imaging protocols for AIS assessment through the accuracy of Cobb angle measurements.MethodsPoly-methyl methacrylate (PMMA) and plaster of Paris (PoP) were used to represent human soft tissue and bone tissue, respectively, to construct a phantom exhibiting a 15° lateral curve of the spine. The phantom was validated by comparing the Hounsfield unit (HU) of its vertebrae with that of human and sheep. Additionally, comparisons of signal-to-noise ratio (SNR) to those from a commercially available phantom. An assessment of the accuracy of the radiographic assessment of the Cobb angle measurement was performed.ResultsThe HU of the PoP vertebrae was 628 (SD = 56), human vertebrae was 598 (SD = 79) and sheep vertebra was 605 (SD = 83). The SNR values of the two phantoms correlated strongly (r = 0.93 (p = 0.00)). The measured scoliosis angle was 14°.ConclusionThe phantom has physical characteristics (in terms of spinal deformity) and radiological characteristics (in terms of HU and SNR values) of the spine of a 10-year-old child with AIS. This phantom has utility for the optimisation of x-ray imaging techniques in 10 year old children.Implications for practiceA phantom to investigate new x-ray imaging techniques and technology in the assessment of scoliosis and to optimise currently used protocols.     

Design concepts and preliminary performances of stationary-sampling whole-body high-resolution positron emission tomography: HEADTOME IV

Design concepts and preliminary performances of a stationary sampling high-resolution whole-body positron emission tomograph, HEADTOME IV, were reported. The system comprises four layers of detector ring which consists of 768 BGO crystals with 3 mm width, 96 photomultiplier tubes arrayed on 825 mm circle. A sufficiently fine sampling-interval allows data sampling without scan motion along the transaxial plane. But an axial motion (Z-motion) is installed to interlace between adjacent planes. Preliminary performance characteristics were an in-plane resolution of 4.5 mm in full width at half maximum (FWHM), an axial resolution of 9.5 mm in FWHM, sensitivities of 14 and 24 kcps/(microCi/ml) for direct and cross planes, respectively, with 20 cm diam. cylindrical flood phantom. A large scale cache memory prepared for each plane allows a realtime correction for the deadtime and the radionuclide decay, and a realtime calculation of the rate constants using the weighted integral method.     

A method to optimize the processing algorithm of a computed radiography system for chest radiography

A test methodology using an anthropomorphic-equivalent chest phantom is described for the optimization of the Agfa computed radiography "MUSICA" processing algorithm for chest radiography. The contrast-to-noise ratio (CNR) in the lung, heart and diaphragm regions of the phantom, and the "system modulation transfer function" (sMTF) in the lung region, were measured using test tools embedded in the phantom. Using these parameters the MUSICA processing algorithm was optimized with respect to low-contrast detectability and spatial resolution. Two optimum "MUSICA parameter sets" were derived respectively for maximizing the CNR and sMTF in each region of the phantom. Further work is required to find the relative importance of low-contrast detectability and spatial resolution in chest images, from which the definitive optimum MUSICA parameter set can then be derived. Prior to this further work, a compromised optimum MUSICA parameter set was applied to a range of clinical images. A group of experienced image evaluators scored these images alongside images produced from the same radiographs using the MUSICA parameter set in clinical use at the time. The compromised optimum MUSICA parameter set was shown to produce measurably better images.