符合线路显像与PET显像中SUV的比较研究

目的比较符合线路显像标准摄取值(SUV)与PET显像的SUV。方法用双探头符合显像仪及PET对模型显像，分别采用不同的重建算法重建，测定图像上热灶的SUV。结果对直径小于30mm热灶，相同大小时，PET、得到的SUV高于符合线路显像；无论对PET还是符合线路显像，随热灶大小增加SUV增加；SUV与重建算法有关；选取的感兴趣区(ROI)越大，获得的SUV越小；由PET图像获得的热灶SUV可见，当热灶大于2倍的系统分辨率时，SUCmax接近热灶的真实值(SUVmax)。结论符合线路显像的SUV低于PET显像；病灶大小、重建算法、ROI大小均影响SUV。     

PET图像勾画肿瘤靶区边界的水模研究

目的 通过水模研究,探讨影响18F-脱氧葡萄糖（FDG）PET图像靶区勾画阈值（TH%）的因素,建立计算阈值的公式,为不同生物学特征的肿瘤选取不同阈值.方法 对特制的拥有5个不同大小靶区、7种不同靶区本底放射性比值的水模进行PET/CT图像采集,由图像得到每个靶区的最大标准摄取值（SUVmax）、靶区边界处的SUV（SUVborder）、本底中1 cm×1 cm大小感兴趣区的平均SUV（SUVbg）以及靶区内径（D）等,使用SPSS 13.0的曲线估计和线性回归分析方法,得到计算阈值的公式,并以此对29个经病理检查确诊的肺癌原发灶或转移淋巴结进行肿瘤靶区勾画,求出体积,比较PET和CT图像勾画的体积间的差别.结果 通过分析水模数据,得出阈值与靶区的大小呈负相关,与靶区的SUVmax呈负相关,与SUVbg呈正相关,阈值的计算公式为TH%=33.1%＋46.8%×SUVbg/SUVmax＋13.9%/D,r=0.994.通过对29个病灶进行比较,发现PET和CT勾画的平均大体肿瘤体积（GTV）分别是（7.36±1.62）ml和（8.31±2.05）ml,两者间差异无统计学意义（t=-1.26,P＞0.05）.结论 靶区的大小、SUVmax及SUVbg均会影响靶区勾画阈值;通过公式TH%=33.1%＋46.8%×SUVbg/SUVmax＋13.9%/D可对不同肿瘤计算合适的阈值.按此公式PET和CT勾画的靶区无明显差异.     

FDG imaging of lung nodules: a phantom study comparing SPECT, camera-based PET, and dedicated PET

PURPOSE: To evaluate 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) imaging of simulated lung nodules in a realistic chest phantom by using attenuation-corrected and non-attenuation-corrected 511-keV single photon emission computed tomography (SPECT), camera-based positron emission tomography (PET), and dedicated PET imaging. MATERIALS AND METHODS: Spheres with diameters of 6, 10, 13, and 22 mm were placed in the lungs of an anthropomorphic chest phantom to simulate nodules. The lungs, nodules, chest wall, and mediastinum were filled with fluorine-18 activities based on the average radionuclide concentrations in those structures from analysis of attenuation-corrected dedicated FDG PET scans. The image sets were evaluated visually and quantitatively by using contrast and signal-to-noise ratios. RESULTS: Attenuation correction reduced the artificially high apparent uptake in the lungs, restored the spherical shape to the nodules, and provided an accurate outer body contour with appropriate intensity. Dedicated PET depicted all four nodules, camera-based PET depicted the three largest nodules, and SPECT depicted the two largest nodules. Lesion contrast was better on the attenuation-corrected images than on the non-attenuation-corrected images. The signal-to-noise ratio generally was improved with attenuation correction. CONCLUSION: Attenuation correction results in many changes in the images and improves lesion detection.     

Detectability of small and flat polyps in MDCT colonography using 2D and 3D imaging tools: results from a phantom study

OBJECTIVE: The objective of this phantom study was to determine the performance of MDCT colonography for the detection of small polyps under ideal imaging conditions and to determine the added value of 3D imaging when used as an adjunct to 2D imaging. MATERIALS AND METHODS: Thirty-six polypoid and 39 flat polyps (44 lesions, 2-5 mm; 31 lesions, 6-8 mm) were placed in three explanted segments of a thoroughly cleaned porcine colon (overall length, 4.5 m) that was distended with air and submerged in a water phantom. MDCT data sets with 4 x 1 mm collimation and 6-mm table feed were reconstructed every 0.7 mm with 1.25-mm effective slice width. The data were reviewed by three radiologists using 2D images in all three projections and with 3D volume-rendered images available as an adjunct to the 2D images. RESULTS: Additional 3D as a problem-solving tool significantly increased the overall sensitivity (96% vs 90%), decreased the total number of false-positive calls (n = 9 vs n = 5), and increased the diagnostic confidence level (p < 0.03) compared with 2D images alone. Small polyps less than or equal to 5 mm (89% vs 95%, p = 0.004) and flat polyps (82% vs 94%, p = 0.001) especially benefited from 3D. Sensitivity was generally higher for polypoid than for flat polyps (99% vs 94%, p = 0.041). CONCLUSION: Under phantom conditions, simulating an ideal clinical setup, MDCT colonography is not limited by spatial resolution and detects polyps less than or equal to 5 mm in size with high sensitivity and specificity. Additional 3D image tools improve diagnostic accuracy and reviewer confidence, especially for the detection of flat and small polyps.     

Quantitative perfusion imaging with pulsed arterial spin labeling: a phantom study.

PURPOSE: Pulsed arterial spin labeling (PASL) is a magnetic resonance (MR) method for measuring cerebral blood flow. Although several validation studies for PASL in animals and humans have been reported, no reports have detailed the fundamental study of PASL using a flow phantom. We compared the true and theoretical flow rates in a flow phantom to confirm the analytical validity of quantitative perfusion imaging with Q2TIPS sequence. METHODS: We built a flow phantom consisting of a 40-mm diameter plastic syringe filled with plastic beads and small plastic tubes 4 mm in diameter. Gd-DTPA-doped 8L water solution (0.1 mM) was circulated between the syringe and a tank through a plastic tube by a constant flow pump while the flow rate was adjusted between 0 and 2.61 cm/s. Q2TIPS sequence parameters were TI(1)=50 ms and TI(2)=1400 ms. Five imaging slices of 50 subtraction images were acquired sequentially in a distal-to-proximal direction using a single-shot echo planar imaging (EPI) technique. The theoretical flow rate calculated based upon the previously reported kinetic model for Q2TIPS was compared with the true flow rate. RESULTS: A good linear relationship was observed between the theoretical, F', and true flow rates, F, in a flow rate range of 1.43 to 1.95 cm/s (F'=1.024*F-1.915, R(2)=0.902). The ratio of theoretical to true flow rate was 92 (+/-) 4%. CONCLUSION: Flow rate was quantified with reasonable accuracy when the entire amount of labeled bolus within the phantom could be recovered. Our experiment confirmed the analytical validity of Q2TIPS and suggested that blood flow measurement may be feasible using the Q2TIPS pulse sequence and kinetic model of the PASL equation.     

Ghost imaging for targeting breast masses with MR imaging: a phantom study

RATIONALE AND OBJECTIVES: The purpose of this study was to test the accuracy of ghost magnetic resonance (MR) imaging for guiding core biopsies of simulated breast masses in a tissue phantom. MATERIALS AND METHODS: A tissue MR phantom implanted with 20 grapes as targets was placed into an interventional breast MR coil. The locations of the centers of the targets were determined, recorded, and saved as ghost images. A nonmagnetic phantom needle was constructed to avoid imprecision secondary to magnetic field inhomogeneity and was used to determine the three-dimensional location of the needle tip in the center of each grape on the ghost image. Once the positions were determined, the true needle was placed and biopsy specimens were taken. The needle was inspected for the presence of pulp after each pass. Each grape was inspected to determine the location of the needle track in relation to the center of the grape. The duration of the procedure was recorded. RESULTS: All grapes were hit by the biopsy needle, as demonstrated either by pulp within the needle or by a needle track within the grape. Seventeen of the 20 grapes (85%) were hit centrally. Three were sampled eccentrically, up to 5-6 mm from the center. Each biopsy took approximately 1 hour. CONCLUSION: These results suggest that ghost imaging may be ideal for needle guidance in core biopsy or preoperative localization, as it extends the period of visibility after a bolus injection of contrast material. Additionally, using a phantom needle for localization appears to overcome imprecision due to magnetic field inhomogeneity of the needle.     

Metallic artifacts caused by dental metal prostheses on PET images: a PET/CT phantom study using different PET/CT scanners

Objective  The objective of this study was to investigate the effects of computed tomography (CT) artifacts caused by dental metal prostheses on positron emission tomography (PET) images. Methods  A dental arch cast was fixed in a cylindrical water-bath phantom. A spherical phantom positioned in the vicinity of the dental arch cast was used to simulate a tumor. To simulate the tumor imaging, the ratio of the ¹⁸F-fluoro-deoxy-glucose radioactivity concentration of the spherical phantom to that of the water-bath phantom was set at 2.5. A dental bridge composed of a gold–silver–palladium alloy on the right mandibular side was prepared. A spherical phantom was set in the white artifact area on the CT images (site A), in a slightly remote area from the white artifact (site B), and in a black artifact area (site C). A PET/CT scan was performed with and without the metal bridge at each simulated tumor site, and the artifactual influence was evaluated on the axial attenuation-corrected (AC) PET images, in which the simulated tumor produced the strongest accumulation. Measurements were performed using three types of PET/CT scanners (scanners 1 and 2 with CT-based attenuation correction, and 3 with Cesium-137 (¹³⁷Cs)-based attenuation correction). The influence of the metal bridge was evaluated using the change rate of the SUVmean with and without the metal bridge. Results  At site A, an overestimation was shown (scanner 1: +5.0% and scanner 2: +2.5%), while scanner 3 showed an underestimation of −31.8%. At site B, an overestimation was shown (scanner 1: +2.1% and scanner 2: +2.0%), while scanner 3 showed an underestimation of −2.6%. However, at site C, an underestimation was shown (scanner 1: −25.0%, scanner 2: −32.4%, and scanner 3: −8.4%). Conclusions  When CT is used for attenuation correction in patients with dental metal prostheses, an underestimation of radioactivity of accumulated tracer is anticipated in the dark streak artifact area on the CT images. In this study, the dark streak artifacts of the CT caused by metallic dental prostheses may cause false negative finding of PET/CT in detecting small and/or low uptake tumor in the oral cavity.     

Maternal-fetal in vivo imaging: a combined PET and MRI study.

An understanding of how drugs are transferred between mother and fetus during the gestational period is an important medical issue of relevance to both therapeutic drugs and drugs of abuse. Though there are several in vitro and in vivo methods to examine this issue, all have limitations. Furthermore, ethical and safety considerations generally preclude such studies in pregnant humans. PET and appropriately labeled compounds have the ability to provide information on both maternal-fetal drug pharmacokinetics and pharmacodynamics. We present here a nonhuman primate animal model and the methodology for combining PET and MRI to identify fetal organs and to measure maternal and fetal isotope distribution using (18)F-FDG and a whole-body imaging protocol to demonstrate proof-of-principle. METHODS: One nonpregnant nonhuman primate was used for determination of the anesthesia protocol and MRI methods and 3 pregnant nonhuman primates (Macaques radiata) weighing 4.5-7 kg were used for the imaging study and anesthetized with propofol (160-300 micro g/kg/min). Anatomic T2-weighted MR images were acquired on a 4-T MR instrument. Subsequently, whole-body PET images were acquired 35 min after injection of (18)F-FDG, and standardized uptake values (SUVs) were calculated. Image processing and coregistration were performed using commercial software. RESULTS: All animals underwent uneventful general anesthesia for a period of up to 7 h. Coregistration of PET and MR images allowed identification of fetal organs and demonstrated that (18)F-FDG readily crosses the placenta and that (18)F accumulates in both maternal and fetal brain, heart, and bladder. Brain SUVs averaged 1.95 +/- 0.08 (mean +/- SD) and 1.58 +/- 0.11 for mothers and fetuses, respectively. Monkeys delivered healthy babies after a normal gestational term of 170 d following the PET/MRI study. CONCLUSION: The pregnant macaque in combination with PET and MRI technology allows the measurement of radioisotope distribution in maternal and fetal organs. This demonstrates the potential for noninvasively measuring the transfer of drugs across the placenta and for measuring the fetal drug distribution. It also opens up the possibility for studying binding and elimination as well as the effects of a drug on specific cellular elements and physiologic processes during the gestational period in a primate model.     

Improving specificity of breast MRI using prone PET and fused MRI and PET 3D volume datasets.

MRI is a sensitive method for detecting invasive breast cancer, but it lacks specificity. To examine the effect of combining PET with MRI on breast lesion characterization, a prototype positioning device was fabricated to allow PET scans to be acquired in the same position as MRI scans--that is, prone. METHODS: To test the hypothesis that fusion of (18)F-FDG PET and MRI scans improves detection of breast cancer, 23 patients with suspected recurrent or new breast cancer underwent a routine whole-body PET scan, a prone PET scan of the chest, and a routine breast MRI scan. The attenuation-corrected prone PET and MRI datasets were registered twice by different operators. The fusion results were judged for quality by visual inspection and statistical analysis. A joint reading of the MRI and PET scans side by side and integrated images was performed by a nuclear medicine physician and a radiologist. Sensitivity and specificity of MRI and combined MRI and PET scans were calculated on the basis of pathology reports or at least 1 y of clinical and radiologic follow-up. RESULTS: All fusions were verified to be well matched using specific anatomic criteria. A total of 45 lesions was assessed. Lesion size range was 0.6 to 10.0 cm. Of the 44 breasts examined, 29 were suspicious for cancer, of which 15 were found to be positive on surgical excision. In lesion-by-lesion analysis, sensitivity and specificity of MRI alone were 92% and 52%, respectively; after MRI and PET fusion, they were 63% and 95%, respectively. The positive predictive value and the negative predictive value for MRI alone were 69% and 85%, respectively; after MRI and PET fusion, they were 94% and 69%, respectively. CONCLUSION: Acquisition of prone PET scans using the new positioning device permitted acquisition of prone scans suitable for fusion with breast MRI scans. Fused PET and MRI scans increased the specificity of MRI but decreased the sensitivity in this small group of patients. Additional data are needed to confirm the statistical significance of these preliminary findings.     

Pseudostenosis in vessels adjacent to intracranial aneurysms on volume-rendered 3D angiograms: a phantom study

RATIONALE AND OBJECTIVE: Among artifacts on three-dimensional (3D) angiograms, pseudostenosis in vessels adjacent to intracranial aneurysms has not been described. By using a phantom, artifacts seen in vessels adjacent to intracranial aneurysms on volume-rendered 3D angiograms were assessed. MATERIALS AND METHODS: Using a 3D angiography system and a C-arm sweep, digital images were obtained with a 512 x 512 matrix. Rotation was 30 degrees /second, and frame rate was 30 frames/second. Phantom aneurysms were designed to simulate intracranial saccular aneurysms and their parent arteries. Phantoms, consisting of a cylinder (inner diameter, 2 or 4 mm) and spheres, 10, 7, 5, 3, or 2 mm in diameter, were placed at 0 degrees and 45 degrees to the axis of rotation. Two radiologists consensually recorded their findings regarding the presence and location of stenosis and its relationship to the angle of rotation. The maximum percentage of stenosis in the pseudostenosis area was measured on multiplanar reconstruction images by using a workstation computer. RESULTS: Pseudostenosis was observed in the cylinder adjacent to the sphere at both 0 degrees and 45 degrees angles; it was on a plane perpendicular to the axis of rotation. Pseudostenosis was most obvious with 10-mm spheres; it was not seen when spheres were 3 mm or less in diameter. The maximum percentage of stenosis of the pseudostenosis increased with sphere size. CONCLUSION: On volume-rendered 3D angiograms, pseudostenosis was seen in the cylinder adjacent to the sphere. The artifact lay on a plane perpendicular to the axis of rotation, and sphere size affected the artifact.     

Evaluation of relaxation time measurements by magnetic resonance imaging. A phantom study

Several circumstances may explain the great variation in reported proton T1 and T2 relaxation times usually seen. This study was designed to evaluate the accuracy of relaxation time measurements by magnetic resonance imaging (MRI) operating at 1.5 tesla. Using a phantom of nine boxes with different concentrations of CuSO4 and correlating the calculated T1 and T2 values with reference values obtained by two spectrometers (corrected to MRI-proton frequency = 64 MHz) we found a maximum deviation of about 10 per cent. Measurements performed on a large water phantom in order to evaluate the homogeneity in the imaging plane showed a variation of less than 10 per cent within 10 cm from the centre of the magnet in all three imaging planes. Changing the gradient field strength apparently had no influence on the T2 values recorded. Consequently diffusion processes seem without significance. It is concluded that proton T1 and T2 relaxation times covering the majority of the biologic range can be measured by MRI with an overall accuracy of 5 to 10 per cent. Quality control studies along the lines indicated in this study are recommended.     

Evaluation of biexponential relaxation processes by magnetic resonance imaging. A phantom study

Despite the complexity of biologic tissues, a monoexponential behaviour is usually assumed when estimating relaxation processes in vivo by magnetic resonance imaging (MRI). This study was designed to evaluate the potential of biexponential decomposition of T1 and T2 relaxation curves obtained at 1.5 tesla (T). Measurements were performed on a phantom of bicompartmental perspex boxes with combinations of different CuSO4 concentrations. T1 determination was based on a 12-points partial saturation inversion recovery pulse sequence. T2 determination was provided by a multiple spin echo sequence with 32 echoes. Applying biexponential curve analysis, a significant deviation from a monoexponential behaviour was recognized at a ratio of corresponding relaxation rates of about 3 and 2, estimating T1 and T2 relaxation, respectively (p less than 0.01, F-test). Requiring an SD less than or equal to 10 per cent for each set of parameters, the biexponential model was accepted when this ratio exceeded a factor of 5 and 8, respectively. Referring to 'expected' T1 and T2 values, however, an accuracy within 20 per cent only was observed at ratios of at least 6 and 15. It is concluded that quantitative estimation of individual and specific relaxation components in complex biologic tissues by MRI may prove very difficult.     

Evaluation of a new low-dose biplanar system to assess lower-limb alignment in 3D: a phantom study

Objective

Knee coronal alignment is routinely assessed on a full-length radiograph of the lower limbs. However, poor positioning of the knee during the procedure affects the accuracy of this kind of measurement, particularly in cases combining knee rotation and flexion. The purpose of this study was to assess the value of a three-dimensional assessment of the hip-knee-ankle (HKA) angle based on a biplanar radiographic system.

Materials and methods

A biplanar slot scanning system was used to take radiographs of three lower-limb synthetic models with similar frontal deviation (5°valgus) but different flexion angulations (0°, 9°, and 18°). Biplane acquisitions were done with lower-limb axial rotations ranging from 20° of internal rotation to 20° of external rotation on each of the lower limb models. Three independent observers performed standard 2D measurements of the HKA angle from each anteroposterior (AP) image and also modeled the lower limb in 3D for each biplane acquisition with dedicated software. The HKA angle was automatically calculated from the 3D models. The results of the 2D and 3D techniques were compared.

Results

Axial rotation provoked 2D HKA measurement errors up to, respectively, 1.4°, 4.7°, and 6.8° for the lower extremities with 0°, 9°, and 18° flexion, while it never affected the 3D HKA measurement for more than 1.5°. Interobserver errors were 0.7° (SD?=?0.5°) for the 2D measurements and 0.6° (SD?=?0.4°) for the 3D measurements.

Conclusions

The 3D modeling allows for a more accurate evaluation of coronal alignment compared to 2D, eliminating bias due to wrong knee positioning.     

Improvement of semi-quantitative small-animal PET data with recovery coefficients: a phantom and rat study

AIM: To evaluate the accuracy of semi-quantitative small-animal PET data, uncorrected for attenuation, and then of the same semi-quantitative data corrected by means of recovery coefficients (RCs) based on phantom studies. MATERIALS AND METHODS: A phantom containing six fillable spheres (diameter range: 4.4-14 mm) was filled with an 18F-FDG solution (spheres/background activity=10.1, 5.1 and 2.5). RCs, defined as measured activity/expected activity, were calculated. Nude rats harbouring tumours (n=50) were imaged after injection of 18F-FDG and sacrificed. The standardized uptake value (SUV) in tumours was determined with small-animal PET and compared to ex-vivo counting (ex-vivo SUV). Small-animal PET SUVs were corrected with RCs based on the greatest tumour diameter. Tumour proliferation was assessed with cyclin A immunostaining and correlated to the SUV. RESULTS: RCs ranged from 0.33 for the smallest sphere to 0.72 for the largest. A sigmoidal correlation was found between RCs and sphere diameters (r(2)=0.99). Small-animal PET SUVs were well correlated with ex-vivo SUVs (y=0.48x-0.2; r(2)=0.71) and the use of RCs based on the greatest tumour diameter significantly improved regression (y=0.84x-0.81; r(2)=0.77), except for tumours with important necrosis. Similar results were obtained without sacrificing animals, by using PET images to estimate tumour dimensions. RC-based corrections improved correlation between small-animal PET SUVs and tumour proliferation (uncorrected data: Rho=0.79; corrected data: Rho=0.83). CONCLUSION: Recovery correction significantly improves both accuracy of small-animal PET semi-quantitative data in rat studies and their correlation with tumour proliferation, except for largely necrotic tumours.     

124I-Epidepride: A PET radiotracer for extended imaging of dopamine D2/D3 receptors

ObjectivesA new radiotracer, ¹²⁴I-epidepride, has been developed for the imaging of dopamine D2/3 receptors (D2/3Rs). ¹²⁴I-Epidepride (half-life of ¹²⁴I = 4.2 days) allows imaging over extended periods compared to ¹⁸ F-fallypride (half-life of ¹⁸ F = 0.076 days) and may maximize visualization of D2/3Rs in the brain and pancreas (allowing clearance from adjacent organs). D2/3Rs are also present in pancreatic islets where they co-localize with insulin to produce granules and may serve as a surrogate marker for imaging diabetes.Methods¹²⁴I-Epidepride was synthesized using N-[[(2S)-1-ethylpyrrolidin-2-yl]methyl]-5-tributyltin-2,3-dimethoxybenzamide and ¹²⁴I-iodide under no carrier added condition. Rats were used for in vitro and in vivo imaging. Brain slices were incubated with ¹²⁴I-epidepride (0.75 μCi/cc) and nonspecific binding measured with 10 μM haloperidol. Autoradiograms were analyzed by OptiQuant. ¹²⁴I-Epidepride (0.2 to 0.3 mCi, iv) was administered to rats and brain uptake at 3 hours, 24 hours, and 48 hours post injection was evaluated.Results¹²⁴I-Epidepride was obtained with 50% radiochemical yield and high radiochemical purity (> 95%). ¹²⁴I-Epidepride localized in the striatum with a striatum to cerebellum ratio of 10. Binding was displaced by dopamine and haloperidol. Brain slices demonstrated localization of ¹²⁴I-epidepride up until 48 hours in the striatum. However, the extent of binding was reduced significantly.Conclusions¹²⁴I-Epidepride is a new radiotracer suitable for extended imaging of dopamine D2/3 receptors and may have applications in imaging of receptors in the brain and monitoring pancreatic islet cell grafting.     

Breast mass detection by US: a phantom study

Phantoms were used to compare imaging of various masses in the glandular region for two types of breasts. In one type, normal glandular tissue contains no fat clumps; in the other type, randomly distributed spherical fat clumps exist. The size distribution and number of fat clumps per unit volume in the phantoms represented those found in actual normal glandular tissue. For a 5-cm path length between the scanning window of the phantom and the centers of the masses, the presence of the fat clumps caused distortions and deviations of the beam, resulting in image distortion. This degradation was reduced when successive slabs of the material containing the fat clumps were removed. Image degradation appeared to be less severe when a 2.25 (rather than a 3.5) MHz transducer was used. The results show that the thickness of tissue containing fat clumps between the scanning transducer and a mass plays a role in the detectability of masses and diagnostic usefulness of the image of a breast. This suggests that imaging of the breast in the compressed configuration should be performed regardless of whether the freely suspended breast is imaged. Also, transducers of lower nominal frequency may have an important role in breast imaging.     

A Rb infusion system for quantitative perfusion imaging with 3D PET

A ⁸²Rb infusion system is described with two important features for imaging with 3D positron emission tomography. First, a generator bypass line is added to flush the patient infusion line at the end of an elution. Second, feedback control is implemented to permit ‘slow-bolus’ constant-activity elutions. A model for the activity eluted from a ⁸²Sr/⁸²Rb generator based on a volume–activity empirical relationship, is used as the basis for performing simulations to demonstrate the efficacy of varying the flow rate through the generator to achieve desired eluted ⁸²Rb activity rate profiles. A ⁸²Rb infusion system was constructed to verify the accuracy of the simulations. The system can deliver accurate constant-activity elutions from 10% to 70% of the total generator activity.     

能窗下限对三维PET图像质量的影响

目的探讨能窗下限对三维PET图像质量的影响。方法1．模型实验。在9个直径各异的圆柱塑料管灌注放射性浓度为121kBq／ml的^18F．脱氧葡萄糖(FDG)，形成热灶；在本底区灌注12．1kBq／ml的^18F-FDG水溶液形成背景区，热灶和背景区的放射性浓度比为10：1。固定发射扫描的能窗上限(ULD)为650keV，能窗下限(LLD)分别设置为350，400及450keV，每次发射采集5min，扫描间隔1h，共12次，OSEM迭代算法重建图像。计算各热灶的对比度和信噪比。2．分别对脑、心脏及中上腹部(LLD：420keV)进行各模型实验条件(或类似条件)的显像。结果1．模型实验结果。面积大小不一热灶的对比度均随采集LLD的升高而增加，而信噪比几乎保持不变。2．临床结果。①脑胶质瘤放疗术后：450keV LLD时PET图像的分辨效果优于350keV，噪声也相对较低，易得出卫星灶的占位诊断。②陈旧性心肌梗死：400keV LLD时PET图像明显比在350keV时清晰。③中上腹部显像：420keV LLD时PET显像示腹主动脉的腔和壁分辨清楚，而350keV则不能。结论适当提高LLD可提高图像的对比度，减少噪声，对提高图像质量有一定帮助。     

The effect of metal artefact reduction on CT-based attenuation correction for PET imaging in the vicinity of metallic hip implants: a phantom study

Background

To determine if metal artefact reduction (MAR) combined with a priori knowledge of prosthesis material composition can be applied to obtain CT-based attenuation maps with sufficient accuracy for quantitative assessment of ¹⁸F-fluorodeoxyglucose uptake in lesions near metallic prostheses.

Methods

A custom hip prosthesis phantom with a lesion-sized cavity filled with 0.2 ml ¹⁸F-FDG solution having an activity of 3.367 MBq adjacent to a prosthesis bore was imaged twice with a chrome–cobalt steel hip prosthesis and a plastic replica, respectively. Scanning was performed on a clinical hybrid PET/CT system equipped with an additional external ¹³⁷Cs transmission source. PET emission images were reconstructed from both phantom configurations with CT-based attenuation correction (CTAC) and with CT-based attenuation correction using MAR (MARCTAC). To compare results with the attenuation-correction method extant prior to the advent of PET/CT, we also carried out attenuation correction with ¹³⁷Cs transmission-based attenuation correction (TXAC). CTAC and MARCTAC images were scaled to attenuation coefficients at 511 keV using a trilinear function that mapped the highest CT values to the prosthesis alloy attenuation coefficient. Accuracy and spatial distribution of the lesion activity was compared between the three reconstruction schemes.

Results

Compared to the reference activity of 3.37 MBq, the estimated activity quantified from the PET image corrected by TXAC was 3.41 MBq. The activity estimated from PET images corrected by MARCTAC was similar in accuracy at 3.32 MBq. CTAC corrected PET images resulted in nearly 40 % overestimation of lesion activity at 4.70 MBq. Comparison of PET images obtained with the plastic and metal prostheses in place showed that CTAC resulted in a marked distortion of the ¹⁸F-FDG distribution within the lesion, whereas application of MARCTAC and TXAC resulted in lesion distributions similar to those observed with the plastic replica.

Conclusions

MAR combined with a trilinear CT number mapping for PET attenuation correction resulted in estimates of lesion activity comparable in accuracy to that obtained with ¹³⁷Cs transmission-based attenuation correction, and far superior to estimates made without attenuation correction or with a standard CT attenuation map. The ability to use CT images for attenuation correction is a potentially important development because it obviates the need for a ¹³⁷Cs transmission source, which entails extra scan time, logistical complexity and expense.