Dedicated phantom tools using traceable 68Ge/68Ga point-like sources for dedicated-breast PET and positron emission mammography scanners.

Since the early 2000s, many types of positron emission tomography (PET) scanners dedicated to breast imaging for the diagnosis of breast cancer have been introduced. However, conventional performance evaluation methods developed for whole-body PET scanners cannot be used for such devices. In this study, we developed phantom tools for evaluating the quantitative accuracy of positron emission mammography (PEM) and dedicated-breast PET (dbPET) scanners using novel traceable point-like 68Ge/68 Ga sources. The PEM phantom consisted of an acrylic cube (100 × 100 × 40 mm) and three point-like sources. The dbPET phantom comprised an acrylic cylinder (ø100 × 100 mm) and five point-like sources. These phantoms were used for evaluating the fundamental responses of clinical PEM and dbPET scanners to point-like inputs in a medium. The results showed that reasonable recovery values were obtained based on region-of-interest analyses of the reconstructed images. The developed phantoms using traceable 68Ge/68 Ga point-like sources were useful for evaluating the physical characteristics of PEM and dbPET scanners. Thus, they offer a practical, reliable, and universal measurement scheme for evaluating various types of PET scanners using common sets of sealed sources.

Age-Dependent Changes in Effective Dose in Pediatric Brain CT: Comparisons of Estimation Methods.

The effective dose (ED) in computed tomography (CT) may be calculated by multiplying the dose-length product (DLP) by a conversion factor. As children grow, automatic exposure control increases the DLP, while the conversion factor decreases; these two changes affect the ED in opposite ways. The aim of this study was to investigate the methods of ED estimation according to age in pediatric brain CT. We retrospectively analyzed 980 brain CT scans performed for various clinical indications in children. The conversion factor at each age, in integer years, was determined based on the values at 0, 1, 5, and 10 years provided by the International Commission on Radiological Protection (ICRP), using a curve (curve method) or lines (linear method). In the simple method, the ED was estimated using the ICRP conversion factor for the closest age. We also analyzed the ED estimated by a radiation dose management system. Although the median DLP at each age increased with age, the median ED estimated by the curve method was highest at 0 years, decreased with age, and then plateaued at 9 years. The linear method yielded mildly different results, especially at 2 and 3 years. The ED estimated by the simple method or the radiation dose management system showed inconsistent, up-and-down changes with age. In conclusion, the ED in pediatric brain CT decreases with age despite increased DLP. Determination of the conversion factor at each age using a curve is expected to contribute to estimating the ED in pediatric CT according to age.

Automatic Exposure Control Attains Radiation Dose Modulation Matched with the Head Size in Pediatric Brain CT.

We investigated the relationship between the head size and radiation dose in pediatric brain computed tomography (CT) to evaluate the validity of automatic exposure control (AEC). Phantom experiments were performed to assess image noise with and without AEC, and indicated that AEC decreased differences in noise between slices of different section sizes. Retrospective analysis was conducted on 980 pediatric brain CT scans where the tube current was determined using AEC. The water equivalent diameter (WED) was employed as an index of the head size, and mean WED for each image set (WEDmean) and WED for each slice (WEDslice) were used for analysis. For the image-set-based analysis, volume CT dose index (CTDIvol) was compared to WEDmean. For the slice-based analysis, the tube current was compared to WEDslice using 20 of the 980 sets. Additionally, CTDIvol and WEDmean were compared between male and female patients matched for age, weight, or WEDmean. CTDIvol increased with increasing WEDmean, and an exponential curve was closely fitted to the relationship. Tube current changed similarly to the change in WEDslice for each image set, and an exponential curve was well-fitted to the plots of tube current against WEDslice when data from the 20 sets were pooled together. Although CTDIvol and WEDmean were slightly but significantly larger for male than female patients after matching for age or weight, a sex-dependent difference in CTDIvol was not found after matching for WEDmean. This study indicated successful dose modulation using AEC according to the head size for each patient and each slice location. The application of AEC to pediatric brain CT is recommended for radiation dose optimization.

Factors affecting dose-length product of computed tomography component in whole-body positron emission tomography/computed tomography.

In whole-body positron emission tomography (PET)/computed tomography (CT), it is important to optimise the CT radiation dose. We have investigated factors affecting the dose-length product (DLP) of the CT component of whole-body PET/CT and derived equations to predict the DLP. In this retrospective study, 1596 whole-body oncology PET/CT examinations with18F-fluorodeoxyglucose were analysed. Automatic exposure control was used to modulate radiation dose in CT. Considering age, weight, sex, arm position (up, down, one arm up), scan range (up to the mid-thigh or feet), scan mode (spiral or respiratory-triggered nonspiral) and the presence of a metal prosthesis as potential factors, multivariate analysis was performed to identify independent predictors of DLP and to determine equations to predict DLP. DLP values were predicted using the obtained equations, and compared with actual values. Among body size indices, weight best correlated with DLP in examinations performed under the standard imaging conditions (arms: up; scan range: up to the mid-thigh; scan mode: spiral; and no metal prosthesis). Multivariate analysis indicated that weight, arm position, scan range and scan mode were substantial independent predictors; lowering the arms, extending the scan range and using respiratory-triggered imaging, as well as increasing weight, increased DLP. The degree of the DLP increase tended to increase with increasing weight. The DLP values were predicted using equations that considered these parameters were in excellent agreement with the actual values. The DLP for the CT component of whole-body PET/CT is affected by weight, arm position, scan range and scan mode, and can be predicted with excellent accuracy using these factors.

Clinical Evaluation of Left Ventricular Diastolic Function Using Phase-contrast Cine Cardiovascular Magnetic Resonance Imaging: A Comparison With Steady-state Free Precession Cine Cardiovascular Magnetic Resonance Imaging and Echocardiography.

OBJECTIVE: We investigated a practical method using phase-contrast (PC) cine cardiovascular magnetic resonance imaging to estimate peak filling rate and early/atrial velocity (E/A) as left ventricular diastolic function indicators. METHODS: Peak filling rate and E/A were estimated in 32 patients using PC imaging with high spatial or high temporal resolution and compared with those estimated using steady-state free precession cine cardiovascular magnetic resonance imaging and echocardiography. RESULTS: Peak filling rate estimated using PC imaging significantly correlated with those estimated using steady-state free precession imaging despite apparent underestimation using PC imaging with high spatial resolution. The E/A estimated using PC imaging significantly correlated with those estimated using echocardiography. CONCLUSIONS: Peak filling rate and E/A measurements using PC imaging with high temporal resolution is convenient and acceptably accurate, suggesting its potential for clinical use.

Traceable point-like 68Ge/68Ga source with a spherically symmetric positron absorber for PET scanners.

A novel traceable point-like 68Ge/68Ga source was developed for calibration of positron emission tomography (PET) scanners. The source was equipped with a spherically symmetric acrylic positron absorber. The physical characteristics of photons emitted from the point-like source were evaluated by Monte Carlo simulation, considering possible geometrical uncertainties. The calibration factor of a clinical PET/CT scanner was evaluated using four manufactured point-like sources as a practical application of the point-like source. The results were consistent with that determined by the conventional cross-calibration method. The traceable point-like 68Ge/68Ga source is expected to be a simple and practical tool for determining the calibration factor and evaluating the physical characteristics of PET scanners.

Clinical evaluation of CT radiation dose in whole-body 18F-FDG PET/CT in relation to scout imaging direction and arm position.

OBJECTIVE: Radiation exposure in CT is modulated by automatic exposure control (AEC) mainly based on scout images. We evaluated CT radiation dose in whole-body PET/CT in relation to scout imaging direction and arm position, and investigated the behavior of AEC. METHODS: Eighty adult patients who underwent whole-body 18F-FDG PET/CT were divided into groups A, B, C, and D. The posteroanterior scout image alone (PA scout) was used for AEC-based dose modulation in groups A and B, while the posteroanterior and lateral scout images (PA + Lat scout) were used in groups C and D. Patients in groups A and C were imaged with their arms beside the head, while those in groups B and D were imaged with their arms at the sides of the trunk. Dose-length product provided by the scanner was recorded. The tube current value, a determinant of radiation dose, for each slice was plotted against slice location to produce a tube current modulation curve. The scan range was divided into seven anatomical regions, and regional tube current was defined as average tube current for each region. Effective dose was calculated for each region and then summed together. RESULTS: Regional tube current was higher in the body trunk and proximal thigh using the PA scout than using the PA + Lat scout, resulting in higher dose-length product and effective dose using the PA scout. A marked dose increase was shown in the shoulder especially using the PA scout. Spike-like high current at the top of the head was often observed in tube current modulation curves using the PA scout but not using the PA + Lat scout. Raising the arms increased tube current in the head and neck and decreased it in the chest and abdomen. Although dose-length product did not differ significantly depending on arm position, raising the arms decreased effective dose significantly. CONCLUSIONS: AEC-based CT dose modulation in whole-body PET/CT is affected by scout imaging direction and arm position, which should be considered to determine an optimal imaging protocol for whole-body PET/CT.

Super-early images of brain perfusion SPECT using 123I-IMP for the assessment of hyperperfusion in stroke patients.

OBJECTIVE: With the advancement of reperfusion therapy in stroke patients, assessment of perfusion status after therapy is gaining importance. Hyperperfusion tends to be underestimated by the conventional early imaging of 123I-IMP brain perfusion SPECT. We evaluated the utility of super-early imaging as an adjunct to early imaging for the assessment of postischemic hyperperfusion in stroke patients. METHODS: Sixty-seven patients who underwent 123I-IMP brain perfusion SPECT within 14 days after the onset of cerebral infarction were retrospectively analyzed. Super-early (4-10 min) and early (15-45 min) images were acquired using a dual-headed gamma camera. Postischemic hyperperfusion was visually assessed using the early images alone and then using both the super-early and early images, and the frequency of postischemic hyperperfusion and the confidence level of the judgement were evaluated. For quantitative evaluation of image contrast, the contrast ratios (the count ratios of the hyperperfused to normal areas) were calculated. RESULTS: The frequency of postischemic hyperperfusion was significantly higher using both the super-early and early images (28/67 patients) than using the early images alone (17/67 patients, p < 0.001). In 56 patients in whom judgement regarding the presence or absence of postischemic hyperperfusion was unchanged, the confidence level was increased in 8 patients using both image sets. The addition of the super-early SPECT images was judged to be useful and marginally useful in 14 and 15 patients, respectively. The contrast ratio was significantly higher on the super-early images (1.48 ± 0.25) than on the early images (1.26 ± 0.18, p < 0.001). CONCLUSIONS: The addition of super-early imaging to the conventional early imaging aids assessment of postischemic hyperperfusion by 123I-IMP brain perfusion SPECT and may contribute to management of stroke patients in the era of reperfusion therapy.

Reliability of 3D arterial spin labeling MR perfusion measurements: The effects of imaging parameters, scanner model, and field strength.

The aim of this study was to investigate the reliability of cerebral blood flow (CBF) measurements obtained by 3D pseudo-continuous arterial spin labeling (pCASL) imaging according to imaging parameters, scanner model, and field strength. We acquired 3D pCASL images in 12 healthy volunteers using four different scanners: two 3.0 T scanners and two 1.5 T scanners. Reliability was evaluated using intraclass correlation coefficient. Our results indicate that the influence of the post-labeling delay and scanner model on CBF measurements should be taken into consideration. If two scanners of the same model are used, scannerdependent differences may be small.

Effects of Different Containers on Radioactivity Measurements using a Dose Calibrator with Special Reference to 111In and 123I.

Low-energy characteristic x-rays emitted by 111In and 123I sources are easily absorbed by the containers of the sources, affecting radioactivity measurements using a dose calibrator. We examined the effects of different containers on the estimated activities. The radioactivities of 111In, 123I, 201Tl, and 99mTc were measured in containers frequently employed in clinical practice in Japan. The 111In measurements were performed in the vials A and B of the 111In-pentetreotide preparation kit and in the plastic syringe. The activities of 123I-metaiodobenzylguanidine and 201Tl chloride were measured in the prefilled glass syringes and plastic syringes. The milking vial, vial A, vial B, and plastic syringe were used to assay 99mTc. For 111In and 123I, measurements were performed with and without a copper filter. The filter was inserted into the well of the dose calibrator to absorb low-energy x-rays. The relative estimate was defined as the ratio of the activity estimated with the dose calibrator to the standard activity. The estimated activities varied greatly depending on the container when 111In and 123I sources were assayed without the copper filter. The relative estimates of 111In were 0.908, 1.072, and 1.373 in the vial A, vial B, and plastic syringe, respectively. The relative estimates of 123I were 1.052 and 1.352 in the glass syringe and plastic syringe, respectively. Use of the copper filter eliminated the container-dependence in 111In and 123I measurements. Container-dependence was demonstrated in neither 201Tl nor 99mTc measurements. The activities of 111In and 123I estimated with a dose calibrator differ greatly among the containers. Accurate estimation may be attained using the container-specific correction factor or using the copper filter.

Examination of a Method to Determine the Reference Region for Calculating the Specific Binding Ratio in Dopamine Transporter Imaging.

The specific binding ratio (SBR) was first reported by Tossici-Bolt et al. for quantitative indicators for dopamine transporter (DAT) imaging. It is defined as the ratio of the specific binding concentration of the striatum to the non-specific binding concentration of the whole brain other than the striatum. The non-specific binding concentration is calculated based on the region of interest (ROI), which is set 20 mm inside the outer contour, defined by a threshold technique. Tossici-Bolt et al. used a 50% threshold, but sometimes we couldn't define the ROI of non-specific binding concentration (reference region) and calculate SBR appropriately with a 50% threshold. Therefore, we sought a new method for determining the reference region when calculating SBR. We used data from 20 patients who had undergone DAT imaging in our hospital, to calculate the non-specific binding concentration by the following methods, the threshold to define a reference region was fixed at some specific values (the fixing method) and reference region was visually optimized by an examiner at every examination (the visual optimization method). First, we assessed the reference region of each method visually, and afterward, we quantitatively compared SBR calculated based on each method. In the visual assessment, the scores of the fixing method at 30% and visual optimization method were higher than the scores of the fixing method at other values, with or without scatter correction. In the quantitative assessment, the SBR obtained by visual optimization of the reference region, based on consensus of three radiological technologists, was used as a baseline (the standard method). The values of SBR showed good agreement between the standard method and both the fixing method at 30% and the visual optimization method, with or without scatter correction. Therefore, the fixing method at 30% and the visual optimization method were equally suitable for determining the reference region.

Two-Dimensional Spoiled Gradient-Recalled Echo Magnetic Resonance Imaging of the Liver Using Respiratory Navigator-Gating Techniques.

OBJECTIVE: We assessed the feasibility of T1-weighted 2-dimensional spoiled gradient-recalled (2D SPGR) acquisition in steady-state imaging of the liver with various respiratory navigator gating techniques. METHODS: A total of 12 healthy volunteers underwent in-phase and out-of-phase 2D SPGR imaging of the liver during breath-holding and free-breathing. Four techniques for respiratory navigation, 2 conventional navigator techniques and 2 self-navigator techniques, were used for free-breathing imaging. RESULTS: Good navigator waveforms were obtained in conventional navigation, whereas fluctuations were evident in self navigation. All of the 4 navigator-based methods provided better images in terms of background signals and visual image quality compared with images obtained with no respiratory control. However, differences remained in comparison with breath-holding. Superiority of self-navigation to conventional navigation was not shown. CONCLUSIONS: Navigator-gating techniques improved 2D SPGR images of the liver acquired during free-breathing, suggesting feasibility and beneficial effects, although navigator-based images were still inferior to breath-hold images.

Influence of the Magnetic Field Strength on Image Contrast in Gd-EOB-DTPA-enhanced MR Imaging: Comparison between 1.5T and 3.0T.

PURPOSE: We quantitatively investigated hepatic enhancement in gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance (MR) imaging at 1.5T and 3.0T. METHODS: A total of 40 patients who underwent Gd-EOB-DTPA-enhanced MR imaging were included in the study. Precontrast and hepatobiliary-phase images acquired at a low flip angle (FA, 12°) and hepatobiliary-phase images acquired at a high FA (30°) were analyzed. From these images, the liver-to-muscle signal intensity ratio (LMR) and liver-to-spleen signal intensity ratio (LSR) were estimated, and the contrast enhancement ratio (CER) was calculated from the liver signal, LMR, and LSR as the ratio of the low-FA hepatobiliary-phase value to the precontrast value. The coefficient of variance in the liver signal was determined to represent image noise. RESULTS: LMR and LSR indicated similar image contrast between 1.5T and 3.0T. A higher FA provided larger LMRs and LSRs, and the degree of the FA-dependent increase was similar between 1.5T and 3.0T. CER did not differ significantly between 1.5T and 3.0T, regardless of the calculation method. A better correlation to CER calculated from the liver signal was found for the LMR-based CER values than for the LSR-based CER. The coefficient of variance in the liver signal was significantly smaller at 3.0T for precontrast and low-FA hepatobiliary-phase images, but not for high-FA hepatobiliary-phase images. CONCLUSION: The indices of hepatic enhancement were similar between 1.5T and 3.0T, indicating that the magnetic field strength does not substantially influence image contrast after administration of Gd-EOB-DTPA.

Cardiovascular magnetic resonance evaluation of left ventricular peak filling rate using steady-state free precession and phase contrast sequences.

BACKGROUND: We investigated a practical method to measure peak filling rate (PFR) as an indicator of diastolic function of the left ventricle. Ten adult volunteers underwent cine MR imaging using steady-state free precession (SSFP) and phase contrast (PC) sequences to measure PFR. Two PC image sets were acquired at the mitral valve orifice, and PFR was determined from the set with high true temporal resolution (temporal PC method) or with high spatial resolution (spatial PC method). SSFP images covering the left ventricle were acquired, and a time-volume curve was generated around the peak filling phase. PFR was determined using parabolic curve fitting on the first-derivative curve of the LV time-volume curve. FINDINGS: PFR values estimated by the PC methods correlated well with those estimated by the SSFP method, despite apparent underestimation. The underestimation was smaller for the temporal PC method (12 %) than for the spatial PC method (28 %). Intra- and inter-observer repeatabilities were better for the PC methods than for the SSFP method. CONCLUSIONS: PFR measurement by PC imaging with high true temporal resolution is convenient and offers excellent repeatability and acceptable accuracy, indicating suitability for clinical use.

Evaluation of a respiratory navigator-gating technique in Gd-EOB-DTPA-enhanced magnetic resonance imaging for the assessment of liver tumors.

OBJECTIVES: We investigated the clinical usefulness of respiratory navigator-gating technique for the assessment of liver tumors in Gd-EOB-DTPA-enhanced magnetic resonance (MR) imaging. METHODS: Eighty patients who underwent Gd-EOB-DTPA-enhanced MR imaging to evaluate known or suspected liver tumors were enrolled. Three-dimensional spoiled gradient-recalled echo images of the liver were acquired in the hepatobiliary phase by the following three methods: breath-hold imaging, navigator-gated low-resolution imaging, and navigator-gated high-resolution imaging. Navigator-gated imaging was performed during free breathing. Spatial resolution was identical between breath-hold imaging and gated low-resolution imaging. Signal intensities in the liver, muscle, and spleen were measured in 20 patients. Image quality was visually evaluated in all 80 patients. The detection rate and lesion conspicuity were assessed for 71 malignant liver lesions identified in 29 patients. RESULTS: The liver-to-muscle and liver-to-spleen signal ratios were significantly lower for gated images compared to breath-hold images. Images of acceptable quality were obtained in most patients by all three methods, and the overall image quality of axial images did not differ significantly among the imaging methods, although superior reformatted coronal images were obtained by gated high-resolution imaging. The detection rates of malignant liver lesions were similar among the three imaging methods, although lesion conspicuity was significantly better for breath-hold imaging compared to gated imaging. CONCLUSIONS: Navigator-gated imaging provided image qualities and detection rates of malignant liver lesions comparable to breath-hold imaging in Gd-EOB-DTPA-enhanced MR imaging; however, no additional benefits of high-resolution imaging were proven for lesion evaluation.

Measurement of Renal Depth in Dynamic Renal Scintigraphy Using Ultralow-Dose CT.

PURPOSE: Renal depths predicted using predefined formulas are commonly used for camera-based evaluation of renal function. We investigated the feasibility and utility of renal depth measurement using ultralow-dose CT images acquired in conjunction with dynamic renal scintigraphy. METHODS: Dynamic renal scintigraphy with Tc-MAG3 was performed in 117 patients (225 kidneys) using a SPECT/CT scanner, and ultralow-dose CT (estimated effective dose of 0.17 mSv) was performed during free breathing immediately before tracer injection. The clarity of the renal contour on the CT images was evaluated visually. The renal depths were measured by 2 methods and compared with depths predicted by 2 previously reported methods. The accuracy of camera-based clearance using predicted and measured depths was evaluated using a single-sample method as a standard. RESULTS: The clarity of the renal contour was poor in 18 of 225 kidneys, and 12 of 117 patients were considered ineligible for depth measurement. The measurement for eligible patients showed excellent intraobserver and interobserver repeatabilities. Although mean depths were similar among the 2 CT measurement methods and 2 prediction methods, absolute differences of more than 1 cm were observed in approximately 20% of kidneys between CT measurement and prediction. CT measurement of renal depth failed to improve the accuracy of camera-based clearance evaluation. CONCLUSION: Ultralow-dose CT allowed measurement of renal depth in most patients. Substantial differences in renal depth between prediction and CT measurement indicated potential usefulness of CT measurement, although no actual improvement in the accuracy of clearance estimation was demonstrated in this study.

Methods of CT Dose Estimation in Whole-Body ¹8F-FDG PET/CT.

UNLABELLED: We evaluated the effective dose (ED) of the CT component of whole-body PET/CT using software dedicated to CT dose estimation and from dose-length product (DLP) values to establish practical methods of ED estimation. METHODS: Eighty adult patients who underwent (18)F-FDG whole-body PET/CT were divided into groups A and B, each consisting of 20 men and 20 women. In group A, ED of the CT component was calculated using CT-Expo for 6 anatomic regions separately, and whole-body ED was obtained by summing the regional EDs (CT-Expo method). DLP was calculated for each of the 6 regions and multiplied by a corresponding conversion factor described in International Commission on Radiological Protection publication 102 to obtain the ED for each region (regional DLP method). Whole-body ED was also calculated as the product of a whole-body DLP value provided by the scanner automatically and a conversion factor (simple DLP method). Moreover, the ED/DLP values were calculated using whole-body ED estimated by the CT-Expo method and the scanner-derived DLP, to optimize the conversion factor. In group B, the optimized conversion factor was applied for the estimation of ED by the simple DLP method. RESULTS: In group A, the regional DLP method allowed an accurate estimation of mean whole-body ED as a result of counterbalance of mild overestimation in men and mild underestimation in women, regarding the CT-Expo method as a standard. The simple DLP method using a conversion factor for the trunk (0.015 mSv/mGy/cm) caused overestimation. On the basis of the ED/DLP values in group A, a modified conversion factor of 0.013 mSv/mGy/cm and sex-specific conversion factors of 0.012 and 0.014 mSv/mGy/cm for men and women, respectively, were determined. In group B, the use of the modified conversion factor improved accuracy, and the use of sex-specific conversion factors eliminated sex-dependent residual errors. CONCLUSION: ED of the CT component of whole-body PET/CT can be assessed by multiplying the scanner-derived DLP by a conversion factor optimized for whole-body PET/CT.

Validation of novel calibration scheme with traceable point-like (22)Na sources on six types of PET scanners.

OBJECTIVE: To improve the reliability and convenience of the calibration procedure of positron emission tomography (PET) scanners, we have been developing a novel calibration path based on traceable point-like sources. When using (22)Na sources, special care should be taken to avoid the effects of 1.275-MeV γ rays accompanying ß (+) decays. The purpose of this study is to validate this new calibration scheme with traceable point-like (22)Na sources on various types of PET scanners. METHOD: Traceable point-like (22)Na sources with a spherical absorber design that assures uniform angular distribution of the emitted annihilation photons were used. The tested PET scanners included a clinical whole-body PET scanner, four types of clinical PET/CT scanners from different manufacturers, and a small-animal PET scanner. The region of interest (ROI) diameter dependence of ROI values was represented with a fitting function, which was assumed to consist of a recovery part due to spatial resolution and a quadratic background part originating from the scattered γ rays. RESULTS: The observed ROI radius dependence was well represented with the assumed fitting function (R (2) > 0.994). The calibration factors determined using the point-like sources were consistent with those by the standard cross-calibration method within an uncertainty of ±4 %, which was reasonable considering the uncertainty in the standard cross-calibration method. CONCLUSION: This novel calibration scheme based on the use of traceable (22)Na point-like sources was successfully validated for six types of commercial PET scanners.