Adaptation of a channelized Hotelling observer model to accommodate anthropomorphic backgrounds and moving test objects in X-ray angiography.

BACKGROUND: Prior implementations of the channelized Hotelling observer (CHO) model have succeeded in assessing the performance of X-ray angiography systems under a variety of imaging conditions. However, often times these conditions do not resemble those present in routine clinical imaging scenarios, such as having complex anthropomorphic backgrounds in conjunction with moving test objects. PURPOSE: This work builds up on prior established CHO methods and introduces a new approach to switch from the already established "multiple-sample" CHO implementation to a "single-sample" technique. The proposed implementation enables the inclusion of moving test objects upon nonuniform backgrounds by allowing only a single sample to represent the test object present condition that is to be used within the statistical test to estimate the detectability index. METHODS: To assess the proposed method, two image data sets were acquired with a clinical X-ray angiography system. The first set consisted of a uniform background in combination with static test objects while the second consisted of an anthropomorphic chest phantom in conjunction with moving test objects. The first set was used to validate the proposed approach against the multiple-sample method while the second was used to assess the feasibility of the proposed method under a variety of imaging conditions, including seven object sizes and seven detector target dose (DTD) levels. RESULTS: For the uniform background data set, considering all detectability indices greater or equal than 1, the ratio between the detectability indices of the proposed single-sample approach versus the multiple-sample method was 0.997 ± 0.056 (range 0.884-1.159). The average single-direction width of the 95% confidence intervals (CIs) of the detectability index estimates for the multiple-sample method was 0.38 ± 0.43 (range 0.03-2.20). For the single-sample approach, the average width was 2.52 ± 0.63 (range 1.11-5.44). For the anthropomorphic background image set, the results were consistent with classical quantum-limited signal-to-noise ratio (SNR) theory. The magnitude of the detectability indices varied predictably with changes in both object size and DTD, with the highest value associated with the highest dose and the largest object size. Additionally, the proposed method was able to capture differences in the imaging performance for a given test object across the field of view, which was associated with the attenuation levels provided by different features of the anthropomorphic background. CONCLUSIONS: A new single-sample variant of the CHO model to assess the performance of X-ray angiography imaging systems is proposed. The new approach is consistent with quantum-limited image quality theory and with a standard implementation of the CHO model. The proposed method enables the assessment of moving test objects in combination with complex, nonuniform image backgrounds, thereby opening the possibility to assess imaging conditions which more closely resemble those used in clinical care.

Eye protection in interventional procedures.

Data suggest that radiation-induced cataracts may form without a threshold and at low-radiation doses. Staff involved in interventional radiology and cardiology fluoroscopy-guided procedures have the potential to be exposed to radiation levels that may lead to eye lens injury and the occurrence of opacifications have been reported. Estimates of lens dose for various fluoroscopy procedures and predicted annual dosages have been provided in numerous publications. Available tools for eye lens radiation protection include accessory shields, drapes and glasses. While some tools are valuable, others provide limited protection to the eye. Reducing patient radiation dose will also reduce occupational exposure. Significant variability in reported dose measurements indicate dose levels are highly dependent on individual actions and exposure reduction is possible. Further follow-up studies of staff lens opacification are recommended along with eye lens dose measurements under current clinical practice conditions.

Technical evaluation of a prototype ratio 29:1 grid for adult patient cardiovascular angiography imaging conditions.

The purpose of this work was to assess technical performance of a prototype high-ratio (r29), 80 line cm-1grid for imaging conditions which mimic those for adult cardiovascular angiography. The standard equipment r15, 80 line cm-1grid was used as a reference. Plastic Water®LR phantoms with thickness in the range 20-44 cm were used to simulate adult patient attenuation and scatter. Grids were tested using x-ray field of view 20 and 25 cm and x-ray source to detector distance (SID) 107 and 120 cm. The primary transmission fraction (TP) was measured using both narrow beam geometry and a lead beam stop (BS) technique. Scatter transmission (TS) was measured with the lead BS technique. The quantum signal to noise ratio improvement factor (KSNR) was used to describe relative grid performance. The experimental conditions required revised theory to assess grid performance. Theory to account for the detector glare and underestimation of scatter intensity by the lead BS method was developed. Also, novelKSNRtheory was developed to allow direct comparison of two grids operated at different SID. MeanTPwas modestly lower for the r29 versus r15 grid (0.69 versus 0.75). When tested under equivalent scatter condition, TSof the r29 grid was approximately ½ that of the r15 grid (0.18 versus 0.34).KSNRof the r29 grid at SID 120 cm compared to the r15 grid at SID 107 cm increased linearly with phantom thickness (range 1.0 to ∼1.16). Findings of this work indicate that the r29 grid used at SID 120 cm is expected to provide improved image quality (or reduced patient radiation dose) when compared to the r15 grid used at SID 107 cm for adult cardiovascular patients and that the potential benefit of the r29 grid increases with patient thickness >20 cm.

Task-specific efficient channel selection and bias management for Gabor function channelized Hotelling observer model for the assessment of x-ray angiography system performance.

PURPOSE: Channelized Hotelling observer (CHO) models have been implemented to assess imaging performance in x-ray angiography systems. While current methods are appropriate for assessing unprocessed images of moving test objects upon uniform-exposure backgrounds, they are inadequate for assessing conditions which more appropriately mimic clinical imaging conditions including the combination of moving test objects, complex anthropomorphic backgrounds, and image processing. In support of this broad goal, the purpose of this work was to develop theory and methods to automatically select a subset of task-specific efficient Gabor channels from a task-generic Gabor channel base set. Also, previously described theory and methods to manage detectability index (d') bias due to nonrandom temporal variations in image electronic noise will be revisited herein. METHODS: Starting with a base set of 96 Gabor channels, backward elimination of channels was used to automatically identify an "efficient" channel subset which reduced the number of channels retained in the subset while maintaining the magnitude of the d' estimate. The concept of a pixelwise Hotelling observer (PHO) model was introduced and similarly implemented to assess the performance of the efficient-channel CHO model. Bias in d' estimates arising from temporally variable nonstationary noise was modeled as a bivariate probability density function for normal distributions, where one variable corresponds to the signal from the test object and the other variable corresponds to the signal from temporally variable nonstationary noise. Theory and methods were tested on uniform-exposure unprocessed angiography images with detector target dose (DTD) of 6, 18, and 120 nGy containing static disk-shaped test objects with diameter in the range of 0.5 to 4 mm. RESULTS: Considering all DTD levels and test object sizes, the proposed method reduced the number of Gabor channels in the final subset by 63-82% compared to the original 96 Gabor channel base set, while maintaining a mean relative performance ( ( d CHO ' / d PHO ' ) × 100 % ) of 95%  ±  4% with respect to the reference PHO model. Experimental results demonstrated that the bivariate approach to account for bias due to temporally variable nonstationary noise resulted in improved correlation between the CHO and PHO models as compared to a previously proposed univariate approach. CONCLUSIONS: Computationally efficient backward elimination can be used to select an efficient subset of Gabor channels from an initial channel base set without substantially compromising the magnitude of the d' estimate. Bias due to temporally variable nonstationary noise can be modeled through a bivariate approach leading to an improved unbiased estimate of d'.

Radiation dose reduction for 3D angiography images in pediatric and congenital cardiology.

OBJECTIVES: The purpose of this study was to investigate the influence of simulated reduced-dose three-dimensional angiography (3DA) on the accuracy and precision of linear measurements derived from 3DA datasets. BACKGROUND: Three-dimensional angiography is performed during X-ray guided interventional procedures to aid diagnosis and inform treatment strategies for children and adults with congenital heart disease. However, 3DA contributes substantially to patient radiation dose and may lead to an increased radiation-induced cancer risk. METHODS: Reduced-dose patient 3DA images were simulated by adding quantum noise to the 2D projection angiograms, then reconstructing the projection angiograms into the 3DA dataset. Dose reduction in the range 33-72% was simulated. Five observers performed 46 vessel diameter measurements along prespecified axes within 23 vessel segments from 11 patient 3DA datasets. Statistical tests were performed to assess the influence of radiation dose reduction on the accuracy and precision of vessel diameter measurements. RESULTS: Vessel diameter measurements were in the range 5.9- 22.7 mm. Considering all vessel segments and observers, the influence of dose level on the accuracy of diameter measurements was in the range 0.02 - 0.15 mm (p .05-.8). Interobserver variability increased modestly with vessel diameter, but was not influence by dose level (p = .52). The statistical test for observer recall bias was negative (p = .51). CONCLUSIONS: Simulated dose reduction up to 72% did not affect the accuracy or precision of the diameter measurements acquired from 3DA images. These findings may embolden 3DA radiation dose reduction for pediatric and congenital heart disease patients.

Determination of iodine detectability in different types of multiple-energy images for a photon-counting detector computed tomography system.

In addition to low-energy-threshold images (TLIs), photon-counting detector (PCD) computed tomography (CT) can generate virtual monoenergetic images (VMIs) and iodine maps. Our study sought to determine the image type that maximizes iodine detectability. Adult abdominal phantoms with iodine inserts of various concentrations and lesion sizes were scanned on a PCD-CT system. TLIs, VMIs at 50 keV, and iodine maps were generated, and iodine contrast-to-noise ratio (CNR) was measured. A channelized Hotelling observer was used to determine the area under the receiver-operating-characteristic curve (AUC) for iodine detectability. Iodine map CNR ( 0.57 ± 0.42 ) was significantly higher ( P < 0.05 ) than for TLIs ( 0.46 ± 0.26 ) and lower ( P < 0.001 ) than for VMIs at 50 keV ( 0.74 ± 0.33 ) for 0.5 mgI/cc and a 35-cm phantom. For the same condition and an 8-mm lesion, iodine detectability from iodine maps ( AUC = 0.95 ± 0.01 ) was significantly lower ( P < 0.001 ) than both TLIs ( AUC = 0.99 ± 0.00 ) and VMIs ( AUC = 0.99 ± 0.01 ). VMIs at 50 keV had similar detectability to TLIs and both outperformed iodine maps. The lowest detectable iodine concentration was 0.5 mgI/cc for an 8-mm lesion and 1.0 mgI/cc for a 4-mm lesion.

Integration of high velocity test object motion into a channelized Hotelling observer for the assessment of x-ray angiography systems.

Assessment of x-ray angiography system performance is typically performed using stationary test objects with simple geometries such as a disk on a uniform background. However, these methods do not represent realistic imaging conditions in interventional cardiology as anatomy and devices are inherently non-stationary due to cardiac motion. In this work, a novel implementation of the channelized Hotelling observer (CHO) was used to assess the influence of motion blur on object detectability. A standard CHO model assumes imaging system stationarity whereby the detectability index [Formula: see text] of a test object is independent of location. However, real angiography systems are inherently non-stationary. While vendor correction gain factors and offset maps are used to compensate for visual non-uniformities, these corrections do not restore stationarity to the images. Methods to accommodate non-stationarity and allow assessment of the influence of motion blur on test object detectability will be presented. The effect of motion blur was quantified with the relative detectability index ([Formula: see text]), where the [Formula: see text] for an object when moving with constant linear velocity was compared to a low velocity 'pseudo-stationary' condition to account for system non-stationarity. The pseudo-stationary condition was used to isolate the influences of spatial non-stationarity and motion blur. Three different test object shapes (disks, spheres and capsules) with linear velocity in the range 0-30 cm · s-1 were tested. For 1 mm diameter objects and linear velocity 30 cm · s-1, [Formula: see text] was degraded by 37%, 33% and 42% for the disk, sphere and capsule respectively, relative to the pseudo-stationary condition. Considering all test objects with diameter greater than 2 mm and linear velocity 30 cm · s-1, [Formula: see text] was degraded by less than 10% due to motion. In summary, this work describes a new approach to assess performance of x-ray angiography systems using the CHO model and moving test objects.

Technical Note: Assessment of scatter originating from the x-ray tube collimator assembly of modern angiography systems.

PURPOSE: While scatter from the patient is assumed to be the primary source of occupational radiation dose associated with fluoroscopically guided interventional procedures, the potential contribution of scatter from the x-ray collimator assembly is unknown. The purpose of this work was to survey clinical x-ray angiography systems to assess the potential contribution of collimator assembly scatter on occupational radiation dose. METHODS: Experimental methods were designed to measure the relative contributions of scatter originating from within the collimator assembly of the x-ray tube to total scatter, which included scatter from a patient-simulating phantom. Measurements were acquired as a function of lateral distance from the x-ray beam center using a posterior anterior (PA) projection and at a fixed location for variable right anterior oblique to left anterior oblique projections in the range -90º to 90º. For one system, the collimator assembly was partially disassembled to assess the scatter contribution of individual components. For two systems, 0.5 mm Pb was added to the inner surface of the collimator assembly cover and tested for efficacy to block collimator assembly scatter. RESULTS: Considering all x-ray systems and only the PA projection, collimator assembly scatter contributed 20-50% to total scatter. For x-ray projection angles of -90º to 90º, the relative contribution of collimator assembly to total scatter was dependent on projection angle and ranged from 5% to 56%. X-ray systems with kerma-area product meters demonstrated higher collimator assembly scatter than those without. Considering all projection angles, the addition of 0.5 mm Pb to the inside of the collimator assembly cover reduced collimator assembly scatter from 28% to 16% of total scatter for both systems. CONCLUSION: Findings from this work suggest that contemporary radiation safety practices and guidelines should be revised to account for scatter originating from the collimator assembly of angiographic x-ray tubes.

Performance assessment of active vs passive pixel x-ray angiography detector systems using a bias-corrected channelized Hotelling observer and adult patient-equivalent experimental conditions.

PURPOSE: Electronic noise associated with passive pixel (PP) x-ray angiography flat panel detectors is known to compromise fluoroscopic image quality. An active pixel (AP) crystalline silicon x-ray detector with potential for reduced influence of electronic noise is commercially available. The purpose of this work was to compare the performance of the AP vs PP x-ray angiography detectors over a detector target dose (DTD) range relevant for invasive cardiology procedures. METHODS: A total of 16 passive pixel detector systems representing two models and two active pixel detector systems of a single model were tested. Iodine contrast (160 mg I ml-1 ) disk-shaped test objects of diameter 0.5-4.0 mm were embedded in 30 × 30 cm2 25-cm-thick PMMA phantom. Detector target dose was 6, 18, and 120 nGy and 1204 test signal present and signal absent images were acquired. A channelized Hotelling observer statistical model (CHO) was used to estimate detectability index (d') of the detectors for the various test objects. The CHO included correction for bias from finite sampling and that due to temporally variable electronic noise. RESULTS: Detectability index estimates demonstrated similar performance between the two models of PP detectors and relatively improved performance for the AP detectors for all DTD levels and test object diameters. For DTD = 120 nGy and the 4.0 mm test object, d' of the AP detectors was 13% and 20% greater than that of the PP detectors. For DTD = 6 nGy, d' of the AP detectors was 42% and 54% greater. CONCLUSIONS: The AP x-ray angiography detector demonstrated superior performance throughout the DTD range tested and especially for DTD consistent with low-dose fluoroscopy. The improved performance of the AP detectors may facilitate reduced patient dose and/or improved image quality.

Determination of Optimal Image Type and Lowest Detectable Concentration for Iodine Detection on a Photon Counting Detector-Based Multi-Energy CT System.

Photon counting detector (PCD) based multi-energy CT is able to generate different types of images such as virtual monoenergetic images (VMIs) and material specific images (e.g., iodine maps) in addition to the conventional single energy images. The purpose of this study is to determine the image type that has optimal iodine detection and to determine the lowest detectable iodine concentration using a PCD-CT system. A 35 cm body phantom with iodine inserts of 4 concentrations and 2 sizes was scanned on a research PCD-CT system. For each iodine concentration, 80 repeated scans were performed and images were reconstructed for each energy threshold. In addition, VMIs at different keVs and iodine maps were also generated. CNR was measured for each type of images. A channelized Hotelling observer was used to assess iodine detectability after being validated with human observer studies, with area under the ROC curve (AUC) as a figure of merit. The agreement between model and human observer performance indicated that model observer could serve as an effective approach to determine optimal image type for the clinical practice and to determine the lowest detectable iodine concentration. Results demonstrated that for all size and concentration combinations, VMI at 70 keV had similar performance as that of threshold low images, both of which outperformed the iodine map images. At the AUC value of 0.8, iodine concentration as low as 0.2 mgI/cc could be detected for an 8 mm object and 0.5 mgI/cc for a 4 mm object with a 5 mm slice thickness.

Head and Neck Radiation Dose and Radiation Safety for Interventional Physicians.

OBJECTIVES: The first aim of this study was to assess the magnitude of radiation dose to tissues of the head and neck of physicians performing x-ray-guided interventional procedures. The second aim was to assess protection of tissues of the head offered by select wearable radiation safety devices. BACKGROUND: Radiation dose to tissues of the head and neck is of significant interest to practicing interventional physicians. However, methods to estimate radiation dose are not generally available, and furthermore, some of the available research relating to protection of these tissues is misleading. METHODS: Using a single representative geometry, scatter radiation dose to a humanoid phantom was measured using radiochromic film and normalized by the radiation dose to the left collar of the radioprotective thorax apron. Radiation protection offered by leaded glasses and by a radioabsorbent surgical cap was measured. RESULTS: In the test geometry, average radiation doses to the unprotected brain, carotid arteries, and ocular lenses were 8.4%, 17%, and 50% of the dose measured at the left collar, respectively. Two representative types of leaded glasses reduced dose to the ocular lens on the side of the physician from which the scatter originates by 27% to 62% but offered no protection to the contralateral eye. The radioabsorbent surgical cap reduced brain dose by only 3.3%. CONCLUSIONS: A method by which interventional physicians can estimate dose to head and neck tissues on the basis of their personal dosimeter readings is described. Radiation protection of the ocular lenses by leaded glasses may be incomplete, and protection of the brain by a radioabsorbent surgical cap was minimal.

Estimating head and neck tissue dose from x-ray scatter to physicians performing x-ray guided cardiovascular procedures: a phantom study.

Physicians performing x-ray guided interventional procedures have a keen interest in radiation safety. Radiation dose to tissues and organs of the head and neck are of particular interest because they are not routinely protected by wearable radiation safety devices. This study was conducted to facilitate estimation of radiation dose to tissues of the head and neck of interventional physicians based on the dose recorded by a personal dosimeter worn on the left collar. Scatter beam qualities maximum energy and HVL were measured for 40 scatter beams emitting from an anthropomorphic patient phantom. Variables of the scatter beams included scatter angle (35° and 90°), primary beam peak tube potential (60, 80, 100, and 120 kVp), and 5 Cu spectral filter thicknesses (0-0.9 mm). Four reference scatter beam qualities were selected to represent the range of scatter beams realized in a typical practice. A general radiographic x-ray tube was tuned to produce scatter-equivalent radiographic beams and used to simultaneously expose the head and neck of an anthropomorphic operator phantom and radiochromic film. The geometric relationship between the x-ray source of the scatter-equivalent beams and the operator phantom was set to mimic that between a patient and physician performing an invasive cardiovascular procedure. Dose to the exterior surface of the operator phantom was measured with both 3 × 3 cm2 pieces of film and personal dosimeters positioned at the location of the left collar. All films were scanned with a calibrated flatbed scanner, which converted the film's reflective density to dose. Films from the transverse planes of the operator phantom provided 2D maps of the dose distribution within the phantom. These dose maps were normalized by the dose at the left collar, providing 2D percent of left collar dose (LCD) maps. The percent LCD maps were overlain with bony anatomy CT images of the operator phantom and estimates of percent LCD to the left, right and whole brain, brain stem, lenses of the eyes, and carotid arteries were calculated. Per expectation, results indicated greater percent dose to superficial versus deep tissues and increasing percent dose to deep tissues with increasing scatter-equivalent beam energy and HVL. The results enable estimation of the scatter dose to tissues of the head and neck of interventional physicians based on occupational dose measured by a personal dosimeter worn at the collar outside the protective apron.

Balloon Valvuloplasty to Predict X-ray Projection Angles that are Perpendicular to Cardiovascular Structures: A TAVI Patient Feasibility Study.

OBJECTIVES: The purpose of this work is to describe methods to measure the 3D angular orientation of cardiovascular structures based on a planar image of a valvuloplasty balloon. These methods facilitate X-ray beam alignment with respect to the anatomy of interest. BACKGROUND: X-ray beam projections which are perpendicular to the long axis of cardiovascular structures are required to support interventional procedures, including transcatheter aortic valve implant (TAVI). METHODS: During the TAVI procedure, the 3D angular orientation of the LVOT of 10 patients was measured from a single planar image of an aortic valvuloplasty balloon and the continuous range of X-ray projection angles which are aligned with the aortic valve plane were calculated (research method). Misalignment of the X-ray beam and TAVI valve frame was measured from images of the deployed valve. The accuracy of the research method was compared to clinical standard method to determine appropriate X-ray projection angles, which utilized CT and aortography. RESULTS: Using the clinical standard method, the median misalignment of the X-ray beam and TAVI valve frame was 8.6° (range 2.6° to 21°). Misalignment was reduced to 2.5° (range 0° to 10°) using the research method. CONCLUSIONS: The 3D angular orientation of cardiovascular structures can be measured accurately from a single X-ray projection image of a known cardiovascular device contained within the anatomy of interest. For TAVI procedures, improved X-ray beam alignment may help facilitate procedural success. © 2016 Wiley Periodicals, Inc.

Direct estimation and correction of bias from temporally variable non-stationary noise in a channelized Hotelling model observer.

Channelized Hotelling model observer (CHO) methods were developed to assess performance of an x-ray angiography system. The analytical methods included correction for known bias error due to finite sampling. Detectability indices ([Formula: see text]) corresponding to disk-shaped objects with diameters in the range 0.5-4 mm were calculated. Application of the CHO for variable detector target dose (DTD) in the range 6-240 nGy frame(-1) resulted in [Formula: see text] estimates which were as much as 2.9× greater than expected of a quantum limited system. Over-estimation of [Formula: see text] was presumed to be a result of bias error due to temporally variable non-stationary noise. Statistical theory which allows for independent contributions of 'signal' from a test object (o) and temporally variable non-stationary noise (ns) was developed. The theory demonstrates that the biased [Formula: see text] is the sum of the detectability indices associated with the test object [Formula: see text] and non-stationary noise ([Formula: see text]). Given the nature of the imaging system and the experimental methods, [Formula: see text] cannot be directly determined independent of [Formula: see text]. However, methods to estimate [Formula: see text] independent of [Formula: see text] were developed. In accordance with the theory, [Formula: see text] was subtracted from experimental estimates of [Formula: see text], providing an unbiased estimate of [Formula: see text]. Estimates of [Formula: see text] exhibited trends consistent with expectations of an angiography system that is quantum limited for high DTD and compromised by detector electronic readout noise for low DTD conditions. Results suggest that these methods provide [Formula: see text] estimates which are accurate and precise for [Formula: see text]. Further, results demonstrated that the source of bias was detector electronic readout noise. In summary, this work presents theory and methods to test for the presence of bias in Hotelling model observers due to temporally variable non-stationary noise and correct this bias when the temporally variable non-stationary noise is independent and additive with respect to the test object signal.