Biomedical subjects

Harrison H Barrett

Publications and source records attributed to Harrison H Barrett.

24 records · Page 2Linked to original sources

PubMed2003

Validating the use of channels to estimate the ideal linear observer.

Image quality can be objectively defined according to how well an observer can perform a task of practical interest given the image. We review a practical model observer for the signal-detection task. The ideal observer for this task is a function of the image probability distributions, which are multidimensional and complicated. This observer is often too difficult to derive or estimate. An alternative to the ideal observer is the ideal linear observer, which can still be unmanageable. Our alternative is the ideal linear observer constrained to a small set of channels: the channelized-Hotelling observer.

Diagnostic Imaging↗

PubMed2003

Performance evaluation of a modular gamma camera using a detectability index.

UNLABELLED: The performance of a modular gamma camera for the task of detecting signals in random noisy backgrounds was evaluated experimentally. The results were compared with a theoretical computer simulation. METHODS: The camera uses a 10 x 10 cm thallium-doped sodium iodide crystal, a 2 x 2 array of 53 x 53 mm photomultiplier tubes, and a parallel-hole collimator (1.5-mm bore width, 23.6-mm bore length). The camera was positioned to look down into a 10-cm-deep water bath that filled its field of view (FOV). The top surface of the water was 5 cm from the front face of the camera. The camera has 3-mm intrinsic spatial resolution (SR) in the center of its FOV and 9-mm system SR for objects 5 cm below the top surface of the water. Uniform and nonuniform random background data were collected by imaging the bath containing 740 MBq (20 mCi) (99m)Tc. Nonuniformities were created by placing water-filled objects in the bath. Each signal dataset was collected by imaging a water-filled plastic sphere, injected with (99m)Tc and set at a specific depth (Z) in the bath. Data were collected for many signal diameters (D) (4, 7, 10, 13, 16, 28 mm) at 1 depth (5 cm) and for 1 signal diameter (10 mm) at several depths (1, 3, 5, 7, 9 cm). Sets of signal-present/signal-absent image pairs (380 pairs, 10(5) events per image) for known contrasts (C) were generated for use in ideal-observer studies in which the detectability (d') was calculated. Contrast-detail (log C vs. log D) plots were created. The theoretical simulation, developed for uniform backgrounds, provided data for comparison. RESULTS: The detectability increased linearly with C and decreased nonlinearly with decreasing D or increasing Z. The C required to achieve a specific d' increased sharply for D < SR. For C = 5, D = 10 mm, and d' = 1.2, the camera consistently detected signals for Z < 6 cm. Similar results were found for nonuniform backgrounds. The theoretical simulation verified the results for uniform backgrounds. CONCLUSION: The methodology presented here provides a way of evaluating gamma cameras on the basis of signal-detection performance for specified lesions, with particular application to scintimammography.

Algorithms↗

PubMed2002

Prototype results of a phase-shifting interferometer capable of measuring the complex index and profile of a test surface.

Results are presented from a prototype phase-shifting interferometer capable of measuring both the real and the imaginary part of the complex index of refraction and the surface profile of a test surface. The three parameters of interest are extracted from the measured data by maximum-likelihood estimation theory. The performance of the system is quantitatively assessed with Cramer-Rao lower bounds. The results are shown to be strongly dependent on the quantization of the interferograms from the 8-bit CCD camera, the incident electric field amplitude, and the relative amplitude and phase difference of each polarized component through each arm of the interferometer.

Equipment Design↗

PubMed2002

Estimation in medical imaging without a gold standard.

RATIONALE AND OBJECTIVES: In medical imaging, physicians often estimate a parameter of interest (eg, cardiac ejection fraction) for a patient to assist in establishing a diagnosis. Many different estimation methods may exist, but rarely can one be considered a gold standard. Therefore, evaluation and comparison of different estimation methods are difficult. The purpose of this study was to examine a method of evaluating different estimation methods without use of a gold standard. MATERIALS AND METHODS: This method is equivalent to fitting regression lines without the x axis. To use this method, multiple estimates of the clinical parameter of interest for each patient of a given population were needed. The authors assumed the statistical distribution for the true values of the clinical parameter of interest was a member of a given family of parameterized distributions. Furthermore, they assumed a statistical model relating the clinical parameter to the estimates of its value. Using these assumptions and observed data, they estimated the model parameters and the parameters characterizing the distribution of the clinical parameter. RESULTS: The authors applied the method to simulated cardiac ejection fraction data with varying numbers of patients, numbers of modalities, and levels of noise. They also tested the method on both linear and nonlinear models and characterized the performance of this method compared to that of conventional regression analysis by using x-axis information. Results indicate that the method follows trends similar to that of conventional regression analysis as patients and noise vary, although conventional regression analysis outperforms the method presented because it uses the gold standard which the authors assume is unavailable. CONCLUSION: The method accurately estimates model parameters. These estimates can be used to rank the systems for a given estimation task.

Diagnostic Imaging↗

PubMed2002

Objective comparison of quantitative imaging modalities without the use of a gold standard.

Imaging is often used for the purpose of estimating the value of some parameter of interest. For example, a cardiologist may measure the ejection fraction (EF) of the heart in order to know how much blood is being pumped out of the heart on each stroke. In clinical practice, however, it is difficult to evaluate an estimation method because the gold standard is not known, e.g., a cardiologist does not know the true EF of a patient. Thus, researchers have often evaluated an estimation method by plotting its results against the results of another (more accepted) estimation method, which amounts to using one set of estimates as the pseudogold standard. In this paper, we present a maximum-likelihood approach for evaluating and comparing different estimation methods without the use of a gold standard with specific emphasis on the problem of evaluating EF estimation methods. Results of numerous simulation studies will be presented and indicate that the method can precisely and accurately estimate the parameters of a regression line without a gold standard, i.e., without the x axis.

Analysis of Variance↗

PubMed2002

Quantitative analysis of acute myocardial infarct in rat hearts with ischemia-reperfusion using a high-resolution stationary SPECT system.

UNLABELLED: The purpose of this study was to develop an in vivo imaging protocol for a high-resolution stationary SPECT system, called FASTSPECT, in a rat heart model of ischemia-reperfusion (IR) and to compare 99mTc-sestamibi imaging and triphenyltetrazolium chloride (TTC) staining for reliability and accuracy in the measurement of myocardial infarcts. METHODS: FASTSPECT consists of 24 modular cameras and a 24-pinhole aperture with 1.5-mm spatial resolution and 13.3 cps/microCi (0.359 cps/kBq) sensitivity. The IR heart model was created by ligating the left coronary artery for 90 min and then releasing the ligature for 30 min. Two hours after 99mTc-sestamibi injection (5-10 mCi [185-370 MBq]), images were acquired for 5-10 min for 5 control rats and 11 IR rats. The hearts were excised, and the left ventricle was sectioned into 4 slices for TTC staining. RESULTS: Left and right ventricular myocardium in control rats was shown clearly, with uniform 99mTc-sestamibi distribution and 100% TTC staining for viable myocardium. Nine of 11 rats with IR survived throughout imaging and exhibited 50.8% +/- 2.7% ischemic area and 37.9% +/- 3.9% infarct in the left ventricle on TTC staining. The infarct size measured by FASTSPECT imaging was 37.6% +/- 3.6%, which correlated significantly with that measured by TTC staining (r = 0.974; P < 0.01). CONCLUSION: The results confirmed the accuracy of FASTSPECT imaging for measurement of acute myocardial infarcts in rat hearts. Application of FASTSPECT imaging in small animals may be feasible for investigating myocardial IR injury and the effects of revascularization.

Animals↗