Lung cancer screening using low dose CT has been shown to reduce lung cancer related mortality and been approved for widespread use in the US. These scans keep radiation doses low while maximizing the detection of suspicious lung lesions. Tube current modulation (TCM) is one technique used to optimize dose, however limited work has been done to assess TCM’s effect on detection tasks. In this work the effect of TCM on detection is investigated throughout the lung utilizing several different model observers (MO). 131 lung nodules were simulated at 1mm intervals in each lung of the XCAT phantom. A Sensation 64 TCM profile was generated for the XCAT phantom and 2500 noise realizations were created using both TCM and a fixed TC. All nodules and noise realizations were reconstructed for a total of 262 (left and right lungs) nodule reconstructions and 10 000 XCAT lung reconstructions. Single-slice Hotelling (HO) and channelized Hotelling (CHO) observers, as well as a multislice CHO were used to assess area-under-the-curve (AUC) as a function of nodule location in both the fixed TC and TCM cases. As expected with fixed TC, nodule detectability was lowest through the shoulders and leveled off below mid-lung; with TCM, detectability was unexpectedly highest through the shoulders, dropping sharply near the mid-lung and then increasing into the abdomen. Trends were the same for all model observers. These results suggest that TCM could be further optimized for detection and that detectability maps present exciting new opportunities for TCM optimization on a patient-specific level.
Lung cancer screening CT is already performed at low dose. There are many techniques to reduce the dose even further, but it is not clear how such techniques will affect nodule detectability. In this work, we used an in-house CAD algorithm to evaluate detectability. 90348 patients and their raw CT data files were drawn from the National Lung Screening Trial (NLST) database. All scans were acquired at ~2 mGy CTDIvol with fixed tube current, 1 mm slice thickness, and B50 reconstruction kernel on a Sensation 64 scanner (Siemens Healthcare). We used the raw CT data to simulate two additional reduced-dose scans for each patient corresponding to 1 mGy (50%) and 0.5 mGy (25%). Radiologists’ findings on the NLST reader forms indicated 65 nodules in the cohort, which we subdivided based on LungRADS criteria. For larger category 4 nodules, median sensitivities were 100% at all three dose levels, and mean sensitivity decreased with dose. For smaller nodules meeting the category 2 or 3 criteria, the dose dependence was less obvious. Overall, mean patient-level sensitivity varied from 38.5% at 100% dose to 40.4% at 50% dose, a difference of only 1.9%. However, the false-positive rate quadrupled from 1 per case at 100% dose to 4 per case at 25% dose. Dose reduction affected lung-nodule detectability differently depending on the LungRADS category, and the false-positive rate was very sensitive at sub-screening dose levels. Thus, care should be taken to adapt CAD for the very challenging noise characteristics of screening.
Purpose: To examine the potential for dose reduction in chest CT studies where lesion
volume is the primary output (e.g. in therapy-monitoring applications).
Methods: We added noise to the raw sinogram data from 15 chest exams with lung lesions
to simulate a series of reduced-dose scans for each patient. We reconstructed the
reduced-dose data on the clinical workstation and imported the resulting image series into
our quantitative imaging database for lesion contouring. One reader contoured the lesions
(one per patient) at the clinical reference dose (100%) and 8 simulated fractions of the
clinical dose (50, 25, 15, 10, 7, 5, 4, and 3%). Dose fractions were hidden from the reader
to reduce bias. We compared clinical and reduced-dose volumes in terms of bias error and
variability (4x the standard deviation of the percent differences).
Results: Averaging over all lesions, the bias error ranged from -0.6% to 10.6%. Variability
ranged from 92% at 3% of clinical dose to 54% at 50% of clinical dose. Averaging over
only the smaller lesions (<1cm equivalent diameter), bias error ranged from -9.2% to 14.1%
and variability ranged from 125% at 3% dose to 33.9% at 50% dose.
Conclusions: The reader’s variability decreased with dose, especially for smaller lesions.
However, these preliminary results are limited by potential recall bias, a small patient
cohort, and an overly-simplified task. Therapy monitoring often involves checking for new
lesions, which may influence the reader’s clinical dose threshold for acceptable
performance.
KEYWORDS: Convolution, Breast, Monte Carlo methods, X-rays, Sensors, Digital breast tomosynthesis, X-ray detectors, Quantum electronics, Computer simulations, Chest
For a rigorous x-ray imaging system optimization and evaluation, the need for exploring a large space of many
different system parameters is immense. However, due to the high dimensionality of the problem, it is often
infeasible to evaluate many system parameters in a laboratory setting. Therefore, it is useful to utilize computer
simulation tools and analytical methods to narrow down to a much smaller space of system parameters and
then validate the chosen optimal parameters by laboratory measurements. One great advantage of using the
simulation and analytical methods is that the impact of various sources of variability on the system's diagnostic
performance can be studied separately and collectively. Previously, we have demonstrated how to separate and
analyze noise sources using covariance decomposition in a task-based approach to the assessment of digital breast
tomosynthesis (DBT) systems in the absence of x-ray scatter and detector blur.1, 2 In this work, we analytically
extend the previous work to include x-ray scatter and detector blur. With use of computer simulation, we also
investigate the use of the convolution method for approximating the scatter images of structured phantoms
in comparison to those computed via Monte Carlo. The extended method is comprehensive and can be used
both for exploring a large parameter space in simulation and for validating optimal parameters, chosen from a
simulation study, with laboratory measurements.
Digital breast tomosynthesis (DBT) shows potential for improving breast cancer detection. However, this technique
has not yet been fully characterized with consideration of the various uncertainties in the imaging chain and
optimized with respect to system acquisition parameters. To obtain maximum diagnostic information in DBT,
system optimization needs to be performed across a range of patients and acquisition parameters to quantify their
impact on tumor detection performance. In addition, a balance must be achieved between x-ray dose and image
quality to minimize risk to the patient while maximizing the system's detection performance. To date, researchers
have applied a task-based approach to the optimization of DBT with use of mathematical observers for tasks in
the signal-known-exactly background-known-exactly (SKE/BKE) and signal-known-exactly background-known statistically
(SKE/BKS) paradigms1-3. However, previous observer models provided insufficient treatment of the
spatial correlations between multi-angle DBT projections, so we incorporated this correlation information into
the modeling methodology. We developed a computational approach that includes three-dimensional variable
background phantoms for incorporating background variability, accurate ray-tracing and Poisson distributions
for generating noise-free and noisy projections of the phantoms, and a channelized-Hotelling observer4 (CHO) for
estimating performance in DBT. We demonstrated our method for a DBT acquisition geometry and calculated
the performance of the CHO with Laguerre-Gauss channels as a function of the angular span of the system.
Preliminary results indicate that the implementation of a CHO model that incorporates correlations between
multi-angle projections gives different performance predictions than a CHO model that ignores multi-angle correlations.
With improvement of the observer design, we anticipate more accurate investigations into the impact
of multi-angle correlations and background variability on the performance of DBT.
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