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- Bornefalk, Hans
(författare)
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Implications of unchanged detection criteria with CAD as second reader of mammograms
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:4, s. 922-929
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we address the use of computer-aided detection (CAD) systems as second readers in mammography. The approach is based on Bayesian decision theory and its implication for the choice of optimal operating points. The choice of a certain operating point along an ROC curve corresponds to a particular tradeoff between false positives and missed cancers. By minimizing a total risk function given this tradeoff, we determine optimal decision thresholds for the radiologist and CAD system when CAD is used as a second reader. We show that under very general circumstances, the performance of the sequential system is improved if the decision threshold of the latent human decision variable is increased compared to what it would have been in the absence of the CAD system. This means that an initial stricter decision criterion should be applied by the radiologist when CAD is used as a second reader than otherwise. First and foremost, the results in this paper should be interpreted qualitatively, but an attempt is made at quantifying the effect by tuning the model to a prospective study evaluating the use of CAD as a second reader. By making some necessary and plausible assumptions, we are able to estimate the effect of the resulting suboptimal operating point. In this study of 12 860 women, we estimate that a 15% reduction in callbacks for masses could have been achieved with only about a 1.5% relative decrease in sensitivity compared to that without using a stricter initial criterion by the radiologist. For microcalcifications the corresponding values are 7% and 0.2%. (c) 2006 American Association of Physicists in Medicine.
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| 2. |
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| 3. |
- Bornefalk, Hans, et al.
(författare)
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On the comparison of FROC curves in mammography CAD systems
- 2005
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 32:2, s. 412-417
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel method for assessing the performance of computer-aided detection systems on unseen cases at a given sensitivity level.
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| 5. |
- Bornefalk, Hans, 1974-
(författare)
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Task-based weights for photon counting spectral x-ray imaging
- 2011
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 38:11, s. 6065-6073
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To develop a framework for taking the spatial frequency composition of an imaging taskinto account when determining optimal bin weight factors for photon counting energy sensitivex-ray systems. A second purpose of the investigation is to evaluate the possible improvement comparedto using pixel based weights.Methods: The Fourier based approach of imaging performance and detectability index d0 is appliedto pulse height discriminating photon counting systems. The dependency of d0 on the bin weightfactors is made explicit, taking into account both differences in signal and noise transfer characteristicsacross bins and the spatial frequency dependency of interbin correlations from reabsorbedscatter. Using a simplified model of a specific silicon detector, d0 values for a high and a low frequencyimaging task are determined for optimal weights and compared to pixel based weights.Results: The method successfully identifies bins where a large point spread function degradesdetection of high spatial frequency targets. The method is also successful in determining how todownweigh highly correlated bins. Quantitative predictions for the simplified silicon detectormodel indicate that improvements in the detectability index when applying task-based weightsinstead of pixel based weights are small for high frequency targets, but could be in excess of 10%for low frequency tasks where scatter-induced correlation otherwise degrade detectability.Conclusions: The proposed method makes the spatial frequency dependency of complex correlationstructures between bins and their effect on the system detective quantum efficiency easier toanalyze and allows optimizing bin weights for given imaging tasks. A potential increase in detectabilityof double digit percents in silicon detector systems operated at typical CT energies (100kVp) merits further evaluation on a real system. The method is noted to be of higher relevancefor silicon detectors than for cadmium (zink) telluride detectors.
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| 6. |
- Bornefalk, Hans
(författare)
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XCOM intrinsic dimensionality for low-Z elements at diagnostic energies
- 2012
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 39:2, s. 654-657
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To determine the intrinsic dimensionality of linear attenuation coefficients (LACs) from XCOM for elements with low atomic number (Z = 1-20) at diagnostic x-ray energies (25-120 keV). H-0(q), the hypothesis that the space of LACs is spanned by q bases, is tested for various q-values. Methods: Principal component analysis is first applied and the LACs are projected onto the first q principal component bases. The residuals of the model values vs XCOM data are determined for all energies and atomic numbers. Heteroscedasticity invalidates the prerequisite of i.i.d. errors necessary for bootstrapping residuals. Instead wild bootstrap is applied, which, by not mixing residuals, allows the effect of the non-i.i.d residuals to be reflected in the result. Credible regions for the eigenvalues of the correlation matrix for the bootstrapped LAC data are determined. If subsequent credible regions for the eigenvalues overlap, the corresponding principal component is not considered to represent true data structure but noise. If this happens for eigenvalues l and l + 1, for any l <= q, H-0(q) is rejected. Results: The largest value of q for which H-0(q) is nonrejectable at the 5%-level is q = 4. This indicates that the statistically significant intrinsic dimensionality of low-Z XCOM data at diagnostic energies is four. Conclusions: The method presented allows determination of the statistically significant dimensionality of any noisy linear subspace. Knowledge of such significant dimensionality is of interest for any method making assumptions on intrinsic dimensionality and evaluating results on noisy reference data. For LACs, knowledge of the low-Z dimensionality might be relevant when parametrization schemes are tuned to XCOM data. For x-ray imaging techniques based on the basis decomposition method (Alvarez and Macovski, Phys. Med. Biol. 21, 733-744, 1976), an underlying dimensionality of two is commonly assigned to the LAC of human tissue at diagnostic energies. The finding of a higher statistically significant dimensionality thus raises the question whether a higher assumed model dimensionality (now feasible with the advent of multibin x-ray systems) might also be practically relevant, i.e., if better tissue characterization results can be obtained.
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| 7. |
- Persson, Mats, 1987-, et al.
(författare)
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Upper limits of the photon fluence rate on CT detectors : Case study on a commercial scanner
- 2016
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Ingår i: Medical physics (Lancaster). - : AMER ASSOC PHYSICISTS MEDICINE AMER INST PHYSICS. - 0094-2405. ; 43:7, s. 4398-4411
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The highest photon fluence rate that a computed tomography (CT) detector must be able to measure is an important parameter. The authors calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest, and abdomen protocols. Methods: The authors scanned an anthropomorphic phantom (Kyoto Kagaku PBU-60) with the reference CT protocols provided by AAPM on a GE LightSpeed VCT scanner and noted the tube current applied with the tube current modulation (TCM) system. By rescaling this tube current using published measurements on the tube current modulation of a GE scanner [N. Keat, "CT scanner automatic exposure control systems," MHRA Evaluation Report 05016, ImPACT, London, UK, 2005], the authors could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gated chest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, the authors simulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner and calculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentiles of the transmitted fluence rate distribution behind the patient are calculated and the effect of omitting projection lines passing just below the skin line is investigated. Results: The highest transmitted fluence rates on the detector for the AAPM reference protocols with centered patients are found for head images and for intermediate-sized chest images, both with a maximum of 3.4 . 10(8) mm(-2) s-1, at 949 mm distance from the source. Miscentering the head by 50 mm downward increases the maximum transmitted fluence rate to 5.7 . 10(8) mm(-2) s(-1). The ECG gated chest protocol gives fluence rates up to 2.3 . 10(8)-3.6 . 10(8) mm(-2) s(-1) depending on miscentering. Conclusions: The fluence rate on a CT detector reaches 3 . 10(8)-6 . 10(8) mm(-2) s(-1) in standard imaging protocols, with the highest rates occurring for ECG gated chest and miscentered head scans. These results will be useful to developers of CT detectors, in particular photon counting detectors. (C) 2016 American Association of Physicists in Medicine.
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