The purpose of this study was to investigate the effect of dose on lesion detection and characterization in breast
tomosynthesis (BT), using human breast specimens. Images of 27 lesions in breast specimens were acquired on a BT
prototype based on a Mammomat Novation (Siemens) full-field digital mammography (FFDM) system. Two detector
modes - binned (2×1 in the scan direction) and full resolution - and four BT exposure levels - approximately 2×, 1.5×,
1×, and 0.5× the total mAs at the same beam quality as used in a single FFDM view with a Mammomat Novation unit
under automatic exposure control (AEC) conditions - were examined. The exposure for all BT scans was equally
divided among 25 projections. An enhanced filtered back projection reconstruction method was applied with a constant
filter setting. A human observer performance study was conducted in which the observers were forced to select the
minimum (threshold) exposure level at which each lesion could be both detected and characterized for assessment of
recall or not in a screening situation. The median threshold exposure level for all observers and all lesions corresponded
to approximately 1×, which is half the exposure of what we currently use for BT. A substantial variation in exposure
thresholds was noticed for different lesion types. For low contrast lesions with diffuse borders, an exposure threshold of
approximately 2× was required, whereas for spiculated high contrast lesions and lesions with well defined borders, the
exposure threshold was lower than 0.5×. The use of binned mode had no statistically significant impact on observer
performance compared to full resolution mode. There was no substantial difference between the modes for the detection
and characterization of the lesion types.
The purpose of this work was to develop a contrast-detail phantom that can be used to evaluate image quality in breast
tomosynthesis (BT) and as a first step use it to evaluate in-plane artifacts with respect to object size and contrast. The
phantom was constructed using a Polylite® resin as bulk material, as it has x-ray mass attenuation properties similar to
polymethyl methacrylate (PMMA), a common phantom material in mammography. Six different materials -
polyoxymethylene (POM), bakelite®, nylon, polycarbonate (PC), acrylonitrilebutadienestyrene (ABS) and polyethene
(PE) - were selected to form the phantom details. For each of the six materials, five spherical objects were manufactured
(diameters of 4, 8, 12, 16, and 20 mm) resulting in 30 objects that were embedded with their centres approximately
aligned at the central plane of a 26 mm thick Polylite® block (210 mm x 300 mm). A 20 mm thick PMMA block was
added to yield a phantom with attenuation properties similar to 45 mm PMMA that could simulate a so-called standard
breast (50 mm thick, 50% glandular tissue). Images of the phantom were acquired using a BT prototype system that
employs filtered backprojection for image reconstruction. The magnitude of the in-plane artifacts was evaluated and was
found to increase linearly with increasing contrast (signal) level and size of the embedded objects. The contrast-detail
phantom was found to be a useful tool for evaluating BT in-plane artifacts and might also be used to study out-of-plane
artifacts and the effect of different acquisition and reconstruction parameters on image quality in BT.
The purpose of this study was to determine how image quality in breast tomosynthesis (BT) is affected when acquisition
modes are varied, using human breast specimens containing malignant tumors and/or microcalcifications. Images of
thirty-one breast lumpectomy and mastectomy specimens were acquired on a BT prototype based on a Mammomat
Novation (Siemens) full-field digital mammography system. BT image acquisitions of the same specimens were
performed varying the number of projections, angular range, and detector signal collection mode (binned and nonbinned
in the scan direction). An enhanced filtered back projection reconstruction method was applied with constant
settings of spectral and slice thickness filters. The quality of these images was evaluated via relative visual grading
analysis (VGA) human observer performance experiments using image quality criteria. Results from the relative VGA
study indicate that image quality increases with number of projections and angular range. A binned detector collecting
mode results in less noise, but reduced resolution of structures. Human breast specimens seem to be suitable for
comparing image sets in BT with image quality criteria.
The purpose of this work was to evaluate and compare the visibility of tumors in digital mammography (DM) and breast tomosynthesis (BT) images. Images of the same women were acquired on both a DM system (Mammomat Novation, Siemens) and a BT prototype system adapted from the same type of DM system. Simulated 3D tumors (average dimension: 8.4 mm x 6.6 mm x 5 mm) were projected and added to each DM image as well as each BT projection image prior to 3D reconstruction. The same beam quality and approximately the same total absorbed dose were used for each breast image acquisition on both systems. Two simulated tumors were added to each of thirty breast scans, yielding sixty cases. A series of 4-alternative forced choice (4-AFC) human observer performance experiments were conducted in order to determine what projected tumor signal intensity in the DM images would be needed to achieve the same detectability as in the reconstructed BT images. Nine observers participated. For the BT experiment, when the tumor signal intensity on the central projection was 0.010 the mean percent of correct responses (PC) was measured to be 81.5%, which converted to a detectability index value (d') of 1.96. For the DM experiments, the same detectability was achieved at a signal intensity determined to be 0.038. Equivalent tumor detection in BT images were thus achieved at around four times less projected signal intensity than in DM images, indicating that the use of BT may lead to earlier detection of breast cancer.
Purpose: To determine how image quality linked to tumor detection is affected by reducing the absorbed dose to 50% and 30% of the clinical levels represented by an average glandular dose (AGD) level of 1.3 mGy for a standard breast according to European guidelines. Materials and methods: 90 normal, unprocessed images were acquired from the screening department using a full-field digital mammography (FFDM) unit Mammomat Novation (Siemens). Into 40 of these, one to three simulated tumors were inserted per image at various positions. These tumors represented irregular-shaped malignant masses. Dose reduction was simulated in all 90 images by adding simulated quantum noise to represent images acquired at 50% and 30% of the original dose, resulting in 270 images, which were subsequently processed for final display. Four radiologists participated in a free-response receiver operating characteristics (FROC) study in which they searched for and marked suspicious positions of the masses as well as rated their degree of suspicion of occurrence on a one to four scale. Using the jackknife FROC (JAFROC) method, a score between 0 and 1 (where 1 represents best performance), referred to as a figure-of-merit (FOM), was calculated for each dose level. Results: The FOM was 0.73, 0.70, and 0.68 for the 100%, 50% and 30% dose levels, respectively. Using Analysis of the Variance (ANOVA) to test for statistically significant differences between any two of the three FOMs revealed that they were not statistically distinguishable (p-value of 0.26). Conclusion: For the masses used in this experiment, there was no significant change in detection by increasing quantum noise, thus indicating a potential for dose reduction.
The purpose of this work was to study how the pixel size of digital detectors can affect shape determination of microcalcifications in mammography. Screen-film mammograms containing microcalcifications clinically proven to be indicative of malignancy were digitised at 100 lines/mm using a high-resolution Tango drum scanner. Forty microcalcifications were selected to cover an appropriate range of sizes, shapes and contrasts typically found of malignant cases. Based on the measured MTF and NPS of the combined screen-film and scanner system, these digitised images were filtered to simulate images acquired with a square sampling pixel size of 10 μm x 10 μm and a fill factor of one. To simulate images acquired with larger pixel sizes, these finely sampled images were re-binned to yield a range of effective pixel sizes from 20 μm up to 140 μm. An alternative forced-choice (AFC) observer experiment was conducted with eleven observers for this set of digitised microcalcifications to determine how pixel size affects the ability to discriminate shape. It was found that observer score increased with decreasing pixel size down to 60 μm (p<0.01), at which point no significant advantage was obtained by using smaller pixel sizes due to the excessive relative noise-per-pixel. The relative gain in shape discrimination ability at smaller pixel sizes was larger for microcalcifications that were smaller than 500 μm and circular.
Thirty images with added simulated pathological lesions at two different dose levels (100% and 10% dose) were evaluated with the free-response forced error experiment by nine experienced radiologists. The simulated pathological lesions present in the images were classified according to four different parameters: the position within the lumbar spine, possibility to perform a symmetrical (left-right) comparison, the lesion contrast, and the complexity of the surrounding background where the lesion was situated. The detectability of each lesion was calculated as the fraction of radiologists who successfully detected the lesion before a false positive error was made. The influence of each of the four parameters on lesion detectability was investigated. The results of the study show that the influence of lesion contrast is the most important factor for detectability. Since the dose level had a limited effect on detectability, large dose savings can be made without reducing the detectability of pathological lesions in lumbar spine radiography.