KEYWORDS: Digital breast tomosynthesis, Breast cancer, Breast, Cancer, Cancer detection, Mammography, Tomosynthesis, Tumors, Image compression, Artificial intelligence
PurposeThe purpose is to describe the Malmö Breast Tomosynthesis Screening Project from the beginning to where we are now, and thoughts for the future.ApproachIn two acts, we describe the efforts made by our research group to improve breast cancer screening by introducing digital breast tomosynthesis (DBT), all the way from initial studies to a large prospective population-based screening trial and beyond.ResultsOur studies have shown that DBT has significant advantages over digital mammography (DM), the current gold standard method for breast cancer screening in Europe, in many aspects except a major one—the increased radiologist workload introduced with DBT compared with DM. It is foreseen that AI could be a viable solution to overcome this problem.ConclusionsWe have proved that one-view DBT is a highly efficient screening approach with respect to diagnostic performance.
Mechanical imaging (MI) uses a pressure sensor array to estimate the stiffness of lesions. Recent clinical studies have suggested that MI combined with digital mammography may reduce false positive findings and negative biopsies by over 30%. Digital breast tomosynthesis (DBT) has been adopted progressively in cancer screening. The tomographic nature of DBT improves lesion visibility by reducing tissue overlap in reconstructed images. For maximum benefit, DBT and MI data should be acquired simultaneously; however, that arrangement produces visible artifacts in DBT images due to the presence of the MI sensor array. We propose a method for reducing artifacts during the DBT image reconstruction. We modified the parameters of a commercial DBT reconstruction engine and investigated the conspicuity of artifacts in the resultant images produced with different sensor orientations. The method was evaluated using a physical anthropomorphic phantom imaged on top of the sensor. Visual assessment showed a reduction of artifacts. In a quantitative test, we calculated the artifact spread function (ASF), and compared the ratio of the mean ASF values between the proposed and conventional reconstruction (termed ASF ratio, RASF). We obtained a mean RASF of 2.74, averaged between two analyzed sensor orientations (45° and 90°). The performance varied with the orientation and the type of sensor structures causing the artifacts. RASF for wide connection lines was larger at 45° than at 90° (5.15 vs. 1.00, respectively), while for metallic contacts RASF was larger at 90° than at 45° (3.31 vs. 2.21, respectively). Future work will include a detailed quantitative assessment, and further method optimization in virtual clinical trials.
Due to variations in amount and distribution of glandular breast tissue among women, the mean glandular dose (MGD) can be a poor measure of the individual glandular dose. Therefore, to improve the basis for risk assessment related to radiation dose from breast X-ray examinations, the distribution should be considered. Breast tomosynthesis (BT) is an imaging technique that may be used as an alternative or complement to standard mammography in breast cancer screening, and it could provide the required 3D-localisation of glandular tissue for estimation of the individual glandular dose. In this study, we investigated the possibility to localize glandular tissue from BT data and use a Monte Carlo simulation routine to estimate the glandular dose for software breast phantoms with different amount and distribution of glandular breast tissue. As an initial evaluation of the method, the local energy absorption in glandular tissue was estimated for seven breast phantoms and the corresponding phantoms recreated from reconstructed BT data. As expected, the normalized glandular dose was found to differ substantially with glandular distribution. This emphasizes the importance of glandular tissue localization for estimation of the individual glandular dose. The results showed good accuracy for estimation of normalized glandular dose using breast phantoms recreated from reconstructed BT image volumes (relative differences between –7.3% and +9.5%). Following this initial study, the method will be evaluated for more phantoms and potentially developed for patient cases. In the future it could become a useful tool in breast dosimetry as a step towards the individual glandular dose.
The large image volumes in breast tomosynthesis (BT) have led to large amounts of data and a heavy workload for breast radiologists. The number of slice images can be decreased by combining adjacent image planes (slabbing) but the decrease in depth resolution can considerably affect the detection of lesions. The aim of this work was to assess if thicker slabbing of the outer slice images (where lesions seldom are present) could be a viable alternative in order to reduce the number of slice images in BT image volumes. The suggested slabbing (an image volume with thick outer slabs and thin slices between) were evaluated in two steps. Firstly, a survey of the depth of 65 cancer lesions within the breast was performed to estimate how many lesions would be affected by outer slabs of different thicknesses. Secondly, a selection of 24 lesions was reconstructed with 2, 6 and 10 mm slab thickness to evaluate how the appearance of lesions located in the thicker slabs would be affected. The results show that few malignant breast lesions are located at a depth less than 10 mm from the surface (especially for breast thicknesses of 50 mm and above). Reconstruction of BT volumes with 6 mm slab thickness yields an image quality that is sufficient for lesion detection for a majority of the investigated cases. Together, this indicates that thicker slabbing of the outer slice images is a promising option in order to reduce the number of slice images in BT image volumes.
Software breast phantoms are increasingly seeing use in preclinical validation of breast image acquisition systems and image analysis methods. Phantom realism has been proven sufficient for numerous specific validation tasks. A challenge is the generation of suitably realistic small-scale breast structures that could further improve the quality of phantom images. Power law noise follows the noise power characteristics of breast tissue, but may not sufficiently represent certain (e.g., non-Gaussian) properties seen in clinical breast images. The purpose of this work was to investigate the utility of fractal Perlin noise in generating more realistic breast tissue through investigation of its power spectrum and visual characteristics. Perlin noise is an algorithm that creates smoothly varying random structures of an arbitrary frequency. Through the use of a technique known as fractal noise or fractional Brownian motion (fBm), octaves of noise with different frequency are combined to generate coherent noise with a broad frequency range. fBm is controlled by two parameters – lacunarity and persistence – related to the frequency and amplitude of successive octaves, respectively. Average noise power spectra were calculated and beta parameters estimated in sample volumes of fractal Perlin noise with different combinations of lacunarity and persistence. Certain combinations of parameters resulted in noise volumes with beta values between 2 and 3, corresponding to reported measurements in real breast tissue. Different combinations of parameters resulted in different visual appearances. In conclusion, Perlin noise offers a flexible tool for generating breast tissue with realistic properties.
KEYWORDS: Monte Carlo methods, Visibility, Breast, Tissues, Spatial resolution, Statistical analysis, Ray tracing, Signal to noise ratio, Sensors, Mammography
Microcalcifications are one feature of interest in mammography and breast tomosynthesis (BT). To achieve optimal conditions for detection of microcalcifications in BT imaging, different acquisition geometries should be evaluated. The purpose of this work was to investigate the influence of acquisition schemes with different angular ranges, projection distributions and dose distributions on the visibility of microcalcifications in reconstructed BT volumes.
Microcalcifications were inserted randomly in a high resolution software phantom and a simulation procedure was used to model a MAMMOMAT Inspiration BT system. The simulation procedure was based on analytical ray tracing to produce primary images, Monte Carlo to simulate scatter contributions and flatfield image acquisitions to model system characteristics. Image volumes were reconstructed using the novel method super-resolution reconstruction with statistical artifact reduction (SRSAR). For comparison purposes, the volume of the standard acquisition scheme (50° angular range and uniform projection and dose distribution) was also reconstructed using standard filtered backprojection (FBP).
To compare the visibility and depth resolution of the microcalcifications, signal difference to noise ratio (SDNR) and artifact spread function width (ASFW) were calculated. The acquisition schemes with very high central dose yielded significantly lower SDNR than the schemes with more uniform dose distributions. The ASFW was found to decrease (meaning an increase in depth resolution) with wider angular range. In conclusion, none of the evaluated acquisition schemes were found to yield higher SDNR or depth resolution for the simulated microcalcifications than the standard acquisition scheme.
Purpose: To investigate detection performance for calcification clusters in reconstructed digital breast tomosynthesis (DBT) slices at different dose levels using a Super Resolution and Statistical Artifact Reduction (SRSAR) reconstruction method. Method: Simulated calcifications with irregular profile (0.2 mm diameter) where combined to form clusters that were added to projection images (1-3 per abnormal image) acquired on a DBT system (Mammomat Inspiration, Siemens). The projection images were dose reduced by software to form 35 abnormal cases and 25 normal cases as if acquired at 100%, 75% and 50% dose level (AGD of approximately 1.6 mGy for a 53 mm standard breast, measured according to EUREF v0.15). A standard FBP and a SRSAR reconstruction method (utilizing IRIS (iterative reconstruction filters), and outlier detection using Maximum-Intensity Projections and Average-Intensity Projections) were used to reconstruct single central slices to be used in a Free-response task (60 images per observer and dose level). Six observers participated and their task was to detect the clusters and assign confidence rating in randomly presented images from the whole image set (balanced by dose level). Each trial was separated by one weeks to reduce possible memory bias. The outcome was analyzed for statistical differences using Jackknifed Alternative Free-response Receiver Operating Characteristics. Results: The results indicate that it is possible reduce the dose by 50% with SRSAR without jeopardizing cluster detection. Conclusions: The detection performance for clusters can be maintained at a lower dose level by using SRSAR reconstruction.
This study aimed to investigate whether decreasing the amount of slices in breast tomosynthesis (BT) image volumes reduce reading time. BT slices were combined into so-called slabs, by reconstructing thin slices and merging them into thicker slabs. Sets of slabs where created from 35 clinical BT volumes with malignant or benignant findings and from 50 BT volumes drawn from screening sets (without any prior review). The image sets were reviewed in two separate sessions while the review time was recorded. A total of five experienced radiologists were employed for the image review. Additionally a VGA study was performed to compare slabbed images with the originals in order to ensure that the image quality was not significantly degraded. One set of 27 pathological cases (13 masses and 14 microcalcification clusters) and one of 22 subtle lesions that had been missed on digital mammography but detected on BT were presented to an experienced radiologist and 2 medical physicists who rated the quality of the slabbed versions relative to the originals. The study could find no significant degradation in image quality when using 2 mm slabs instead of 1 mm slices. There was no significant decrease in reading time on clinical cases (P = .133), but on screening images there was a significant decrease of 7.7 ± 9.6 s from an average level of 32.2 ± 14.5 s (P < .0001). This suggests that increasing slab thickness can reduce the time radiologists spend studying normal images by 20%.
Purpose: To investigate the visibility of single spiculations in digital breast tomosynthesis (DBT). Method: Simulated spheres (6 mm diameter) with single spiculations were added to projection images acquired on a DBT system (MAMMOMAT Inspiration, Siemens). The spiculations had a cylindrical shape and were randomly, diagonally aligned (at four different positions: ± π/4 or ± 3π/4) at a plane parallel to the detector. They were assumed to consist of a fibroglandular tissue composition. The length of the spiculations was 5 mm while the diameter varied (0.12 – 0.28 mm). Reconstructed central slices of the lesion, separated by insertion in fatty or dense breasts (100 images in each), were used in 4-alternative forced choice (4AFC) human observer experiments. Three different reconstructions were used: filtered back projection (FBP) with 1 mm thick slices and a statistical artifact reduction reconstruction (SAR) method generating 1 and 2 mm thick slices. Five readers participated and their task was to locate the spiculation in randomly presented images from the whole image set (4 diameters × 100 images). The percent correct (PC) decision was determined in both fat and dense tissue for all spiculation diameters and reconstructions. Results: At a PC level of 95% the required diameter was about 0.17 – 0.22 mm in dense tissue, and 0.18 – 0.26 mm in fatty tissue (depending upon reconstruction). Conclusions: SAR was found to be a promising alternative to FBP. The visibility of single spiculations was determined. The required diameter depends on both tissue composition and reconstruction.
The pressure distribution over a compressed breast is in general heterogeneous. In this study we investigated the pressure
distribution over compressed breasts with tumor masses. Twenty-two women either recalled for work-up of findings suspicious for breast cancer in the screening program or with clinically suspected findings were included in the study.
Twenty-one lesions turned out to be malignant and one benign. The distribution of compression pressure was measured using thin FSR (Force Sensing Resistor) pressure sensors attached to the compression plate. The pressure over the breast
was ascertained by acquiring an x-ray image of the compressed breast with the pressure sensors present. The pressure
data and the mammogram were used to create a composite image with pressure data displayed as a color overlay. The
malignant tumor area generally matched an elevated pressure area and this pressure was generally higher than the
pressure over surrounding parenchyma. In 11 out of 22 (50%) subjects the maximum pressure over the breast was located over the tumor. Only 4 out of 22 (18%) masses had a lower tumor mean pressure compared to the mean pressure
over the breast (including one small < 10 mm tumor and one benign structure). The results suggest that tumors are
stiffer, thus, absorbing more pressure compared to the surrounding parenchyma and that this property can be quantified.
Refined pressure techniques could possibly be used to demonstrate the relative elasticity distribution in breast tissue, which might provide valuable differential diagnostic information.
In breast tomosynthesis (BT) a number of 2D projection images are acquired from different angles along a limited arc.
The imaged breast volume is reconstructed from the projection images, providing 3D information. The purpose of the
study was to investigate and optimize different reconstruction methods for BT in terms of image quality using human
observers viewing clinical cases. Sixty-six cases with suspected masses and calcifications were collected from 55
patients.
KEYWORDS: 3D modeling, Signal detection, Performance modeling, Image filtering, Breast, 3D image processing, 3D acquisition, Medical imaging, Image segmentation, Digital filtering
The development of model observers for mimicking human detection strategies has followed from symmetric signals in
simple noise to increasingly complex backgrounds. In this study we implement different model observers for the
complex task of detecting a signal in a 3D image stack. The backgrounds come from real breast tomosynthesis
acquisitions and the signals were simulated and reconstructed within the volume. Two different tasks relevant to the
early detection of breast cancer were considered: detecting an 8 mm mass and detecting a cluster of microcalcifications.
The model observers were calculated using a channelized Hotelling observer (CHO) with dense difference-of-Gaussian
channels, and a modified (Partial prewhitening [PPW]) observer which was adapted to realistic signals which are not
circularly symmetric. The sustained temporal sensitivity function was used to filter the images before applying the
spatial templates. For a frame rate of five frames per second, the only CHO that we calculated performed worse than the
humans in a 4-AFC experiment. The other observers were variations of PPW and outperformed human observers in
every single case. This initial frame rate was a rather low speed and the temporal filtering did not affect the results
compared to a data set with no human temporal effects taken into account. We subsequently investigated two higher
speeds at 5, 15 and 30 frames per second. We observed that for large masses, the two types of model observers
investigated outperformed the human observers and would be suitable with the appropriate addition of internal noise.
However, for microcalcifications both only the PPW observer consistently outperformed the humans. The study
demonstrated the possibility of using a model observer which takes into account the temporal effects of scrolling through
an image stack while being able to effectively detect a range of mass sizes and distributions.
The purpose of this study was to evaluate four different viewing procedures as part of improving viewing conditions of
breast tomosynthesis (BT) image volumes. The procedures consisted of free scroll volume browsing, and a combination
of initial cine loops at three different frame rates (9, 14 and 25 fps) terminated upon request followed by free scroll
volume browsing. Fifty-five normal BT image volumes in MLO view were collected. In these, simulated lesions (20
masses and 20 clusters of microcalcifications) were randomly inserted, creating four unique image sets for each
procedure. Four readers interpreted the cases in a random order. Their task was to locate a lesion, mark and assign a five
level confidence scale. The diagnostic accuracy was analyzed using Jackknife Free Receiver Operating Characteristics
(JAFROC). Time efficiency and visual search behavior were also investigated using eye tracking. The results indicate
that there was no statistically significant difference in JAFROC FOM between the different viewing procedures,
however the medium cine loop speed seemed to be the preferred viewing procedure in terms of total analyze time and
dwell time.
Our aim was to evaluate if there is a benefit in diagnostic accuracy and efficiency of viewing breast tomosynthesis (BT)
image volumes presented horizontally oriented, but also to evaluate the use of a systematic search strategy where the
breast is divided, and analyzed consecutively, into two sections. These image presentations were compared to regular
vertical image presentation. All methods were investigated using viewing procedures consisting of free scroll volume
browsing, and a combination of initial cine loops at three different frame rates (9, 14, 25 fps) terminated upon request
followed by free scroll volume browsing if needed. Fifty-five normal BT image volumes in MLO view were collected.
In these, simulated lesions (20 masses and 20 clusters of microcalcifications) were randomly inserted, creating four
unique image sets for each procedure. Four readers interpreted the cases in a random order. Their task was to locate the
lesions, mark and assign a five level confidence scale. The diagnostic accuracy was analyzed using Jackknife Free
Receiver Operating Characteristics (JAFROC). Time efficiency and visual search behavior were also investigated using
eye tracking. Results indicate there was no statistically significant difference in JAFROC FOM between the different
image presentations, although visual search was more time efficient when viewing horizontally oriented image volumes
in medium cine loops.
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.
Breast tomosynthesis is currently an investigational imaging technique requiring optimization of its many combinations
of data acquisition and image reconstruction parameters for optimum clinical use. In this study, the effects of several
acquisition parameters on the visual conspicuity of diagnostic features were evaluated for three breast specimens using a
visual discrimination model (VDM). Acquisition parameters included total exposure, number of views, full resolution
and binning modes, and lag correction. The diagnostic features considered in these specimens were mass margins,
microcalcifications, and mass spicules. Metrics of feature contrast were computed for each image by defining two
regions containing the selected feature (Signal) and surrounding background (Noise), and then computing the difference
in VDM channel metrics between Signal and Noise regions in units of just-noticeable differences (JNDs). Scans with
25 views and exposure levels comparable to a standard two-view mammography exam produced higher levels of feature
contrast. The effects of binning and lag correction on feature contrast were found to be generally small and isolated,
consistent with our visual assessments of the images. Binning produced a slight loss of spatial resolution which could
be compensated in the reconstruction filter. These results suggest that good image quality can be achieved with the
faster and therefore more clinically practical 25-view scans with binning, which can be performed in as little as 12.5
seconds. Further work will investigate other specimens as well as alternate figures of merit in order to help determine
optimal acquisition and reconstruction parameters for clinical trials.
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.
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