Hannie Petersson

Application of the fractal Perlin noise algorithm for the generation of simulated breast tissue

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.

Monte Carlo simulation of breast tomosynthesis: visibility of microcalcifications at different acquisition schemes

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.

Can mechanical imaging increase the specificity of mammography screening

AbstractObjectivesThis study aimed to investigate the effects of adding adjunct mechanical imaging to mammography breast screening. We hypothesized that mechanical imaging could detect increased local pressure caused by both malignant and benign breast lesions and that a pressure threshold for malignancy could be established. The impact of this on breast screening was investigated with regard to reductions in recall and biopsy rates.Methods155 women recalled from breast screening were included in the study, which was approved by the regional ethical review board (dnr 2013/620). Mechanical imaging readings were acquired of the symptomatic breast. The relative mean pressure on the suspicious area (RMPA) was defined and a threshold for malignancy was established.ResultsBiopsy-proven invasive cancers had a median RMPA of 3.0 (interquartile range (IQR) = 3.7), significantly different from biopsy-proven benign at 1.3 (IQR = 1.0) and non-biopsied cases at 1.0 (IQR = 1.3) (P < 0.001). The lowest RMPA for invasive cancer was 1.4, with 23 biopsy-proven benign and 33 non-biopsied cases being below this limit. Had these women not been recalled, recall rates would have been reduced by 36% and biopsy rates by 32%.ConclusionsIf implemented in a screening situation, this may substantially lower the number of false positives.Key Points• Mechanical imaging is used as an adjunct to mammography in breast screening. • A threshold pressure can be established for malignant breast cancer. • Recalls and biopsies can be substantially reduced.

VOLUMETRIC LOCALISATION OF DENSE BREAST TISSUE USING BREAST TOMOSYNTHESIS DATA

This study attempted to use combined data from reconstructed digital breast tomosynthesis (DBT) volumes and density estimation of projection images to localise dense tissue inside the breast, using the assumption that the breast can be treated as consisting of only two types of tissue: fibroglandular (dense) and adipose (fatty). To be able to verify results, software breast phantoms generated using fractal Perlin noise were employed. Projection images were created using the PENELOPE Monte Carlo package. Dense tissue volume was estimated from the central projection image. The density image was used to determine the number of dense voxels at each pixel location, which were then placed using the DBT image as a template. The method proved capable of accurately determining the composition of 75±5 % of voxels.

VALIDATION OF A SIMULATION PROCEDURE FOR GENERATING BREAST TOMOSYNTHESIS PROJECTION IMAGES

In order to achieve optimal diagnostic performance in breast tomosynthesis (BT) imaging, the parameters of the imaging chain should be evaluated. For the purpose of such evaluations, a simulation procedure based on the Monte Carlo code system Penelope and the geometry of a Siemens BT system has been developed to generate BT projection images. In this work, the simulation procedure is validated by comparing contrast and sharpness in simulated images with contrast and sharpness in real images acquired with the BT system. The results of the study showed a good agreement of sharpness in real and simulated reconstructed image planes, but the contrast was shown to be higher in the simulated compared with the real projection images. The developed simulation procedure could be used to generate BT images, but it is of interest to further investigate how the procedure could be modified to generate more realistic image noise and contrast.

Breast Density Assessment Using Breast Tomosynthesis Images

In this work we evaluate an approach for breast density assessment of digital breast tomosynthesis DBT data using the central projection image. A total of 348 random cases both FFDM CC and MLO views and DBT MLO views were collected using a Siemens Mammomat Inspiration tomosynthesis unit at Unilabs, Malmo. The cases underwent both BI-RADS 5th Edition labeling by radiologists and automated volumetric breast density analysis VBDA by an algorithm. Preliminary results showed an observed agreement of 70i¾?% weighted Kappa, i¾?i¾?=i¾?0.73 between radiologists and VBDA using FFDM images and 63i¾?% i¾?i¾?=i¾?0.62 for radiologists and VBDA using DBT images. Comparison between densities for FFDM and DBT resulted in high correlation ri¾?=i¾?0.94 and an observed agreement of 72i¾?% i¾?i¾?=i¾?0.76. The automated analysis is a promising approach using low dose central projection DBT images in order to get radiologist-like density ratings similar to results obtained from FFDM.

Detection of calcification clusters in digital breast tomosynthesis slices at different dose levels utilizing a SRSAR reconstruction and JAFROC

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.

Towards determination of individual glandular dose

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.

BEAM QUALITY CORRECTION FACTORS FOR KAP METERS FOR LIGHTLY AND HEAVILY FILTERED X-RAY BEAMS

Kerma-area product (KAP) meters have a pronounced energy dependence when measuring air KAP for lightly filtered X-ray beams (RQR). Today, it is also common with more heavily filtered beams. In this work, the energy dependence for lightly as well as heavily filtered beams (RQC) was investigated for several KAP meter models. The relative energy dependence of the readings of an external and an internal KAP meter was determined relative to an ionisation chamber, which had been calibrated at the primary standards laboratory. As a complement to the measurements, the sensitivity of a KAP meter for various X-ray beam qualities was modelled using Monte Carlo simulations of photon transport and absorption. The result showed a variation in relative energy dependence of up to 30 % for KAP meters for RQC beam qualities compared with RQR qualities. A reduced sensitivity of KAP meters for heavily filtered beams in comparison with lightly filtered ones was found, and it is important that the beam-specific radiation quality correction factors are applied to correct the registered KAP values.

Evaluation of the possibility to use thick slabs of reconstructed outer breast tomosynthesis slice images

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.