Abstract:
The present invention relates to systems and methods for medical imaging. Digital images are processed to provide phase images of a region of interest to aid in the diagnosis and treatment of various conditions. A preferred embodiment of the invention provides improved mammography screening for cancerous or precancerous conditions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Provisional Patent Application No. 60/648,637 filed Jan. 28, 2005. The entire contents of the above application is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002]     This invention was supported, in whole or in part, by grant 1R21CA89673-01A1 from The National Institutes for Health. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     There are numerous features in the human body in which medical imaging techniques can be used effectively to assist in the diagnosis and treatment of medical conditions. Various diseases or conditions involve calcified materials, structures or deposits within the human body that are indicative of the medical condition of the patient. These can include features of the human skeleton, such as the spine, calcified deposits within the arterial system such as obstructions to blood flow in the coronary arteries, or microcrystalline deposits in breast tissue that can become cancerous. Breast cancer, for example, is one of the most frequently diagnosed malignancies and the second largest cause of cancer deaths in American females. Several improvements in diagnostic protocols have enhanced our ability for earlier detection of breast cancer, resulting in improvement of therapeutic outcome and an increased survival rate for breast cancer victims. Triple assessment is involved in identification of breast cancer. They are (1) clinical examination, (2) radiological assessment using mammography or ultrasound for example and, (3) pathological assessment using cytology or biopsy.  
         [0004]     Although an impressive array of body-imaging techniques, such as x-ray imaging, x-ray computed tomography, magnetic resonance imaging, thermal infrared imaging (TIR), ultrasound, and radioisotope imaging are currently available to yield useful information, there are important limitations of safety, resolution, cost, and lack or limited specificity to key chemicals or structures necessary for functional body monitoring.  
         [0005]     On the other hand, x-ray mammography, the current standard for monitoring breast cancer, has been shown to be effective in screening asymptomatic women to detect breast cancers. Abnormalities detected in mammography are classified as: Spiculated masses, Stellate lesions, Circumscribed masses, and microcalcification. Mammography is extremely useful in identifying pre-cancerous microcalcifications. Microcalcifications are found within the duct wall or lumen. Malignant microcalcifications are usually linear or branching whereas benign micro calcifications are rounded and punctuate.  
         [0006]     This apparent positive benefit has resulted in a number of leading health care societies recommending that all women be screened using mammography on at least biennial basis. In order for mass screening to be cost effective, methods need to be developed to achieve it with high accuracy and speed. Moreover, as the microcalcifications are imbedded in dense soft tissue, the diagnosis of mammograms is subjective and solely depends on the interpretations of the radiologist of the mammogram. At times, even for qualified personnel, it is difficult to interpret screening mammograms in large numbers. So an appropriate use of imaging processing techniques to enhance the important features of mammograms improves the specificity and objectivity of clinical cancer diagnosis.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention relates generally to the field of medical imaging in which digital images can be acquired and used in the diagnosis and/or treatment of medical conditions. A preferred embodiment of the invention uses image processing techniques to separate the phase information from the acquired digital image to provide an enhanced diagnostic image. A preferred embodiment of the invention is particularly useful for the identification and imaging of those features of the animal or human body which cause high spatial frequency features in the acquired image. Hard tissue structures such as bone or calcified or crystalline masses, lesions or cysts can cause such a high spatial frequency response making them suitable for phase component imaging.  
         [0008]     Mammograms are now being acquired in digital format thereby allowing the use of digital image processing techniques such as the fast Fourier Transform, to enhance the identification of microcalcifications. A preferred embodiment of the present invention employs phase-only image reconstruction of digital mammogram that uses only high spatial frequency components, that show microcalcifications and contours of lesions and other masses of interest in a dark background. The phase-only information can be processed with averaged amplitude information to reconstruct the original digital image.  
         [0009]     Preferred embodiment of the invention involve the phase imaging to provide images of obstructions within the arterial system, including the coronary arteries, to detection off kidney stones, of hairline fractures and other abnormalities within the skeletal system including the spinal column.  
         [0010]     A preferred embodiment of the invention employs a digital imaging detector to acquire images from a region of interest. A patient support such as a table can be used to position a region of interest of the patient relative to an energy source such as an x-ray tube of a radiographic imaging system such as a computed tomography system or a mammography system. The imaging device provides image data to a data processor such as a computer having a memory, an image processor, a display and a user interface. A software program can be employed to provide phase-only image processing in accordance with the invention.  
         [0011]     The foregoing and other features and advantages of the system and method for phase based digital imaging will be apparent from the following more particular description of preferred embodiments of the system and method as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  illustrates a method of forming an image of a region of interest of a patient in accordance with the preferred embodiment of the invention.  
         [0013]      FIG. 1B  illustrates a digital radiographic acquisition system in accordance with the present invention.  
         [0014]      FIGS. 2A and 2B  show an original image and a phase-only image with edge enhancement, respectively.  
         [0015]      FIG. 3  shows a phase-only image of a phantom with simulated microcalcification.  
         [0016]      FIGS. 4A and 4B  show a contrast detail digital image of a phantom with embedded gold particles obtained using a full field digital mammography system and a phase-only image, respectively.  
         [0017]      FIGS. 5A, 5B  and  5 C are an original mammogram, a phase-only image and a contrast adjusted phase-only image, respectively.  
         [0018]      FIGS. 6A, 6B  and  6 C are an original mammogram, a phase-only image and a contrast adjusted phase-only image, respectively.  
         [0019]      FIGS. 7A, 7B  and  7 C are an original mammogram, a phase-only image and a contrast adjusted phase-only image, respectively.  
         [0020]      FIGS. 8A, 8B  and  8 C are an original mammogram, a phase-only image and a contrast adjusted phase-only image, respectively.  
         [0021]      FIG. 9  illustrates a process sequence in accordance with a preferred embodiment of the invention.  
         [0022]      FIGS. 10A-10E  show and original mammogram and processed images in accordance with the invention.  
         [0023]      FIGS. 11A-11E  show an original mammogram and processed images in accordance with the invention.  
         [0024]      FIGS. 12A-12C  show an original mammogram and processed images in accordance with the invention.  
         [0025]      FIGS. 13A-13C  show an original mammogram and processed images in accordance with the invention.  
         [0026]      FIGS. 14A-14C  show an original mammogram and processed images in accordance with the invention.  
         [0027]      FIGS. 15A-15C  show an original mammogram and processed images in accordance with the invention.  
         [0028]      FIGS. 16A-16J  show an original mammogram and processed images in accordance with the invention.  
         [0029]      FIG. 17  illustrates a process sequence in accordance with the preferred embodiment of the invention.  
         [0030]      FIGS. 18A-18C  show a reference image, auto correlation spot and peak value, respectively.  
         [0031]      FIGS. 19A-19C  show an image, auto correlating spot and cross correlation peak value, respectively.  
         [0032]      FIG. 20  shows a phantom image with invisible micro calcification.  
         [0033]      FIG. 21  shows a phantom image with invisible micro calcification.  
         [0034]      FIG. 22  shows a phantom image with invisible microcalcification.  
         [0035]      FIG. 23  shows a reference phantom image.  
         [0036]      FIG. 24  shows a target image.  
         [0037]      FIG. 25  shows a cross-correlation peak value.  
         [0038]      FIG. 26  illustrates a process sequence in accordance with a preferred embodiment of the invention.  
         [0039]      FIGS. 27A-27C  show original images for spectral phase subtraction.  
         [0040]      FIGS. 28A-28C  show processed images from  FIGS. 27A-27C .  
         [0041]      FIGS. 29A-29C  show contrast detail of phantom and processed images.  
         [0042]      FIGS. 30A-30C  show residual images based on  FIGS. 20-22 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]     A preferred embodiment of the present invention is the phase characteristics of Fourier transform of medical images for computer aided diagnosis (CAD). We propose phase-only image reconstruction, original image reconstruction from phase-only information, phase-only correlations, spectral phase subtraction techniques for comprehensive CAD.  
         [0044]     The method for phase-only image reconstruction is shown in  FIG. 1A . Original digital image (digital mammogram, digital chest x-ray or in general any digital radiograph) is Fast Fourier Transformed using a programmed FFT sequence stored on a computer. The phase angle of the FFT spectrum is calculated. The phase of low spatial frequencies in the Fourier spectrum is zero or close to zero, while the phase of high spatial frequencies is in the neighborhood of ±π. From this phase angle, a phase-only function with unit amplitude transmittance is generated. The phase-only function is inverse Fourier transformed using another FFT operation to obtain a phase-only image. This phase-only image predominantly contains high spatial frequency components.  
         [0045]     Microcalcifications are tiny regions of calcium in the breast. In digital mammograms these microcalcifications appear in small clusters of a few pixels with relatively high intensity compared with their neighboring pixels that belong to soft dense tissues in the breast. Given that the microcalcifications belong to high spatial frequency components of the Fourier spectrum of a digital mammogram, detection of microcalcifications is achieved by reconstructing the phase-only image. The low spatial frequency components (corresponding to the soft dense tissue) have zero phases and are suppressed in the phase-only image. Another important change seen on the mammogram is the presence of mass, which may occur with or without associated calcifications. A mass is any group of cells clustered together more densely than the surrounding tissue. The size, shape and margins (edges) of the mass help the radiologist in evaluating the likelihood of cancer. Since a mass differs in its gray-value with respect to the surrounding tissue in the mammogram, edges of the mass correspond to high spatial frequencies. The phase-only information of a mammogram with such masses shows the shape and edges of the mass.  
         [0046]     Digital radiographs can be acquired using a system  10  such as that illustrated in  FIG. 1B . This includes an x-ray source  12 , a table  14  on which a patient  15  lies, a detector system  16  for detecting x-ray radiation that is transmitted through the patient. The detector system is connected to a computer  18  having a memory  20  and an image processor  22 , a display  24  and a user interface  26  such as a keyboard. The computer  18  can be connected to a public access network such as the Internet, a local area network, or connected remotely to a remote network, server, computer workstation, or other databases. The detector system includes a digital detector such as a charge coupled device, a CMOS imaging detector, an amorphous silicon detector or other digital image detector employing a scintillator, or alternatively, it can be a detector that converts x-rays into electrical signals.  
         [0047]      FIGS. 2A-2B ,  FIG. 3  and  FIGS. 4A-4B  show results of phase-only image reconstruction of digital phantoms. A digital phantom with invisible simulated microcalcifications which differ in brightness with respect to surrounding pixels are not visible while  FIG. 3  shows a phase-only image reconstructed with only microcalcifications having good contrast.  FIG. 4A  is Contrast digital phantom with embedded tiny gold particles obtained from Full Field Digital mammography machine.  FIG. 4B  is the phase-only image of  FIG. 4A . It clearly shows embedded small gold particles as tiny bright spots while little bigger size gold particles are shown with their shape and edges.  FIG. 5A ,  FIG. 6A ,  FIG. 7A , and  FIG. 8A  show clinical digital mammograms and  FIG. 5B ,  FIG. 6B ,  FIG. 7B , and  FIG. 8B  show the corresponding phase-only images. The microcalcifications are shown as bright spots in the dark background providing a many-fold increase in the contrast compared to the original image.  
         [0048]     The advantage of this technique in detection of microcalcifications over conventional digital image processing techniques is, it doesn&#39;t depend on the density of soft tissue in the breast that appear as a background (DC components) in the mammogram. In other words the technique is self adaptive to the changes in the background as the phase of low spatial frequency is zero. On the other hand other image processing techniques that involve high pass and band pass filters, the filter size and threshold have to be adjusted depending on the type of background in the mammogram. The system of the present invention provides a phase only that image preserves the morphology and texture.  
         [0049]     The phase-only image reconstruction can in general be applies to any digital radiographic image, digitized radiographic image, and Magnetic Resonance images (MRI) and Computed Tomography (CT) such as coronary calcifications in Cardiac CT images to extract and view essential features of the image hidden in the background of the image.  
         [0050]     A method of reconstructing an original digital mammogram from its phase-only information is shown in  FIG. 9 . It is common practice in mammography to obtain  4  different views of breast x-ray images, namely LCC, LMLO, RCC and RMLO. All of the images are Fourier transformed using an FFT sequence. The phase angle and spectual-magnitude of the Fourier spectrum for each image are extracted. The average magnitude is obtained by averaging the spectual magnitude over an ensemble of these images. The extracted phase-only information of each image is multiplied to this average magnitude. The resulting product of each image is then inverse Fourier transformed using the programmed FFT sequence to reconstruct the original image. These reconstructed original images preserve all the essential features of the respective original images including the morphology and texture.  
         [0051]      FIG. 10A  and  FIG. 11A  are original digital mammograms of the same patient but of different views; RMLO and LMLO respectively.  FIG. 10B  and  FIG. 11B  show the original image reconstructed from its phase-only information but with magnitude of  FIGS. 11A and 10A  degradation. It is important to note that the phase-only information is essential in reconstructing the original image as it preserves all the significant features of the image and not the spectral magnitude.  FIG. 10C  and  FIG. 11C  show the reconstructed original images of  FIGS. 10A and 11A  reconstructed from their respective phase-only information but with representative magnitude averaged over individual spectral magnitudes of  FIGS. 10A and 11A . It is evident from these figures that  FIGS. 10C and 11C  resemble more closely to the original image than  FIG. 10B  and  FIG. 11B . Thus the magnitude averaged over the large ensemble of similar images gives a nearly original image reconstruction. The sequence shown in  FIG. 9  uses digital mammograms of LCC, LMLO, RMLO and RCC views of a patient to generate the average magnitude.  FIG. 12A ,  FIG. 13A ,  FIG. 14A  and  FIG. 15A  show the LCC, LMLO, RCC, RMLO views of the patient while  FIG. 12C ,  FIG. 13C ,  FIG. 14C , and  FIG. 15C  show corresponding reconstructed original images from the average magnitude.  FIG. 12B ,  FIG. 13B ,  FIG. 14B , and  FIG. 15B  show corresponding phase-only images.  
         [0052]     Radiologists are often under tremendous pressure while giving decisions based on mammogram readings. The tiny microcalcifications hidden in the background of dense soft tissue are clearly visible in some mammograms, barely visible in some and not visible at all in some. This is mostly due to density of soft tissue in the breast which varies from person to person and with age. For example younger women have denser breast tissues providing a bright background in the mammogram. It can be very difficult to interpret mammograms in these cases. It would be helpful to the radiologist if a training tool is available which can be used to extract information about microcalcifications and other masses from a known mammogram case, add this information to different backgrounds provided by the other mammograms from the same or other patients and see whether the added information can be detected.  
         [0053]     The preferred method of reconstructing an image using its phase-only information and spectral magnitudes of images is useful for training the radiologist in his decision making process. For example, the subtle microcalcifications and other important features such as cysts and masses can be extracted from a mammogram using phase only image reconstruction. Using the process sequence shown in  FIG. 9  this phase only information can be multiplied with the magnitude extracted from any other mammogram and used to determine whether they are able to detect the features being added. The results are shown in  FIG. 16 .  
         [0054]     In clinical diagnosis, as well as in radiotherapy planning and evaluation, several images of one patient obtained using different imaging modalities or at different times, need to be compared. Although visual comparisons of available radiographic image with subsequent radiographic images are still standard practice as part of routine clinical evaluation, computerized analysis of these images has recently attracted the interest of both medical physicists and physicians alike. In this invention phase-only correlation and spectral phase subtraction techniques are used for tracking the development of useful information in digital radiographic images with respect to a selected time period.  
         [0055]     Phase only correlation (POC) methods use the phase information of a reference image that is correlated with the phase information of an acquired image. Due to the absence of low spatial frequency components in the phase-only information the POC method produces a sharp correlation peak. The POC method is consequently preferred to the amplitude-only correlation and complex Fourier spectrum correlation techniques. This sharp correlation peak feature of POC technique is used for measuring translational, rotational and scale shifts in the medical images.  
         [0056]     Phase-only information obtained from the phase of the Fourier transform suppresses the background due to soft dense breast tissue (low spatial frequency components) and predominantly contains information about essential features such a microcalcifications, shape and edges of masses and cysts (high spatial frequency components) in mammograms. The POC method can be used to correlate the phase-only information of a prior mammogram with phase-only information of a current mammogram.  
         [0057]     The POC method shown in  FIG. 17  can be used to calculate the discriminate ratio (percentage change in the said essential features of interest) between an image and its subsequent image (current) obtained at different time interval. Usually this time interval between the prior and the current image can be one month to a year depending on the seriousness of the case. When the image in  FIG. 18A  is used both as a reference and a target image in the sequence shown in  FIG. 17 , the output correlation peak is called auto correlation peak as shown in  FIG. 18C  with the peak value=3.627. When the image in  FIG. 18A  is slightly distorted as shown in  FIG. 19A  and used as a target image in the process sequence, the maximum correlation peak value, called cross correlation peak, drops to 1.827 ( FIG. 19C ). The ratio of maximum of cross correlation peak value to the auto correlation peak, drops to 1.827. The ratio of maximum of cross correlation peak value to the auto correlation peak value is called discrimination ratio (DR). For this example the DR is 50.37%, and indicated the percent change in the high spatial frequency components of the target image with respect to the reference image.  
         [0058]     For example, when the patient is normal, there may not be any clusters of microcalcifications present in the breast and the corresponding mammogram (say MAMO 1 ) will not show any sign of microcalcifications. When the patient obtains her next mammogram (say MAMO 2 ) after a year of two, and she developed some microcalcifications in the breast, which are a sign of a cancer at a preliminary stage. Certainly the radiologist may or may not be able to detect these microcalcifications in the mammogram, MAMO 2 . If the radiologist detects them, another mammogram (say MAMO 3 ) can be recommended after a month or so. By this time, she may have a more advanced stage of the cancer and develops not only a cluster of microcalcifications but also some masses like cysts in the breast. The radiologist after reading the mammogram, MAMO 3 , now recommends her for ultrasound scanning followed by biopsy. The phase of the Fourier spectrum of the mammogram in all three cases (MAMO 1 , MAMO 2  and MAMO 3 ) will be different and will often reflect only the changes in important features of the mammograms. However in the practical case, the random noise present in each mammogram may prevent reflection of actual changes in features of the two mammograms as random noise is also found in the high frequency components. This random noise is function of many parameters that can depend on the imaging system. Under preferred conditions, the amount of random noise in each mammogram may be more of less the same and cancel out when a comparison is drawn between these mammograms. Thus, when the high spatial frequencies due to subtle microcalcifications in MAMO 2  are compared to high spatial frequencies in MAMO 1 , it more or less reflects the actual changes in important features (microcalcifications that are sign of precancerous tissue of the two mammograms). In the Fourier spectrum of MAMO 3 , the high spatial frequencies will increase due to the presence of clusters of microcalcifications and masses like cysts and/or tumors. A preferred embodiment of the present invention provides tracking of these changes in the important features of mammograms (MAMO 1 , MAMO 2  and MAMO 3 ) using the POC as well as spectral phase subtraction technique.  
         [0059]     The POC technique is analyzed with binary images as shown in  FIG. 19A-19C  and phantom images with invisible microcalcifications as shown in  FIG. 20 ,  FIG. 21  and  FIG. 22 . As discussed earlier these images,  FIGS. 20-22 , represent the stages of cancer over a period of time, MAMO 1 , MAMO 2  and MAMO 3  respectively.  FIG. 20  consists of invisible bright random white spots that represent the random noise in the gray background that represents the soft dense tissue in the mammogram. In addition to these noise features of  FIG. 20 ,  FIG. 21  consists of an invisible cluster of bright spots with a definite pattern to represent the formation of microcalcifications at this state. Besides the noise features of  FIG. 20  and represent microcalcifications of  FIG. 21 ,  FIG. 22  consists of features that resemble masses or cysts. This represents the advanced stage of cancer over a period of time.  
         [0060]     If the reference image is same as the acquired image, the maximum value of the correlation peak that is obtained following the phase-only correlation (POC) method given in  FIG. 17  is called the autocorrelation peak value. If the reference image and acquired image are different, then the maximum correlation peak value is called the cross correlation peak value.  
         [0061]     The image in  FIG. 20  can be used both as a reference as well as the acquired image to obtain the maximum autocorrelation peak value i.e. 3.3180. When the image in  FIG. 20  is used as a reference and  FIG. 21  as the acquired image, the maximum cross correlation peak value is 2.3734. Therefore the discrimination ratio is 0.7153. This indicates that there is 71.53% correlation between these two images. This correlation is due to only the important features of interest, i.e. high spatial frequencies (due to random noise) present in both the images as the background due to low frequencies are not included in the correlation method and are suppressed as phase-only information of the images is used in the correlation process. The 30 percent drop in the correlation is due to the cluster of simulated microcalcifications that are present in the image of  FIG. 21 .  
         [0062]     When the image in  FIG. 20  is used as a reference image and  FIG. 22  as the acquired image the maximum cross correlation peak value is 1.5667. Therefore the discrimination ratio is 0.4722. This indicates that there is only 47.22% correlation between these two images. This correlation is due to only the essential features of interest, i.e. high spatial frequencies (due to random noise) present in both the images, as the background due to low frequencies are not included in the correlation method and are suppressed as phase-only information of the images is used in the correlation process. The 52.78% drop in the correlation is due to the cluster of simulated microcalcifications as well as masses or cysts that are present only in the image of  FIG. 22 .  
         [0063]     The image in  FIG. 21  is used both as both reference as well as a reference image to obtain the maximum autocorrelation peak value i.e. 3.3202. When the image in  FIG. 21  is used as reference and  FIG. 22  as the acquired image, the maximum cross correlation peak value is 1.7364. Therefore the discrimination ratio is 0.5230. This indicates that there is about 52.30% correlation between these two images. This correlation is due to only the essential features of interest, i.e. high spatial frequencies (due to cluster of simulated microcalcifications as well as random noise) present in both images as the background due to low frequencies are not included in the correlation method, and are suppressed as phase-only information of the images issued in the correlation process. The 48% drop in the correlation is due to the presence of masses or cysts that are present only in the image of  FIG. 22 . The image in  FIG. 23  is used both as both reference as well as the acquired image to obtain the maximum autocorrelation peak value i.e. 3.3614. When the image in  FIG. 23  is used as reference and  FIG. 24  as the acquired image, the maximum cross correlation peal value is 0.5540. Therefore the discrimination ration is 0.1668. This tells us that there is only 17% correlation between these two images. This correlation is due to only the non important features of interest related to high spatial frequencies (due to the lines and letters that don&#39;t relate to the embedded gold particles) present in both the images as the background due to low frequencies are not included in the correlation method, and are suppresses as phase-only information of the images is used in the correlation process. The 83% drop in the correlation is due to addition of tiny bright spots to the image in  FIG. 24 .  
         [0064]     A process sequence for spectral phase subtraction is shown in  FIG. 26 . Since the phase of the Fourier transform contains the important features of an image, the changes in the feature of subsequent images obtained over a period of time can be tracked using this technique. The phase-only information of prior image is subtracted from the phase-only information of the current image. This residual information is inverse Fourier transformed to reconstruct the residual image. Our results show that these residual images clearly display only the changes in the important features (high spatial frequencies) between these two images.  
         [0065]     The residual phase-only image shown in  FIG. 29A  is obtained by subtracting phase-only information of  FIG. 20  from phase-only information of  FIG. 21  following the method given in  FIG. 26 . This residual image clearly shows only the changes in the important features, i.e., changes in high spatial frequencies, (cluster of simulated microcalcifications) between these images. The background present in two images is suppressed because we used both images canceled out during the subtraction process.  
         [0066]     The residual phase-only image shown in  FIG. 29B  is obtained by subtracting phase-only information of  FIG. 20  from phase-only information of  FIG. 22  following the method given in  FIG. 26 . This residual image clearly shows only the changes in the important features, i.e. changes in high spatial frequencies, (cluster of simulated microcalcifications and masses with their shape and edge) between these images. The background present in two images is suppressed because we used phase-only information for subtraction, while high spatial frequency component due to random noise that is present in both images canceled out during the subtraction process.  
         [0067]     The residual phase-only image shown in  FIG. 29C  is obtained by subtracting phase-only information of  FIG. 21  from phase-only information of  FIG. 22  following the method given in  FIG. 26 . This residual image clearly shows only the changes in the important features, i.e. changes in high spatial frequencies, (masses with their shape and edge) between these images. The background present in two images is suppressed because we used phase-only information for subtraction, while high spatial frequency features due to random noise as well as cluster of simulated microcalcifications that are present in both the images canceled out during the subtraction process.