Study of the potential for a combined optical and x-ray mammography system using a compressed breast phantom

Breast Cancer and Mammography

• In UK, 40,000 new cases of breast cancer are diagnosed each year
• Approximately 11,500 women die of breast cancer each year in England and Wales
• The NHS Breast Screening Programme (NHSBSP) invites women from the age of 50 to be screened for breast cancer
  This screening involves a mammogram during which the patients breast is compressed and an image is produced of it using low energy x-rays

The main drawback of mammography is that it is unable to distinguish abnormalities as malignant or benign. This means that any patient who is diagnosed with an abnormality will have to undergo further testing.

Using diffuse optical tomography (DOT) to detect breast cancer

• Near-infrared light is emitted into the breast. Some of the light will be absorbed by areas of the breast such as blood
• The transmitted light is detected and the time of flight and intensity of the photons detected will be processed and images produced from the data
• The images produced will identify tumours as they are more vascular than normal breast tissue

The main drawback of using diffuse optical tomography to detect breast cancer is that the images have poor spatial resolution

Combining DOT with mammography

• X-ray images can be used to locate abnormalities and provide information on their exact position and size
• Optical images can be used to classify abnormalities as malignant or benign
• This would reduce the need for further testing

Aims of this project

• Design and construct compressed and uncompressed breast phantom which can be used in both x-ray and optical imaging
• Image the compressed phantom using the two techniques
• Combine the images
• Design a combined mammography system

Breast Phantoms

• Phantoms represent a part of the body such as the breast by simulating its properties
• They are used in research and as quality assurance tools
• Phantoms are typically made of epoxy resin, acrylic and wax

Designing the phantoms

• The linear attenuation coefficient of the optical phantom material (epoxy resin) used by the BORL department of UCL was found experimentally

  	µ (cm-1) at 60keV
  Optical phantom material	0.228 ± 0.006
  Normal breast tissue	0.201

• This material was found to be suitable to use as the normal breast tissue of the phantom
• To provide a suitable contrast between the normal breast tissue and the tumours of the phantom, a difference comparable to that of normal breast tissue and tumours had to be achieved

• Linear attenuation coefficient of epoxy resin with talc and chalk found experimentally
• It was found that the addition of 5% talc to the epoxy resin increased its linear attenuation coefficient by 0.011cm-1
• To achieve the required contrast, a difference of approximately 0.015cm-1 at 60keV was needed

• The optical properties of epoxy resin and solutions with chalk and talc were found experimentally
• It was concluded that talc did not effect the optical properties of the epoxy resin but chalk did

Final design

• Shape and thickness of the compressed breast phantom based on a compressed breast
• The tumours in the phantom contain 10% talc as this produces a sufficient contrast
• The absorption coefficient of tumours is 2.5 times that of normal breast tissue of the phantom
• 4 tumours of 2 different diameters were set in the phantom at ¼ and ½ the depth of the phantom
• 2 markers were also put inside the phantoms to be used to align images of the phantom. The markers contain 20% talc and have optical properties 10 times greater than those of the normal breast tissue of the phantom

For details of the compressed breast phantom, click here. The uncompressed breast phantom is the same as the compressed except that it is double the thickness.

Images of the phantom

To see the x-ray image of the phantom, click here

An example of the optical images obtained of the phantom is shown here:

Double-sided interface		Single-sided interface
10mm	20mm	10mm	20mm

The double-sided interface images were obtained when sources and detectors were above and below the phantom. The single-sided interface images were obtained with all the sources and detector on one side of the phantom. The images are shown at 10mm and 20mm below the top of the phantom because these are the depths at which the middle of the tumours were set.

The images produced using the double-sided interface were of better quality and provided more information. For this reason, the design of the combined system is based on having sources and detectors above and below the breast.

To see a selection of the combined images, click here

Design for a combined system

• The design is based on a standard mammogram unit with the addition of a double –sided optical interface
• First an x-ray image is produced of the breast, then, without releasing compression, the optical interfaces are moved onto the compression plates so that the optical data can be acquired
• The compression plates would have to be made of glass to reduce the interference in the optical images

Summary

• A compressed and an uncompressed breast phantom have been produced
• The compressed phantom has been imaged using x-ray mammography and DOT
• Sources/detectors above and below phantom produced best optical images
• Combination possible
• Need improvements in the optical image quality and a reduction in the optical data acquisition time before the technique could be considered in routine diagnosis of breast cancer

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Abstract