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Lasers bring new light to breast imaging.

A technique that uses laser light instead of x-rays to image the breast could lead to earlier diagnoses for women with breast cancer. In CT laser mammography, a laser is used to examine the breast using computed tomography techniques, creating a diagnostic image without ionizing radiation or breast compression. The ultrafast laser imaging technology offers resolution and accuracy that should enable radiologists to spot tumors 1 mm in size, potentially allowing physicians to detect cancer in its earliest stages.[1]

Breast cancer is the most common form of cancer in American women and is the leading cause of cancer-related deaths among women between 15 and 54 years old. In 1996, the American Cancer Society estimated that 184,300 women would be diagnosed with breast cancer and that 44,300 women would die from it.

Other than a strong family history of breast cancer, no risk factor is as important as simply growing older.[2] Women younger than 39 have a 1 in 222 chance of developing breast cancer, but women aged 60 to 70 have a 1 in 15 chance of developing the disease. Assuming a lifespan of more than 85 years, the birth-to-death risk of developing breast cancer is 1 in 8.

The mortality rate of breast cancer is directly related to the stage at which the cancer is detected. In its earliest stages, breast cancer is generally painless and too small to be detected by palpation alone. As a breast cancer metastasizes, it begins to feel like a lump or thickening of the breast. Between 75% and 90% of breast cancers are discovered accidentally by women already in the advanced stage of the disease.

Effective treatment of breast cancer requires that the disease be detected in its early, non-palpable stages. Early detection can improve the survival of breast cancer patients and also improves the probability of a cure.[3]

Currently, virtually all mammography is performed using screen-film systems and dedicated mammographic x-ray units. Although it is effective in detecting cancers, traditional x-ray mammography has several drawbacks. For example, with conventional mammography it is essential to maintain adequate tissue compression force to achieve a quality image. Force should range from 25 to 40 pounds of pressure to reduce thickness of the tissue that must be penetrated by x-rays, thus reducing scatter radiation and improving image quality.[4]

However, even careful mammography with adequate compression can produce indeterminate findings, resulting in unnecessary biopsies and surgery. False negative mammograms have been found in up to 63% of women younger than 45 years old in one study; in another study, false negative films resulted in delays in diagnosis that ultimately led to diagnosis of the disease in advanced stages.[3]

In addition, because of several inherent limitations of film as a recording device for a mammographic image, conventional screen-film mammography is limited in its ability to detect cancers in patients with radiodense breast tissue. Fatty breast tissue appears black on a radiograph and everything else -- glands, tumors, connective tissue -- appears white. Tumors stand out nicely against a backdrop of fat, but they are hard to spot in the 40% of women who have dense, fibrous breasts. Due to the increased cellularity and subsequent radiodensity of their breast tissues, premenopausal women or those undergoing hormone replacement therapy are not candidates for conventional x-ray mammography.[5]

Dense breasts occur mainly in:

* Premenopausal women in their late 30s or early 40s with metabolically active breast tissue.

* Postmenopausal women on hormone replacement therapy.

* Women with fibrocystic disease.

* Women who have undergone radiation treatment.

These women need breast cancer detection techniques that are independent of the radiological density of the breast tissue. For these women, CT laser mammography (CTLM) is a promising new diagnostic tool.[6] Advantages of CTLM include:

* It produces cross-sectional images of the breast.

* It produces a digital image that can be manipulated by computer.

* It does not require breast compression.

* It does not use x-rays and can be performed as often as desired or necessary without increased risk of radiation exposure.

* It can differentiate a cyst from a solid mass.

* It is not affected by breast augmentation.

The CTLM Breast Imaging System

CT laser mammography uses near-infrared laser illumination and a novel scanner design to examine the breast for cancer. The foundation of this new technology is the interaction of radiation with atoms. Radiation in the visible and near-infrared spectrums of light has longer wavelengths than x-rays. These longer wavelengths do not penetrate matter with the same degree of straight-line propagation as the x-ray. The visible and near-infrared radiation causes the bound electrons in the atoms and molecules to vibrate and re-emit the radiation in many different directions relative to the original path of the impinging photons. This phenomenon is called "scattering."

Scattering has been a limiting factor in the use of light and near-infrared radiation to image the body. Although transmission images can be obtained by passing light or near-infrared radiation through a body part, the scattering phenomenon limits the spatial resolution obtained. However, research in the field of lasers and biochemistry was advanced significantly with the discovery of methods of producing ultra-short pulses of light. Lasers were developed that could produce a pulse of light that lasted only 0.00000000000001 seconds are emitted at a rate of 76 million times per second.[1] These incredibly short, incredibly fast pulse lengths were important advances in the field of electrooptics.

Because near-infrared light readily penetrates breast tissue, ultrafast lasers with newly developed high-speed detectors are able to sense changes in the absorption of the laser pulse energy as it passes through tissue. In CTLM systems, a high-power laser is used in combination with a solid state laser to produce pulsing near infrared light. The visible green light is pumped into a titanium sapphire mode-locked laser, which produces ultra-short pulses of radiation in the near-infrared range. These pulses are used in the actual scanning procedure. Laser illumination is maintained as a single beam, using large area photodiodes. Both the detectors and the laser beam orbit around the object being scanned, and the circular array of multiple detectors acquires data at several hundred positions in the 360 [degrees] orbit. Data acquisition time for each detector varies from microseconds to milliseconds, and all detectors are gated to sample at the same moment. Collimation devices improve spatial resolution.

The CTLM system uses the high-speed optical pulses from the laser to create contiguous, cross-sectional images based upon the scattering and absorption properties of the breast tissue. The laser beam projection apparatus and the detector ring move 2 mm at a time to acquire each slice plane of data. Data from each individual slice plane is used to reconstruct an image of the interior structure of the breast with a thickness of 2 mm. The process of moving the laser beam and detectors is repeated until the entire breast is imaged from nipple to the chest wall.

The contiguous, cross-sectional slices allow the CTLM system to create a three-dimensional image of the breast that is viewed on a computer screen. This image can be processed to provide cross-sectional slices from any desired projection, allowing the physician to differentiate a cyst from a solid lesion without using ultrasound. CT laser mammography also can be used to examine breast prostheses, which should reduce the need for MRI. These multiple views provide a deck of information about each breast instead of a simple view.

Patient Experience with CTLM

A patient receiving a CTLM scan lies in a prone position on a scanning bed, with one breast suspended in the scanning chamber. Because breast compression is not required, there is no discomfort during the examination. Lack of breast compression is a significant motivation for women to have the CTLM procedure performed.

According to one woman scanned by CTLM in a research study, "I've had at least two mammograms a year for the last 8 years due to a history of breast abnormalities, and several times the breast compression has ruptured my cysts. So, for me, the CTLM, which requires no breast compression and uses no radiation, is a dream come true."[7] Another woman in the study said, "I have fibrocystic breast disease that sometimes requires me to have as many as four mammograms a year. I often worried about the harmful effects of all that radiation. I found the CTLM to be a drastic improvement to the uncomfortable mammograms I've had to endure over the years and it eliminated my concerns about the radiation since it uses laser technology."[7]

The Financial Perspective

From a financial standpoint, CTLM is cost-effective compared with traditional x-ray mammography. In fact, the procedure offers several financial incentives for hospital radiology departments or mammography centers:

* A CTLM procedure can differentiate a cyst from a solid lesion, thus eliminating a second procedure, aspiration and/or ultrasound.

* The laser technique is anticipated to have a higher specificity than mammography, thus reducing the number of unnecessary biopsies.

* Breast density does not affect CTLM, thus opening the examination to younger women with denser breasts for whom x-ray mammography has been less effective.

* The time required for a bilateral CTLM exam is about the same as or slightly shorter than the time required for a conventional mammogram. A bilateral exam requires less than 15 minutes from undressing and scanning to redressing.

* Because CTLM does not use ionizing radiation, it would allow more frequent exams of high-risk women, with the potential for earlier detection of life-threatening lesions.

* The digital nature of CTLM images allows computer interpretation of the images and has the potential to improve the accuracy of the interpretation and reduce the time necessary for interpretation.

CT laser mammography is the newest tool in the race to detect breast cancer as early as possible, when the chances of survival are higher. The technique is currently in the first phase of human clinical trials under approval by the U.S. Food and Drug Administration. These tests will evaluate the clinical effectiveness of CTLM as a potential diagnostic tool for breast imaging. Although CTLM may not replace conventional x-ray mammography, it will become another powerful diagnostic weapon in the fight against breast cancer.

References

[1.] Grable J. Mammography Today. 1995.

[2.] Williams MB, Fajardo LL. Digital mammography: performance considerations and current detector designs. Academic Radiology. 1996;3:5.

[3.] Newman J. Early detection techniques in breast cancer management. Radiol Technol. 1997;68:309-324.

[4.] Electronic scale detects force changes in mammography compression devices. Radiol Technol. 1994;66:138.

[5.] Sevick-Muraca EM. Computations of time-dependent photo migration for biomedical optical imaging. Methods in Enzymology. 1994;20.

[6.] American Cancer Society Breast Cancer Network. Internet WWW page at URL: <http://www.cancer.org/riskfact.htm.> Accessed April 1997.

[7.] IDSI Media Clips. Women find breast exams by new CT laser device a welcome change. PR Newswire. Dec. 2, 1996.

Sharon Ager, R.T. (R), is a recent graduate of the Genessee Hospital School of Radiologic Technology in Rochester, N.Y., where she is employed as a staff radiologic technologist.

Cynthia Daniels, B.S., R.T. (R), is editor of this section of the Journal, dedicated to publishing the written works of students in radiologic science educational programs. Ms. Daniels is chairman of the ASRT Committee on Student Writing Competitions. She is coordinator of the radiography program at Barnes Jewish Hospital in St. Louis, Mo.

Articles published in the "Student Scope" column are eligible to compete for the Mallinckrodt-Radiologic Technology Writing Award. Writing guidelines may be obtained by contacting Christine Morrison, c/o the American Society of Radiologic Technologists, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.
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Author:Ager, Sharon; Daniels, Cynthia
Publication:Radiologic Technology
Date:May 1, 1998
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