POSITRON-EMISSION MAMMOGRAPHY (PEM): New technology: Almost ready

Happy Mother's Day to all of you; I hope you had breakfast in bed, got flowers and ate candy for lunch!

May's Tech-talk is a continuation of April's discussion on PET scanning. May gets into a little more detail on Positronic Emission Mammography (PEM). New modalities are always coming down the pipe. Some are more valuable than others but all add a fuller dynamic to our tool box of diagnosis, prognosis and treatment options.

Enjoy and pass it on, keep in touch,

Anne

OBJECTIVES:

Mammography is the one breast cancer screening test proven to reduce mortality, yet on many occasions, it still does not capture the full picture. Detection using the high sensitivity of breast-MRI (magnetic resonance imaging) is very promising, yet it often casts too wide a net. For women at high risk for breast cancer or women who have breast cancer, the limitations of these two imaging techniques cause unnecessary procedures, anxiety and cost. However, many radiologists and physicians can now optimize the precision of molecular imaging through positron-emission tomography (PET) and focusing it on breast cancer detection and diagnosis.

Using PEM within the Breast Imaging Process:

Due to challenges inherent in other modalities, many ultimately unnecessary biopsies can be performed. Right now, PEM is being used as an adjunct to current breast imaging techniques. Used in concert with existing modalities, PEM should be able to reduce the number of biopsies, improve surgical planning, and better detect any recurrent disease, at least on the local level.

PEM is currently being used for patients with newly diagnosed breast cancer who are facing surgery. Before scheduled surgery is undertaken, PEM can provide physicians with one more imaging analysis to help determine whether there is multifocality and multicentricity. Multifocality means that there is more than one location of cancer along a dual system in a radial pattern. Multicentricity means that the cancer is in completely different locations, either in the same breast or the other breast. The difference between multifocal and multicentric is vital because, in most cases, multifocality means that breast conservation may be possible, where multicentricity is a contraindication to breast conservation. PEM likely will improve the quality of surgical decisions because it is very good at staging and determining the local extent of disease.

PEM systems can serve as a legitimate substitute for patients who cannot undergo breast MRI for reasons including implants, claustrophobia, or obesity. PEM has been found to be helpful in cases involving women with dense breast tissue that can make tumours hard to spot with mammography due to reduced specificity and sensitivity. Traditionally, whole breast ultrasound and breast MRI represent alternatives for very dense breast tissue. Both these modalities are useful but have their own issues with dense tissues, cost and specificity. PEM may be proving to achieve a lower false positive rate than MRI and has no issue with dense tissue.

The Procedure:

PEM involves the injection of the radiopharmaceutical FDG. A patient needs to fast for four hours before the procedure. Afterward, a serum blood sample is taken to determine blood glucose level. Then FDG is injected intravenously. Tumour cells will take up much more glucose than normal cells, and PEM imaging will reveal the FDG concentrations that suggest malignancy.

The imaging is acquired one hour after the FDG injection, when the patient’s body has had enough time to absorb the radiopharmaceutical. Images are obtained in a manner similar to mammography. Both breasts are imaged, with mediolateral oblique and craniocaudal views taken. Each view takes approximately 10 minutes to accomplish; thus imaging requires roughly 40 to 45 minutes. During imaging, the breast is immobilized but requires only slight compression.

Patients report that they tolerate the procedure very well because of this reduced amount of compression. The purpose of compression in a mammography procedure is to reduce the thickness of tissue to x-ray, which increases the sensitivity and decreases the background noise and radiation. But during a PEM procedure, the compression is more for isolation purposes. The imaging window for PEM is minutes, not seconds, so, only light compression for immobilization is necessary.

During the imaging, the PEM detectors are positioned close to the breast. As a result, image acquisition is much more efficient than regular full body PET scanning. In addition, with only a small attenuation of counts, the spatial resolution is greatly improved compared with whole-body PET imaging. Most PEM systems include an articulating arm with two parallel detectors at the end, which allows for flexibility in imaging of any small body part, although these systems are designed for breast imaging. Some PEM scanners include components and a compact design that make it useful for the imaging not only the breast but appendages such as the head, hand, or foot.

The viewing station should be highly mobile, with a small footprint, a camera and detectors. Having it situated on casters, allows it to be easily rolled into and out of a room. This mobility is significant because it allows the camera to be wheeled into a standard imaging room, next to a large, whole-body system, for example, and to be used in concert with that larger system.

PEM Proto-type:

Tissue Diagnosis: DCIS and IDC:

Figure 1:Mammography

Figure 2: PEM

Figure 3: Full Body PET

Figure 4: CT

PEM Images of Various Cancers

Figure 5: IDC with central necrosis

Figure 6: Multi focal breast Ca PEM & Mammo

Figure 7: Occult DCIS in a single duct

SUMMARY:

Since the advent of formal breast imaging for the purpose of diagnosing breast cancer we have done nothing but accept change. That is the nature of our specialty: X-ray, Xerography, film/screen Mammography, breast screening, US, Digital Mammography, MRI, PET, PET/CT and PEM; a virtual never ending stream of change. With every change we save few more lives and give a few more women a better prognosis and a better quality of life.

As breast imaging technologists we are required to adjust our work habits, learn more modalities and accept more adjustments to our specialty than any other modality in DI. With every change comes more knowledge, more understanding of our job and greater satisfaction with the outcomes of our examinations. In the end, knowledge always equals power!