**2.4 Scintimammography**

456 Advances in Cancer Therapy

The injection of a contrast agent can enhance the ability of MRI to detect specific features or in the case of dynamic contrast-enhancement MRI the functionality of the tissue can be investigated. The contrast agents used are paramagnetic agents with gadolinium (Gd-DTPA) being the most common. The increased vascularity of tumours produces a preferential uptake of contrast agent and the technique can be used to improve their contrast from surrounding normal tissue. In dynamic contrast-enhancement MRI, scans are repeatedly acquired following the contrast injection and the dynamic nature of contrast uptake can be examined, which may improve the differentiation of benign and malignant

The main advantages of MRI are that the modality is suitable for women with denser breasts and the technique is non-ionising. It is possible to take images in any orientation and determine multi-focal cancers. The technique can also show breast implants and ruptures. The disadvantages of MRI are that a contrast agent is required to provide adequate specificity and that it is immobile, expensive, and unsuitable for some women. The modality cannot image calcifications and can induce feelings of claustrophobia, and require long scan

Ultrasound is defined as a frequency of sound above the threshold of human hearing (i.e. > 20 kHz). The frequency range used in medical ultrasound imaging is 1 – 15 MHz. This allows wavelengths less than 1 mm to be measured and thus produces good spatial resolution. Ultrasound waves interact with tissue in a variety of ways but it is the reflection and transmission at interfaces between tissues of different acoustic impedance that is utilised in medical imaging. If there is a large acoustic mismatch between two tissues then a large fraction of the ultrasound intensity will be reflected. If there is a small difference in the acoustic impedance then most of the intensity will be transmitted. The time between pulse generation and the detection of an echo provides the depth of the reflecting interface, and thus images can be generated. Measuring the magnitude and time difference between different reflected signals can be used to determine the type, depth and size of different

Ultrasound pulses are generated and detected by a hand-held transducer, based on an array of small piezoelectric crystals. The very low acoustic impedance of air means that any boundary between air and tissue results in a near 100% reflection, so to ensure that the ultrasound waves are coupled into the body an impedance matching gel is used between the breast and the transducer. As there is a large difference between the acoustic impedance of a liquid filled cyst and normal breast tissue, around 23% of the ultrasound wave is reflected at such a boundary, making this technique particularly useful for the diagnosis of such a lesion. The small differences in acoustic impedance between adipose tissue and glandular tissue mean that this technique is of particular use for younger women with denser breasts, where x-ray mammography is often unsuitable. Doppler ultrasound utilises Doppler shifts from Rayleigh backscattered ultrasound waves to determine the velocity at which red blood cells are moving. Doppler ultrasound can be used to monitor blood flow and as a result can

Ultrasound is relatively inexpensive and a versatile technique. It can provide excellent contrast resolution, which means suspicious areas are easy to differentiate from normal tissue. X-ray mammograms are frequently followed up with ultrasound imaging to determine whether a lesion that appeared on a mammogram is a cyst or a solid mass. Since

be used as an indicator of vascularisation of a malignant tumour in the breast.

disease.

**2.3 Ultrasound** 

tissues.

times in comparison to x-ray mammography.

Scintimammography or nuclear medicine imaging is sometimes used alongside x-ray mammography in the diagnosis of breast disease since this technique is able to determine if a located lesion is malignant. The technique involves injection of a radioactive tracer into the patient. The tracer emits radiation, which is detected using a gamma camera. Appropriate image reconstruction algorithms enable the distribution of the tracer within the body to be mapped. Since the tracer accumulates differently in malignant and benign tissues it can be used to distinguish between the two conditions. Several radioactive compounds have been investigated, although only one, technetium-99m sestamibi (MIBI), is approved by FDA for use in breast imaging. A nuclear medicine investigation of the breast usually takes between 45 and 60 minutes. In a typical examination, the radioactive tracer (Tc-99m sestamibi) is injected into the patient's arm. The patient lies face down on a special table with her breast suspended through a hole. The images of the breast are taken from several angles using a gamma camera.

The advantages of scintimammography are that it can be used on patients with dense breasts and it can image large palpable lesions that do not appear with other imaging modalities. The modality involves the use of ionising substance that is injected into a patient (invasive) and is time consuming. Whilst it is 90% accurate for abnormalities over 1cm it is only 40- 60% accurate for smaller size abnormalities. The technique can be used to test tissue remaining from a mastectomy and can also be used to check for metastases in the auxiliary lymph nodes.
