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The use of Magnetic Resonance Imaging (MRI) of the breast is gaining popular support, but is still mainly limited to large research centres in the UK and is not yet

considered ‘standard’ investigation. Over the past few years however, it has developed an increasing role for investigating the ‘difficult breast’ i.e. a patient who has equivocal findings using conventional methods. There has also recently been a drive to try and standardise breast MRI with the aim of introducing some quality assurance measures and guidelines with regards to its use [Anonymous, 1999b].

Unlike mammography and ultrasound that rely mainly on variable densities to differentiate between normal, benign and malignant tissue, breast MRI utilises the difference in vascularity between such tissues following the administration of an intravenous contrast agent such as dimeglumine gadopentate (Magnevist®). It is well validated that invasive tumours rely on new vessel growth (neovascularisation), in order to increase in size [Folkman et al., 1989]. These new vessels develop under the influence of various angiogenic growth factors e.g. vascular endothelial growth factor, VEGF [Locopo et al., 1998]. By introducing an agent into the circulation which produces an increased signal intensity in tissue that has a rich vascularity e.g. breast cancer, it is possible to make tumours ‘stand out’ from the background. Breast cancers therefore generally appear bright on contrast enhanced Tl-w eighted images. Unfortunately however, some benign lesions, such as fibroadenomas, also have increased vascularity and therefore can also appear as an enhanced lesion.

There has therefore been much work aimed at increasing the accuracy of MRI not only for the initial detection of breast lesions but also in differentiating between benign and malignant tumours. A plethora of MR techniques have been tried and there is still much discussion as to which imaging parameters are best [Weinreb and Newstead, 1995]. Opinions are divided in a variety of areas including choice of contrast agent, pulse sequences, orientation of plane, post processing techniques and whether both breasts are imaged together or individually. There is also variation in imaging equipment including magnet field strength and use of dedicated breast coils.

Despite these ‘differences in opinion’ there is no doubt that breast M RI is extremely good at detecting breast cancer with most studies reporting a sensitivity of over 95% [Davis and McCarty, Jr., 1997;Ercolani et al., 1998;Heywang-Kobrunner et al., 1997]. Specificity can however vary greatly and has been reported as anywhere between 37% and 91% [Drew et al., 1999;Harms et al., 1993]. With further

advancements in both hardware and software to improve image quality and increasing experience of radiologists, these figures may further improve.

There are several clinical situations where interest in the possible use of breast MRI is most concentrated. These indications are however undoubtedly set to expand as further validation is achieved:

• Investigation o f women with equivocal findings on mammography or ultrasound:

Occasionally, a ‘suspicious’ area is seen on mammography in only one view therefore making conventional biopsy impossible. Where ultrasound is unhelpful, MR imaging may have a role in helping to either confirm or exclude an underlying breast cancer or other breast pathology [Lee et al., 1999].

Follow up o f breast cancer patients who have undergone breast conservation surgery and radiotherapy: In this situation, both clinical examination and mammography have traditionally been difficult to interpret due to scarring and radiation change. MRI may be more sensitive in picking up recurrence in patients where there is clinical suspicion [Mumtaz et al., 1997], but its use as a blanket screening tool has been questioned [Coulthard et al., 1999].

Evaluation o f the breasts o f women with silicon implants: With the increased popularity of breast augmentation and reconstructive procedures following breast cancer surgery, the incidence of breast cancer patients with silicon implants is set to rise. Mammography in these patients is generally difficult and also less accurate than for the general population as the implants are radio-opaque causing some of the breast is to be obscured. Ultrasound has been found to be a useful adjunct to mammography in these patients [Carlson et al., 1993]. With MR imaging creating a cross sectional view, the implant can easily be differentiated from the ‘natural’ breast. MR also has an increasing role in assessing implants for possible rupture [Harms, 1998;Middleton, 1998].

Screening o f high risk women: There has been for a long time, much debate as to the best way to achieve early and reliable detection of breast cancers in young

women at high risk due to family history. This is especially true for those women carrying a predisposing genetic mutation such as B R C A lor BRCA2. This debate has mainly been fuelled by the low sensitivity of mammography in the younger patient due to the relative density of the breast caused by glandular tissue [Edeiken, 1988]. MR imaging has therefore been proposed as a possible method of screening these women in an attempt to detect early and therefore potentially curable cancers [Orel and Schnall, 1999]. Multi-centre trials to test this hypothesis are currently underway in England, the United States, Canada and Holland. A recent report has shown that MR imaging is more accurate in detecting breast cancer than mammography in these ‘high risk’ patients [Kuhl et al., 2000], although it will be some time before it is known if there is a true survival benefit. Also there is a very real question as to whether the results of these trials can be transferred to the general population.

• Evaluation o f extent o f disease: Although triple assessment has a high sensitivity for diagnosing breast cancer, mammography and/or ultrasound is not reliably accurate in determining the extent of the tumour. MR imaging may therefore have a role in evaluating multifocality or multicentricity in patients selected for breast conservation surgery. This may have an impact on therapeutic approach [Conrad et al., 1999fis c h e r et al., 1999], the subsequent incidence of local recurrence [Davidson et al., 1997] or re-excision rates due to positive tumour margins.

• Determining the early effect o f primary chemotherapy: For patients who are undergoing primary (neoadjuvant) chemotherapy, it is sometimes difficult to assess the clinical response with either clinical examination or mammography. Some researchers have claimed good accuracy in determining the extent of residual disease following such treatment [Abraham et al., 1996]. This may be of use in determining the subsequent surgical approach i.e. mastectomy versus breast conservation surgery.

There is little doubt that breast MR imaging will continue to be used in the evaluation of breast cancer for a selected group of patients. At present however, there are many questions still to be answered with regards to its true accuracy in some of

the clinical situations mentioned above. Also standardisation of not only equipment and imaging protocols but also (and probably most importantly) the training and subsequent experience of radiologists, is still a long way off. Another major problem is development of MR guided biopsy systems for sampling areas of enhancement that cannot be seen with either mammography or ultrasound. Only recently have ‘open access’ breast coils become more readily available with the newer generation magnets. Such attachments however are not available for every existing MRI. There are also still problems with needle artefacts and rapid loss of contrast enhancement [Heywang-Kobrunner et al., 2000]. Once these problems have been ironed out, and the cost effectiveness calculated, MR imaging of the breast may become as routine an investigation as the other imaging modalities. At present however, the high accuracy of mammography and ultrasound will continue to be invaluable in the detection and evaluation of breast cancer.