Magnetic resonance imaging (MRI) has come to light as a valuable tool in breast imaging. It is an ultimate supplementary imaging modality in addition to mammography and ultrasonography in the evaluation of breast disease. Breast MRI is most commonly obtained to evaluate and characterize malignancy. Screening breast MRI has shown higher sensitivity rates in the detection of breast cancer compared to mammography, ultrasonography or both combined in asymptomatic high-risk women. The purpose of screening is to find smaller and node-negative cancers, which reduces breast cancer-specific mortality. This article aims to review the indications for screening and diagnostic breast MRI, discuss the technique and some of the most commonly encountered artifacts.
The most common indication for obtaining breast MRI in clinical practice is to evaluate for breast cancer. Studies have shown increased sensitivity in diagnosing breast cancer when MRI supplements mammography in comparison with mammography alone or combined mammography and ultrasonography. Annual screening MRI of the breasts is recommended in high-risk women such as:
- Those with the lifetime risk of 20% or greater
- BRCA mutation carriers
- Untested first degree relative of BRCA carrier
- History of chest irradiation between ages 10 and 30 years.
- Women with genetic syndromes predisposing for breast cancer
In patients already diagnosed with breast cancer, MR imaging is crucial for further evaluation including:
- Preoperative assessment of the extent of cancer in newly diagnosed patients for surgical planning
- Screening of contralateral breast in conditions where there is an increased risk of bilaterality such as lobular carcinoma
- Breast cancer staging for treatment planning
- Monitoring tumor response to neoadjuvant therapy such as chemotherapy or hormonal therapy
Preoperative assessment for multifocal and multicentric lesions is important for appropriate treatment, which can decrease re-excision rate and recurrence rate.
Other indications for breast MRI include:
- Women with dense breasts, as they have one to two-fold increased risk of breast cancer than the general population
- To differentiate scar versus recurrence
- Woman with suspected inflammatory breast cancer - MRI demonstrates skin enhancement
- Axillary nodal metastasis with unknown primary or occult breast lesion on mammography and ultrasound
- To assess the stability of silicone breast implants
- Patients with nipple discharge with inconclusive findings on other imaging studies
Brown et al. conducted an online survey among primary care providers (PCP) and specialists regarding the frequency of ordering breast MRI which showed that the specialists order more breast MRI studies compared to PCPs. The most common indications for ordering breast MRI in the survey were to evaluate clinical finding with negative mammogram and ultrasound, new breast cancer diagnosis, cancer staging, and monitor response to chemotherapy. To improve and maintain the credibility of MRI in breast imaging, American College of Radiology (ACR) has formulated appropriateness criteria to provide the ordering physicians with appropriate diagnostic imaging studies for their patients.
Some challenges that present with the use of breast MRI include high cost, lack of availability in some of the breast centers, longer examination time, need for intravenous contrast medium, and false positive reads. False positives result in additional imaging, biopsy, or unnecessary surgical interventions.
A 1.5 Tesla magnet is widely used for breast imaging. The patient would be in a prone position. A specialized breast coil is utilized to obtain images. Intravenous administration of contrast medium such as gadolinium (0.1 mmol/kg) is required in almost all instances, with the exception of the evaluation of silicone implants. Both breasts are imaged routinely to assess for symmetry and background parenchymal enhancement. MRI of the breasts should ideally be performed during days 7 to 14 of the menstrual cycle to minimize the effect of hormones on background parenchymal enhancement and to reduce the rate of false positives. The field of view (FOV) should be carefully selected to ensure adequate anatomical coverage of the region of interest while maintaining spatial resolution. Ideal FOV extends from the clavicle to the infra-mammary fold, including axilla. The interactions of protons in tissue primarily create MR images. The difference in the tissue characteristics of diseased breast parenchyma in comparison to adjacent normal breast tissue creates signal changes on MR images. Homogenous fat suppression is another vital requirement for cancer detection. Inadequate fat suppression or field inhomogeneity can render the images uninterpretable for subtle lesions.
Breast cancer is the most common indication for obtaining MRI. Dynamic contrast-enhanced MRI is required to distinguish benign versus malignant lesions of the breast. It enables assessment of tumor morphology and enhancement kinetics. T1 weighted sequences with fat suppression are most commonly used to obtain contrast-enhanced images. Malignant lesions are usually larger in size, have spiculated or irregular margins and shape with heterogeneous enhancement. Round or oval mass with circumscribed margins, dark septa, and homogenous enhancement indicate benign lesion. Three different types of kinetic curves exist. Most of the times, malignant lesions demonstrate rapid uptake with rapid washout of the contrast medium (type III) due to their high vascularity. Benign lesions demonstrate slow, continuous uptake (type I). Rapid uptake followed by a plateau or persistent enhancement can represent either benign or malignant lesion (type II). However, in some instances malignant lesions can show slow, persistent uptake and benign lesions can have malignant-type enhancement pattern. Therefore, enhancement patterns and washout curves must always be supplemented with morphologic features of the lesion.
Abbreviated MRI protocols have been proposed for cancer screening and diagnosis with reduced scan time and cost. New established MRI techniques have been reported in the literature such as diffusion-weighted imaging and proton MR spectroscopy imaging. Additional newer techniques such as blood oxygen level dependent (BOLD) and hyperpolarised MRI are still under investigation. These newer techniques are expected to improve the accuracy and specificity of breast MRI.
An artifact occurs due to misinterpretation of the object in the FOV. A well-trained MRI technologist is necessary to obtain good quality MR images. The interpreting radiologists must be aware of these artifacts to resolve them and improve the image quality. The following briefly discusses some of the most commonly encountered artifacts in breast imaging.
Artifacts are broadly classified based on patient factors and technical factors.
1. Motion artifact
Motion can be physiological due to respiration and pulsation of the blood vessels or patient movement which can be voluntary or involuntary. Patient motion can affect the entire series of images or sometimes only a few images in a series especially when it is due to pulsation of blood vessels. Motion artifact always occurs in the phase-encoding direction.Fix: Educate the patient about the importance of holding still. Patients with pain or claustrophobia may need adequate analgesia or sedation.
Improper positioning of the breasts within the breast coil can result in signal distortion and inhomogeneous fat suppression. Large breasts can get compressed against the coil and produce abnormal signal hyperintensity at the site of contact. Fix: Breasts should be properly placed within the dedicated breast coil. Comfortable positioning of the patient with arms by the side or above the head.
3. Metal artifact
Ferromagnetic (iron, nickel, cobalt) and non-ferromagnetic (titanium) metals have higher magnetic susceptibility. Metallic objects such as biopsy clips, jewelry, skin tattoos, metallic snaps on clothing, deposits from core biopsy needle or electrocautery devices produce field inhomogeneity which presents as signal void along with distortion of the image. MRI of the post-surgical breast for local recurrence remains challenging due to the metal artifact.Fix: Thorough examination of the patient with the removal of any metallic objects is required; using a lower field strength scanner such as 1.5 Tesla magnet with a short echo time (TE).
1. Zipper artifact
Zipper artifact, also known as radiofrequency interference occurs when there is an external radio-frequency (RF) signal that is not consistent with the phase encoding gradient. The images have a noisy background which manifests as alternating dark and bright bands across the image. The source of the external RF signal can be within the imaging unit or outside such as television, radio, or patient monitoring equipment. The imaging unit has a metal shield called the Faraday cage that normally prevents the external RF signal from entering the unit. Breach in this metal shield result in zipper artifact.Fix: Periodic quality control check of the Faraday cage is necessary. Ensure the door to the scan room is closed correctly. Use of monitoring equipment that is MR compatible.
2. Wrap-around artifact
When the FOV is smaller than the signal producing tissue, it results in a wrap-around artifact. The signal generated by the tissue outside the FOV gets superimposed on the contralateral side of the image. Wrap-around artifact, also known as aliasing, occurs along the phase-encoding direction.Fix: Aliasing can be reduced by increasing the FOV or by phase oversampling. Saturation bands can also be used to suppress the signal from excited tissues outside the FOV.
3. Chemical shift artifact
There are two kinds of chemical shift artifact. In the first kind, the differences in the precession frequency of the protons in water and fat generate an image with bright and dark bands on either side of the tissue. This type always occurs in the frequency encoding direction. The second kind of chemical shift artifact occurs on gradient-echo images, where the water and fat protons move in and out of phase at fixed time points, depending on the strength of the magnet (1.5 T or 3T). As a result, there will be signal void at the fat-fluid interface, which presents as a dark rim around the tissue.Fix: Chemical shift artifact (first kind) can be reduced by increasing the bandwidth per pixel. Use of fat suppression technique, longer TE and changing the frequency encoding direction also can reduce this artifact.
4. Moire fringes or Zebra artifact
Zebra artifact is a combination of wrap-around artifact and field inhomogeneity. It is seen along the edges with larger FOV as alternating bands of bright and dark signal. It can also occur when a body coil is used instead of the breast coil.Fix: Selecting appropriate FOV to reduce aliasing or phase oversampling. Shimming of the magnet to reduce field inhomogeneities.
It is essential to understand the importance of breast MRI and its indications for screening and diagnostic purposes which aids in timely diagnosis, management, and better patient outcome. Identifying the potential artifacts related to the technique and ways to overcome them will yield good quality images and avoid unnecessary interventions.
Enhancing Healthcare Team Outcomes
Breast cancer ranks as the most common malignancy in women. Annual screening mammography is recommended to every woman starting at age 40. MRI of the breast is an ideal complementary imaging study to mammography and ultrasound for the evaluation of breast disease. The primary care provider and nurse practitioner must be aware of the screening recommendations and indications for breast MRI. The interpreting radiologists must be aware of the commonly encountered artifacts of breast MRI, to prevent them and improve the quality of the scan. Dynamic contrast-enhanced MRI is the most sensitive non-invasive imaging modality to differentiate benign and malignant breast lesions. The malignant lesions are small and often node-negative when identified with MRI compared to mammography alone. This diagnostic modality leads to early detection of breast cancer, better patient management, and outcome. Overall, the mortality rate for breast cancer has been decreasing due to screening and improved management, which is best accomplished with an interprofessional team approach using the various physician specialties (radiology, oncology, and the PCP) along with the nursing staff and radiology techs.