A Review of MRI of the Breast and MRI-Guided Stereotactic Biopsy

Suzan Lowe, BA, RT(R)(MR)


  *Senior Research Technologist, Department of Radiology-Magnetic Resonance Imaging, The University of Michigan Health System, Ann Arbor, Michigan.
  Address correspondence to: Suzan Lowe, BA, RT(R)(MR), Senior Research Technologist, The University of Michigan, Department of Radiology UH-B2B 407, 1500 East Medical Center Drive, Ann Arbor, MI 48109. E-mail: srohrer@med.umich.edu.

Disclosure: Ms Lowe reports having no financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

Changes in technology are altering the role of magnetic resonance imaging (MRI) in breast imaging. MRI continues to show improvement in the detection and diagnosis of breast cancer, even in its earliest stages. It is especially beneficial for imaging lesions that are occult on other modalities. To better understand the role of MRI in conventional breast imaging and as a modality for guiding biopsies this article will provide a review of basic MR breast imaging techniques, an overview for technologists of MRI-guided stereotactic breast biopsy procedures, and highlights of the pros and cons of MRI breast biopsy guidance as an alternative to surgical biopsy. The role of the MRI technologist, various safety considerations, and technical developments are also discussed.

Introduction
 on-palpable breast lesions are common findings on mammography which has long been the gold standard for breast cancer screening.1,2 However, not all lesions can be depicted by mammography alone. Magnetic resonance imaging (MRI) has the potential to identify occult disease that cannot be detected by physical examination or seen on mammography or sonography.3 Technical improvements have enhanced routine MR breast imaging and have enabled MRI-guided stereotactic biopsies in cases where suspicious MRI findings have been identified. In many cases the need to perform a more resource-intense surgical biopsy is eliminated. The superior anatomical detail, multiplanar capability, detection of contrast enhancement, and the availability of stereotactic image guidance makes MRI a very useful tool for both routine breast imaging and biopsy guidance of lesions, especially those visible only on MRI.

The Utility of MR in Breast Imaging
Patients can benefit from MRI for various reasons. MRI is highly sensitive for detecting invasive breast cancers. It can be useful in screening high-risk patients and those with a strong family history of breast cancer. The American Cancer Society supports MRI in conjunction with mammography for screening high-risk patients.1 In the United States at least 10 000 new cases of breast cancer are diagnosed every year in patients with a high genetic risk.4 It has been reported that breast MRI may identify cancers not visualized on mammography in 33% of patients.5 Ten percent of patients with a malignant finding on mammography will have another malignant lesion that is not detected by mammography or sonography.6

Magnetic resonance imaging can improve visualization where other modalities fall short. Dense breast tissue is demonstrated better by MRI than on mammography or sonography. As breast tissue density increases the sensitivity for cancer detection on mammography decreases.7 Even the density of silicone does not completely obscure breast cancer findings on MRI as it can on mammography.8 Angiogenesis is well demonstrated and can signal invasive tumor or inflammatory changes.7 One disadvantage is that MRI is not as highly sensitive to ductal carcinoma in situ (DCIS) because calcifications are harder to detect with MRI and DCIS enhancement patterns can vary.

Figure 1MRI of the Breast
There is no set standard or single protocol for MRI of the breast. However, the majority of MRI breast imaging protocols for cancer evaluation include T2-weighted, T1-weighted pre- and post-contrast-enhanced sequences, and the additional use of fat suppression techniques. Fat suppression is vital to detecting lesions that might otherwise appear isointense to fat. Enhancing lesions and fat both appear hyperintense on post-contrast imaging. Protocol variations strongly influence image quality and sensitivity. High-quality MRI allows the breasts to be evaluated before biopsy, reducing the number of biopsies performed unnecessarily on benign lesions (Figure 1).8

Magnetic resonance sequences may be 2-dimesnsional or 3-dimensional (3D), spin echo, or gradient echo depending on site preferences. 3D sequences benefit from increased signal-to-noise ratio (SNR), which allows for thinner slices. Isotropic voxel dimensions make it easier to reconstruct 3D scans with less distortion. Stereotactic image guidance requires 3D imaging.

Imaging can be performed unilaterally or bilaterally. Bilateral imaging is favored when there is concern for asymmetry or a question of cancer involvement in both breasts. The sagittal plane tends to require a higher number of slices than the axial plane for coverage of both breasts. Increasing coverage can also increase scan time. Scanning bilaterally can lower spatial resolution.

T2-weighted sequences are fluid sensitive and may show fluid-filled abnormalities such as cysts or fibroadenomas.9 T1-weighted sequences are used to evaluate anatomic structures, architectural distortion, and changes in enhancement after contrast.

Non-enhanced and enhanced T1-weighted imaging with identical parameters is essential for performing subtraction after the administration of contrast. Subtraction is extremely sensitive to motion and misregistration. It is particularly helpful when uniform fat suppression cannot be achieved by other methods.

3-dimensional gradient echo T1-weighted sequences typically provide the greatest spatial and temporal resolution for dynamic imaging.9 Temporal resolution should be approximately 60 to 120 seconds per dynamic acquisition.7 Evaluating kinetic information requires higher temporal resolution whereas evaluating morphology requires greater spatial resolution. Morphologic assessment includes things such as size, shape, density, and borders whereas kinetic assessment focuses on the enhancement features. Irregular borders, peripheral enhancement, increased angiogenesis, asymmetries, and architectural distortions are some examples of concerning characteristics that are best demonstrated with high-resolution T1-weighted imaging. Maintaining the balance between temporal and spatial resolution is essential to high-quality breast imaging.

Figure 2Dynamic contrast enhancement (DCE) sequences are used to generate time-intensity curves that show the uptake and washout of contrast material (Figure 2). During DCE a single dose (0.1 mmol/kg) of gadolinium contrast is injected intravenously as a rapid bolus using a power injector. The imaging has to be performed fast enough to catch the arterial phase. Sufficient temporal resolution can catch a lesion as it enhances before the enhancement of surrounding breast parenchyma has time to catch up and mask the lesion. During dynamic scanning the volume is scanned repeatedly over time. The arrival and uptake of contrast is plotted over time as a time-signal intensity curve. The kinetic data are used to show enhancement patterns that help establish a differential diagnosis between benign and malignant lesions. The early phases of DCE are best for evaluating the borders of a lesion.7 Invasive cancers tend to enhance rapidly following contrast compared to most benign abnormalities that usually show slower enhancement rates.8 Peak enhancement is often reached within the first 2 minutes in invasive lesions.7 Malignant cancers tend to demonstrate strong rapid enhancement during the early phases followed by rapid signal loss or a signal plateau. The enhancement pattern of DCIS can be an exception. Delayed phase enhancement varies and is often misleading with only 70% of DCIS showing rapid enhancement early on.7

Figure 3Time-intensity curves and enhancement patterns alone are not 100% accurate. Many other factors have to be taken into account and caution should be used. Note that not all enhancing lesions turn out to be malignant.10 Fibroadenomas and hyperplasia are examples of benign findings that can demonstrate enhancement patterns similar to malignant findings. Both false-positive and false-negative results can occur and prevent patients from receiving the appropriate treatment. Previous surgery or biopsy, radiation therapy, and changes during the patient's menstrual cycle can cause false-positive MRI enhancement.8 Abnormalities demonstrating false-negative enhancement could be misdiagnosed resulting in the lack or delay of treatment. Figure 3 provides an example of a time-intensity curve showing early and rapid enhancement.

Advanced MRI may include diffusion-weighted imaging and spectroscopy. Diffusion-weighted imaging is used to measure the restriction of water molecules. Diffusion restriction between normal and pathologic tissue can be quantified by measuring apparent diffusion coefficient values. This can improve specificity or be used to evaluate early response to neoadjuvant chemotherapy. MR spectroscopy is most often used to measure choline/fat or choline/creatine ratios in the breast. In normal breast tissue creatine is present and choline levels are typically low. Abnormal ratios can indicate the presence of disease.

Routine MRI is often performed as part of the patient's complete workup before a biopsy is performed under MRI guidance. The number of sequences and scan time during a biopsy are greatly reduced when MRI is performed in advance. Scanning is often limited to high-resolution pre- and post-contrast 3D T1-weighted imaging for quick localization of the fiducial markers and lesion that are essential for stereotactic guidance.

Types of Breast Biopsy Procedures
Different kinds of biopsies can be performed: fine needle aspiration (FNA), core needle biopsy (CNB), vacuum-assisted device (VAD), and wire localization.11 Each has its own advantages and disadvantages. MRI guidance can be used to assist with all 4 types of biopsy.

Fine Needle Aspiration 
Fine needle aspiration uses a very small needle (22-25 gauge) to extract cells from superficial lesions into a syringe. FNA requires a trained cytopathologist to evaluate the sample. Benign and atypical findings on FNA can raise the question of false-negative findings which can be as high as 32%.12 This may result in a second procedure to confirm the original findings. In the United States, FNA has largely been replaced by core needle procedures.2

Core Needle Biopsy
Core needle biopsies use a larger needle (11-14 gauge) connected to a trough that is covered by a probe that samples tissue using a spring-loaded or vacuum-assisted sampling gun. These devices can access lesions and collect tissue samples from deep within the breast. CNB is fairly less invasive than a surgical biopsy and can be performed in many outpatient settings.

Vacuum-Assisted Device
Vacuum-assisted devices use a disposable probe that connects to a suction unit to remove tissue samples by pulling tissue into the cutting chamber. The samples are suctioned from the patient while the probe remains in place. Better sampling results are achieved by the collection of larger, less fragmented samples compared to non-vacuum-assisted core procedures. Insufficient sampling occurs far less frequently with vacuum-assisted CNB than with FNA. Larger tissue sampling slightly increases the risk of bleeding and hematoma which are expected to occur in less than 1% of patients.11 Another advantage of VADs is being able to take multiple samples from different directions during a single needle placement, allowing more samples to be procured in less time. The same probe allows for marker deployment when the biopsy is complete. Vacuum assistance has been reported to decrease the rate of marker migration.13 VAD systems require extra equipment and accessories that can be costly, but the advantages typically outweigh the additional expenses.

Wire Localization
Wire localization deploys a guide wire through a needle. The hollow needle has a hooked wire inside that anchors into the suspicious area. The wire must be placed as close to the lesion as possible to ensure accurate sampling. Once the area is localized the needle is retrieved, leaving only the wire in place to guide the surgeon. The end of the wire lets the surgeon know exactly where to extract tissue. The wire is removed along with the tissue samples.

Comparing Sonography and MRI for Biopsy Guidance
Sonography is another imaging modality that can be used to guide breast biopsy procedures. Sonography shares many similarities with MRI. Similar to MRI it provides high-quality anatomical detail, soft tissue contrast, and real-time imaging. They both have multiplanar capabilities and can be used with stereotactic assistance. Both lack the use of ionizing radiation and are considered suboptimal for visualizing calcified lesions or clusters of microcalcifications. Lesions near the chest wall, high in the axilla, or those located in the distal breast region may be better served by sonographic guidance.12 Sonography also allows the patient to be imaged in a supine position, which is not possible with MRI.

Targeted or second-look sonography imaging can often identify lesions once thought to be detectable by MRI only. In such a case, if a biopsy is required, it should be performed with sonography guidance rather than MRI guidance because it tends to be more cost effective, easier for the patient to tolerate, and more widely available than MRI for guiding biopsies.

Magnetic resonance imaging does have some advantages over sonography that should be considered when selecting the best modality to guide a biopsy. MRI allows imaging of a larger field of view and better visualization of structures deep within the breast.14 Better visualization means a better chance of targeting the most suspicious part of a lesion and obtaining the best sample for pathologic testing. Easy visualization of vascular structures reduces the risk of complications during an MRI-guided biopsy.14

The American College of Radiology (ACR) has established guidelines to help uniformly assess the criteria for performing stereotactically guided breast biopsy for each modality including MRI. The ACR recommends that lesion visualization and access, availability of the imaging modality, efficiency, safety, and the practitioner's experience are all used to determine the best guidance technique.15

Stereotactic Guidance and Computer-Aided Detection Systems
Stereotactic biopsy guidance improves the efficiency, accuracy, and ease of targeting breast lesions. Stereotactic planning systems are computer based and use a fixed coordinate system. It works by assigning the lesion a specific location that is identified in terms of X, Y, and Z coordinates that are relative to the fiducial marker or set point of reference.16 The computer uses the X, Y, and Z coordinates to calculate the exact needle position required to target the designated lesion. Effectively and accurately planning the best trajectory and entry point increases targeting success and minimizes risks to the patient. An example is provided in Figure 4.

Figure 4

Stereotactic planning is just one of many options that are available on most computer-aided detection (CAD) systems. Other CAD applications for MRI of the breast include, but are not limited to, standardized reporting, multimodality viewing, archiving, maximum intensity projections, subtraction, reformatting, curve analysis, color overlay, and automatic detection.

Preparing the Patient for MRI-Guided Stereotactic Biopsy
Before any type of MRI-guided biopsy is performed the patient must be screened for MRI-related contra-indications. The patient's renal function, allergies, medications, and medical history are evaluated and the patient's consent is obtained. The MRI-guided biopsy procedure should be discussed with the patient as well as any other alternatives. The patient should be informed of any potential risks including the chance of infection or injury. It is important that blood thinners and anticoagulants are discontinued at least 3 days prior to a biopsy to decrease the risk of complications due to excessive bleeding.

Hormonal changes can make some breast lesions difficult to see.17 It is important to know that normal breast tissue can enhance more or less at certain times during a woman's menstrual cycle. Imaging is normally recommended during the 7 to 14 days mid cycle. Any history of hormone replacement therapy should also be noted. This can help reduce the chance of missing a lesion during routine MRI or during a biopsy.

The patient's ability to tolerate the procedure also must also be taken into consideration. During the biopsy the patient is to lay prone for approximately 45 minutes which can cause back and neck pain. Patients need to be made aware of the importance or remaining still throughout the procedure. Even subtle shifting or occasional movement is enough to cause substantial motion artifacts that are visible in the phase encoding direction. Patients experiencing claustrophobia or pain issues may need extra reassurance or intervention. Taking a few extra minutes early on to ensure the patient's comfort during the biopsy is certainly worthwhile. Positioning supports can be placed under the patient's head and arms to decrease discomfort or strain through the neck and shoulders. Patients that cannot maintain their positioning may have to undergo another type of biopsy.

Once it is determined that the patient can safely undergo an MRI biopsy, the patient should be changed into a gown or clothing that is free of any metal. An intravenous (IV) is placed prior to starting in the patient's arm or hand so that gadolinium contrast can be given later in the procedure.

The Universal Protocol for Preventing Wrong Site, Wrong Procedure, Wrong Person SurgeryTM from the Joint Commission Board of Commissioners requires a "time out" to be conducted in the biopsy suite just prior to beginning the procedure. The entire procedure team is expected to be present. A mandatory check including confirming the correct patient's identity, correct side and site, agreement on the procedure to be performed, correct patient position, and the availability of special equipment or requirements must be completed and documented.15

MRI-Guided Stereotactic Biopsy Procedure
During the stereotactic MRI-guided biopsy, the patient is placed in a dedicated breast coil in a prone or slightly oblique decubitus position. Prone positioning reduces motion artifacts. Positioning the patient as close to isocenter as possible is ideal for minimizing field inhomogeneities. Different equipment designs allow the patient to enter the scanner head first or feet first. The feet first prone position may be preferred because it makes it easier to access the patient and may even help reduce claustrophobia.

Each breast is suspended into a separate compartment in the coil. The breast coil works like an antenna to receive radio frequency waves that are used to create MRIs. Mild compression is applied to immobilize the breast during the procedure. Applying too much compression during imaging can obscure lesions and reduce blood flow, further restricting IV contrast. Not immobilizing the breast enough can result in failed or multiple attempts to sample the lesion.

As much care should be taken to position the patient correctly for the MRI as would be taken to position the patient during mammography. The positioning of the patient should be checked to make sure that no skin folds exist in the breast tissue. A common mistake is to exclude tissue from the upper chest or to include unnecessary abdominal tissue. Medial breast tissue should also be pulled away from the center support of the breast coil to make sure that all breast tissue falls freely within the field of view. The patient should be positioned to include as much of the breast and axilla as possible. Each breast has to be centered to the center of the coil to avoid artifacts and uneven fat suppression.

The imaging field of view must cover all of the breast and the fiducial markers on the grid device. Depending on the location of the lesion, the equipment may be set up to allow for a medial or lateral biopsy approach. Some systems are even designed to allow for a cranial approach.

Figure 5Figure 5 shows a breast biopsy system. The patient is moved into the scanner and initially imaged to localize the lesion and fiducial markers. A fiducial marker is simply a tube filled with gadolinium. In order to be well visualized on MRI, the fiducial marker is filled with a gadolinium to saline ratio of 1:100. Copper sulfate can also be used. Any air bubbles should be expelled to ensure accuracy. The proper filling of these markers should be checked routinely.

Subsequent 3D T1 imaging is performed with and without contrast to focus on the lesion. The pre-contrast scan will serve as a mask for subtraction. Gadolinium contrast is given to enhance the lesion, making it easier to determine the best location to sample. Sagittal, axial, or coronal imaging can be performed; submillimeter to 1-mm resolution is highly recommended.

The images are immediately sent to a CAD system with special stereotactic software that measures the lesion in relationship to the fiducial markers on the biopsy grid. The radiologist uses the post-contrast images to identify the fiducial markers and the lesion to be targeted while the computer automatically calculates the entry point of the needle. The X coordinate is the superior-inferior position, the Y coordinate is the anterior-posterior position, and the Z coordinate identifies the depth the needle must travel within the breast to the reach the lesion.4 Targeting software is used to find the shortest distance and best needle trajectory for accurate sampling.5 Some systems will display this information electronically on a monitor in the scan room or a print out can be made of the coordinates. The radiologist will decide which length targeting set will be used. An 8-, 11-, or 14-gauge needle or probe can be used depending on the location of the lesion. The needle will be positioned perpendicular to the biopsy grid at the location determined by the coordinates. Several different grid configurations can be used. The accuracy of needle placement should be ±1 mm using stereotactic biopsy guidance.12 Using a pillar post on the biopsy device allows the needle to be angled upon insertion into the breast. This can be particularly helpful for lesions that are hard to reach or close to the chest wall.

The same calculations can also be made manually without CAD assistance. This can increase calculation errors and be time consuming. Stereotactic assistance is preferred because it provides very efficient, consistent, and precise spatial localization.3

Once the coordinates have been determined, the patient is then moved out of the magnet. The skin is cleaned and a local anesthetic such as lidocaine is injected into the breast tissue to numb it before the biopsy needle is inserted. The anesthetic is not injected deep enough to interfere with the site being targeted. The field must now remain sterile during the duration of the biopsy. A scalpel is used to make a very small incision in which the needle will enter. Most patients experience only mild discomfort and pressure. Additional non-prescription pain medicines for use after the procedure are rarely necessary.

Using the predetermined coordinates, the radiologist will advance the biopsy needle to the specified depth. The depth can be visualized by the increments along the needle. The patient is imaged again with the needle in place to confirm needle placement in relation to the lesion before tissue samples are collected. If any adjustments are made, the patient must be imaged again. If no change was made to the needle position, the inner stylet is removed and the vacuum-assisted cutting probe is inserted. The needle tip is carefully positioned to allow the alignment of the lesion within the cutting chamber.

Multiple samples are collected from different positions surrounding the lesion. The sampling locations are identified as the positions on a clock are. For instance, samples may be taken from the 2 o'clock and 8 o'clock positions. If a vacuum device is not used, a spring-loaded biopsy gun can be used instead. Once collected the samples will be placed in a formalin filled specimen container and sent for analysis. The sample will be accompanied by the radiologist's interpretation for concurrence with the pathology results. The number and size of samples must be sufficient for the pathologist to make a definitive diagnosis. Eight to 10 samples are usually enough. An adequate sample allows the pathologist to report the following factors: tumor type, tumor grade, and other bio-indicators that will be used to determine the patient's course of treatment and predicted response to treatment.2 The cutting probe is removed when the sampling is complete. A confirmation scan is performed to verify the sample was obtained.

A titanium marker/clip is placed through the hollow of the needle into the area that was sampled before the needle is finally removed. The patient is moved back into the scanner and a final scan may be performed to confirm placement of the marker. This makes the area easier to identify for further interventions or follow-up care.

When the procedure is complete the patient is moved out of the scanner and the targeting set is removed and disposed of. The patient is able to sit up. The incision is covered with steri-strips and manual compression is applied for 5 to 10 minutes following the procedure to stop bleeding and minimize bruising. The area may additionally be iced to reduce swelling and pain. The patient is given post-procedure instructions and can expect to return to normal activities within a day. The entire MRI-guided stereotactic breast biopsy can generally be performed in 30 to 60 minutes. Images from an MRI-guided stereotactic breast biopsy are included in Figure 6.

Figure 6

MRI Technologist's Role and Training
The procedure involves a learning curve for technologists as they truly become efficient in performing MRI-guided stereotactic biopsies. Technologists may work alone or be assisted by mammography and nursing staff. The MRI technologist's role can vary depending on the make up of the interventional team. They generally have a significant role in equipment set-up, patient positioning, and making sure all of the necessary supplies are available. Biopsy grids, targeting sets, ice packs, and sterile trays with scalpels, syringes, forceps, specimen containers, and gauze are a few of the essentials that must be on hand. MRI technologists have the added responsibility of helping to make sure all equipment and instruments are MRI compatible.

The ACR has established training guidelines for radiologists and technologists performing stereotactic-guided biopsy procedures. The following are just a few of the ACR's recommendations.

Biopsy procedures demand technical skill, accuracy, plus high-quality and efficient scanning techniques to achieve the best results. The MRI technologist needs a solid knowledge of imaging techniques and parameters. This includes knowing how to obtain effective fat suppression, good contrast weighting, achieving an acceptable balance between spatial and temporal resolution, and correctly positioning the patient in addition to providing good patient care throughout the procedure.

Susceptibility Effects and Fat Suppression
Susceptibility effects have to be recognized and eliminated as much as possible. Susceptibility causes signal voids and misregistration at air tissue interfaces where the local magnetic fields are different. Susceptibility effects can be reduced by scanning on a lower field strength magnet. Smaller lesions (<5 mm) can be obscured by magnetic susceptibility from the biopsy needle.5 The orientation of the needle in the magnet can increase the effect. Susceptibility increases when the needle is positioned perpendicular to the main magnetic field.6 Magnetic susceptibility can affect how accurately the stereotactic coordinates are calculated or how images are interpreted by creating distortion or obscuring the area of interest. To compensate for susceptibility-related field inhomogeneities a larger pixel bandwidth and smaller fat water shift can be used with a slight cost in SNR.

Achieving uniform fat suppression in the breast can be especially difficult at high field strengths. Improvements in fat suppression can be made by applying volume shims to improve field homogeneity. Center frequency must be properly positioned on the water peak instead of the fat peak during the pre-scan. Suboptimal fat suppression is more likely to occur in patients with higher ratios of fatty tissue to breast parenchyma.

MRI Safety Considerations
The MRI environment itself can be a challenge to perform biopsies in. MRI safety has to be maintained at all times. This means that anyone entering the MRI room during the procedure must be screened for various implants in their body and previous injuries involving metal. They must be fully aware of the danger presented by ferromagnetic objects. Metallic objects such as needles and scalpels can be attracted to the magnet. Also, metal objects within the patient can be dislodged by the magnetic field.

Figure 7Any equipment being used for the biopsy must be specially designed to work in the MRI suite. Susceptibility effects and ferromagnetism strongly influence instrument design. Distortion factors include the size and shape of the object, the object's orientation in the body, and imaging technique.18 The demand for these compatible tools and devices is increasing along with the demand for MRI-guided biopsies. Needles with higher nickel and titanium contents have been produced to decrease distortion and ferromagnetic pull. Allowing the biopsy needle to shift even slightly could prevent adequate tissue sampling or cause injury to the patient. It also prevents any type of ferromagnetic needle from staying in the patient during scanning. Non-ferromagnetic needles have been criticized for being more expensive and less effective than steel needles.4 It can be a challenge to find MRI equipment and instruments that are compatible with the magnetic field without sacrificing functionality (Figure 7).

Technical Developments
The inherent benefits of MRI and technical advances are helping to improve the utility of stereotactic MRI-guided biopsies. Improvements in pulse sequence design provide better spatial and temporal resolution which can increase visualization of smaller lesions and provide better conspicuity.

Many breast coil designs offer multiple channel phased array configurations that make parallel imaging techniques possible. Parallel imaging can greatly reduce scan time. Most of the newer coils are designed accommodate biopsy equipment. There are even coils that are integrated into tables designed specifically for imaging the breast and performing biopsy procedures (Figure 8).

Figure 8

Open MRI scanners have the additional advantage of increasing access to the patient during the procedure. This can decrease or eliminate the need for the table to move in and out of the scanner during the procedure. Even short closed bore systems require the patient to be moved in and out of the scanner. Most biopsies are still performed on closed bore scanners. Not all open scanners are capable of providing image quality adequate for conducting biopsies. Powerful gradients are required for breast imaging on any type of system. Many open or closed bore high-field 1.5 Tesla (T) and 3T scanners provide plenty of SNR and resolution for breast imaging and biopsy guidance. Scanners with field strength of 1T or less are generally not recommended for breast imaging.

As 3T scanners become increasingly common for clinical use, it is important to know that the increase in field strength offers greater SNR, which can be traded for other benefits including resolution and faster scanning. It also increases specific absorption rates, B1 inhomogeneity, susceptibility effects, and T1 relaxation times. All of these factors should be taken into account when building breast imaging protocols at 3T.

Advantages of MRI-Guided Stereotactic Breast Biopsy
Magnetic resonance imaging-guided stereotactic biopsies offer several advantages over surgical biopsies. The procedure can be performed relatively quickly on an outpatient basis where the patient may feel more at ease. The MRI-guided biopsy is more cost-effective compared to a surgical biopsy. The cost can be approximately 50% of the cost of surgical biopsy and takes less time to perform.8 Because the MRI-guided biopsy is less invasive than surgery there is less risk to the patient. Procedure-related risks and complications are extremely rare and far less likely to be clinically significant compared to surgical biopsies. Local anesthetics can be used instead of general anesthesia which may be better tolerated by the patient. The accuracy of stereotactic MRI-guided biopsies is comparable to a surgical biopsy. Post-procedure scarring tends to be minimal. Less scarring has a cosmetic benefit but also reduces the chance that the scar could obscure underlying tissue on mammography. Recovery time is minimal. The diagnosis is often made faster because of better access to biopsy suites than operating rooms.

Disadvantages of MRI-Guided Stereotactic Breast Biopsy
Even though there may be several reasons to perform a stereotactic MRI-guided biopsy, it is not always the way to go. MRI is not the modality of choice for biopsying lesions that can be easily identified by mammography or sonography.11 Even though MRI-guided biopsies are less costly than surgical options, other modality-guided interventions may be even more cost-effective. The number of facilities equipped to perform MRI-guided biopsies is still somewhat limited. Claustrophobic patients also may have a difficult time being inside the MRI scanner.

Technical failures are infrequent, but they do happen. It is possible to miss a lesion all together or to get an inadequate sampling. Equipment that is not used properly decreases targeting accuracy, which may lead to a second needle pass. Targeting smaller lesions has a greater chance for error whereas larger lesions frequently offer better success rates. Some sites do not recommend MRI-guided biopsies of lesions that are less than 10 mm in size because there is an increased chance of uncertain results.6 Small lesions or lesions located near the chest wall tend to pose the greatest challenge because they are more difficult to target.

A surgical biopsy may still be performed if the results from the MRI-guided biopsy are inconclusive or if an adequate sample was not obtained. If breast cancer is still suspected after negative biopsy findings are reported, a surgical biopsy is usually needed to confirm the results.

Significant complications include hematomas requiring drainage, excessive bleeding, infection, and other injuries. Accidentally advancing the biopsy needle into the chest wall could injure muscle or cause the lung to collapse. The risk of tumor seeding is similar to that of an open surgical biopsy.12

Conclusions
Technical advances are escalating the role of MRI in breast imaging and biopsy guidance. MRI is increasingly being used to localize and confirm breast lesions that are occult on sonography or mammography. The superior anatomical detail, multiplanar capability, and detection of contrast enhancement rarely allow invasive cancer to go undetected on MRI. High-risk patients and those with characteristics that are difficult to image with mammography or sonography can benefit from some the advantages of MRI. Used in conjunction with mammography and sonography, MRI may not only replicate findings but offer additional information.

Magnetic-resonance imaging-guided stereotactic biopsy offers a high-quality alternative to surgical biopsy for lesions visible only on MRI. It provides a cost-effective means of reducing surgical biopsies while achieving comparable results. The advantages of MRI-guided stereotactic breast biopsy include excellent sample retrieval, low occurrence of complications, procedure efficiency, better accessibility, and quicker recovery. As a modality, MRI offers an additional means of detecting and confirming breast cancer, which ultimately improves the management of breast disease.

References
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