MRI of the Thorax

CHAPTER 5 MRI of the Thorax



Chapter outline



MRI OF THE BREAST


CARDIAC MRI


MRA OF THE GREAT VESSELS OF THE THORAX


MRI OF THE BRACHIAL PLEXUS



MRI of the breast



Scan Considerations


MRI has a role in the detection and management of some breast cancers. Routine mammography can miss two of three breast cancers. Because of increased vascularity, breast malignancies typically enhance brighter and more intensely than benign tissues after intravenous injection of contrast. It is imperative to scan both breasts simultaneously in about 90 seconds per phase to evaluate for breast cancer. All vendors offer high-definition bilateral volume breast imaging, which makes this possible.


The GE VIBRANT (Volume Imaging for BReast AssessmeNT), or vendor equivalent, allows you to perform bilateral breast examinations in both the sagittal and axial planes. VIBRANT is a high-resolution fast spoiled gradient echo (T1) with ASSET (parallel imaging) and “Special,” a fat-saturation technique that uses an inversion recovery pulse to eliminate fat. The scan time with VIBRANT is equal to the scan time of a single-breast scan in the past. It obtains high temporal resolution without sacrificing spatial resolution. VIBRANT eliminates fat to enhance and better identify lesions.



Patient Preparation and Positioning


Patients are screened with both MRI screening and a breast screening form. The patient’s history and pertinent mammogram and ultrasound imaging information are important for radiologist interpretation.


Breast MR is menstrual cycle and hormone dependent. Breast MR is best performed in midcycle (week 2, day 7-14). Imaging the patient at this optimal time in her cycle does not completely exclude enhancement of benign breast parenchyma. Benign background hormonal enhancement is one of the greatest challenges facing interpretation of breast MR in premenopausal women.


The patient is prepared with a 22-G or less intravenous line in the antecubital vein. A dose of 0.1   mmol per kilogram of body weight of gadolinium contrast is suggested (see Contrast section). A power injector should be prepared with contrast and 20   mL of saline. The suggested flow rate is 2   mL/second for contrast and 2   mL/second for saline. An injector delay is determined at the time of scanning to allow for a mask and additional postcontrast phases. Timing is essential.


Before positioning the patient, breast markers should be placed on the patient’s nipples and any scars or biopsy sites. The technologist should explain the examination to the patient and emphasize the importance of holding still and not changing position, particularly between pre- and post-contrast scans. The patient is positioned prone in a


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Figure 5-1 Correct positioning in dedicated bilateral breast coil


high-definition (HD) breast coil. High-definition breast array coils are available from all vendors and have biopsy capabilities. The breasts should be hanging freely in the coil and not touching the sides. Place gauze between the patient and the coil when necessary to avoid any possibility of bright signal from coil burnout. The patient should be made as comfortable as possible, making sure to support the feet as well as the head and arms. Communicating with the patient during the examination is imperative. The patient’s intravenous line should be hooked up to the power injector before the examination begins.


A typical bilateral breast examination is composed of a calibration scan, which must extend past anatomy by 50% superiorly and inferiorly. FOV is patient-dependent Bilateral shim volumes should be applied, covering as much of the breasts as possible and taking care not to include the heart or lungs. A three-dimensional VIBRANT (T1 FSPGR) axial non–fat-saturated scan is acquired to evaluate the fatty structures of the breast. The axial plane is preferred for bilateral evaluation by the radiologist. Multiphase pre- and post-VIBRANT scans with “special” (spatial fat saturation) are performed, which include a mask and five or six post-contrast phases. The phase timing is determined by the VIBRANT non–fat-saturated scan. Autosubtraction is imperative to remove pre-contrast tissue. A sagittal VIBRANT post-contrast delayed scan should be performed. Sagittal T2 fat-saturation scans can be performed bilaterally as well as a bilateral axial T2 fat-saturation or STIR. Fat and water separation imaging, IDEAL, can be used to replace all fat-saturated imaging. The total scan time is around 30 to 45 minutes.


The technologist and nurse must be “in synch.” The programmable “inject delay time” should be doubled-checked to ensure that it is correct. The technologist should inform the patient of the injection before it is administered, reinforcing the importance of holding still. Timing is everything in breast imaging.


Evaluation of contrast image enhancement is important to determine lesion kinetics. This can help to determine malignant from nonmalignant lesions. Image timing is extremely important because malignant lesions can enhance and wash out their contrast in about 90 seconds. The “time-intensity curve” of dynamic phases can help determine malignant from benign. Computer-assisted diagnosis (CAD) for MR is available from several vendors. These systems provide time-intensity curves, multiplanar reformatting, and subtraction imaging options; angiogenesis maps; maximum intensity projections (MIP); volume summaries; and vascular maximum intensity projection images.


To evaluate a region of interest, a cursor is placed over the focally enhanced area, and a kinetic curve is produced and evaluated. In the early post-contrast phase (60 to 90 seconds after injection), the washout rate, or enhancement velocity, is quantified. Radiologists routinely analyze both the morphologic features of the lesion and the kinetic curves. Morphologic features are reliable tools in diagnosis and treatment planning.



Sequence Significance


Nonenhanced, nonfat saturation VIBRANT (T1-SPGR) images identify high signal hemorrhagic or proteinaceous cysts and high signal proteinaceous material within dilated ducts.


VIBRANT pre-contrast images are subtracted from the post-contrast images so only areas of enhancement are prominent.


Because both fat and water can appear bright on T2, fat-saturated sequences are performed. T2 fat saturation and/or STIR are used to determine high-signal masses such as cysts, lymph nodes, or fibroadenomas. Carcinomas are usually isointense or hypointense in this sequence.



Bilateral Breast Imaging


Acquire three-plane pilot per site specifications.


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Figure 5-2 Axial breast


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Figure 5-3 Sagittal breast


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Figure 5-4 Coronal breast



COIL: Multi-channel vendor-specific breast coil


POSITION: Prone, feet first with the breasts suspended in the coil


Place the cushion under the ankles, with the arms extended comfortably toward the head.


LANDMARK: Midcoil


IMMOBILIZATION: Shield the patient from touching the sides of the magnet by using sponges or sheet.



Tip:


Place MR markers on the patient’s nipples and surgical scar sites before positioning.



Acquisition of Axial images of the Breasts



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Figure 5-5 Sagittal image of the breast with axial locations


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Figure 5-6 Midline axial image of the breasts


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Figure 5-7 Midline axial anatomy of the breasts



SLICE ACQUISITION: Plot superior to inferior


SLICE ALIGNMENT: Perpendicular to the sternum


ANATOMIC COVERAGE: Chest wall through the nipple, including the axillary margins



Tip:


Breasts are often asymmetric. Cross-reference bilateral sagittal pilots to ensure complete coverage of both breasts. An additional post-contrast VIBRANT without using SPF (swap phase and frequency) will best visualize the axilla.



Key:


Adi, adipose; Gl, glandular lobule; Les, lesion; Ni, nipple; Ri, rib; Ster, sternum.



Acquisition of Sagittal Images of the Breasts



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Figure 5-8 Axial image of the breast with sagittal locations


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Figure 5-9 Midline sagittal image of a single breast


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Figure 5-10 Midline sagittal anatomy of the breast



SLICE ACQUISITION: For bilateral three-dimensional acquisitions, scan left to right through both breasts.


SLICE ALIGNMENT: Straight, no angle


ANATOMIC COVERAGE: Midthoracic cavity to the nipples, including the axillary margins




Tip:


Breasts are often asymmetric. Cross-reference axial pilots to ensure complete coverage of both breasts.



Key:


A, adipose; C, clavicle; GL, glandular lobule; IM, intercostal muscle; LD, lactiferous duct; N, nipple; PRM, pectoralis major muscle; R, rip; SL, suspensory ligament.



Table 5-1 MRI of the Breast

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Table 5-2 Site Protocol: MRI of the Breast

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Cardiac MRI



Cardiac Nomenclature


In 2002 the Cardiac Imaging Committee of the Council on Clinical Cardiology of the American Heart Association issued a statement for health care professionals. In an effort to standardize the nomenclature for cardiac imaging, they recommended “all imaging modalities should define, orient and display the heart using the long axis of the left ventricle and selected planes oriented at 90-degrees angles relative to the long axis” (Manuel D, et al: Standardized myocardial segmentation and nomenclature for tomographic imaging of the heart, Circulation 105: 539, 2002).


The nomenclature of short, vertical long, and horizontal long axis is suggested for cardiac planes used in cardiac MR. For the purpose of industry-wide consistency, this nomenclature will be adhered to in this text. See Figure 5-11 below.


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Figure 5-11 Planes of the heart



Key:


HLA, horizontal line axis; SA, short axis; VLA, vertical line axis.

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Jan 10, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on MRI of the Thorax

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