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The spectroscopic peaks of choline and sodium are smaller by several orders of magnitude relative to the water peak. [...] the SNR provided by higher field strengths, such as 3T, permit these metabolites to emerge with a more readily quantifiable peak.
Breast MRI at 3T Rebecca Rakow-Penner, MS, Brian Hargreaves, PhD, Gary Glover, PhD, and Bruce Daniel, MD B reast magnetic resonance imag- ing (MRI) is recommended for women at high risk for breast cancer1,2 and is increasingly being used for surgical planning3 and treatment monitoring.4,5 The current state of breast MRI focuses on collecting morphologic and dynamic information at a 1.5T mag- netic field strength. High-resolution images yield information regarding tumor morphology, while rapidly col- lected images acquired after contrast injections generate dynamic contrast- enhanced (DCE) information. These 2 techniques are used to improve the sensitivity and specificity of breast MRI. efficiencies being the same and ignor- throughout the entire object. B0 homo- Many malignant tumors tend to present ing relaxation effects). In practice, with geneity specifically suffers at interfaces with a spiculated border as well as with a finite repetition time (TR), the SNR between soft tissue and air, known as a faster contrast uptake and clearance than gains are <2. Overall, the SNR gain at magnetic susceptibility effect.6 The non- benign tissue. Thus, morphology and 3T allows for potential improvements spherical shape of the breast over the ster- DCE information are both important. At over the 1.5T protocol in image resolu- num exacerbates this effect. For example, 1.5T, publications note breast MRI sen- tion and faster data collection. In addi- most brain imaging does not suffer as sig- sitivities between 77% and 100% and tion, the greater spectral dispersion of nificantly from susceptibility effects specificities between 81% and 99%.2 fat and water at 3T may improve and/or because the head is mostly spherical and When compared with more standard reduce the duration of fat-saturation in the center of the bore. Susceptibility protocols at 1.5T, breast MRI at a pulses. In addition, at 3T, exploratory artifacts manifest when the shape of the 3T magnetic field strength (Figure 1) techniques such as spectroscopic imag- imaged object no longer maintains its provides several opportunities for im- ing and blood-oxygen-level–dependent spherical and symmetric shape; the air- proving diagnostic image quality. At 3T, (BOLD) contrast imaging become more tissue interface causes an artifact because breast MRI also opens new paths for feasible (Table 1). of the sharp change in magnetic field. For investigating novel techniques. By dou- breast imaging, this problem persists as bling the magnetic field strength, the Physics of transitioning each nonspherical breast is offset from signal-to-noise ratio (SNR) approxi- from 1.5T to 3T the center of the field, with a large air- mately doubles (contingent on the coil The SNR is directly related to the main tissue surface area in the center of the magnetic field, B0. As noted above, the field. Beyond the center of the field, this Ms. Rakow-Penner is an MD/PhD can- SNR at 3T is nearly double that at 1.5T. artifact also manifests around the breast’s didate in Biophysics, Dr. Hargreaves Increased SNR is the main motivation for circumference at the breast-body junc- is Assistant Professor of Radiology, 3T breast MRI. By increasing field tion. These approximately perpendicular Dr. Glover is Professor of Radiology, and Dr. Daniel is Associate Professor of strength, however, there are other impor- “corners” share the property of a nonuni- Radiology, Department of Radiology, tant considerations. First, when increas- form change in magnetic field. The sus- Stanford University, Stanford, CA. ing B0, it is ever more challenging to ceptibility artifact scales linearly from keep the magnetic field homogeneous 1.5T to 3T; thus, susceptibility artifacts at 6 ■ APPLIED RADIOLOGY © www.appliedradiology.com March 2009 BREAST MRI AT 3T Table 1. Breast MRI at 3T compared with 1.5T Effect Protocol modification Increased SNR Enjoy improved image quality or tradeoff with a faster imaging technique such as parallel imaging Increased contrast between tissue and gadolinium Match T1-weighting to 1.5T Increased separation between spectral peaks Employ improved fat saturation and spectroscopic pulse (improvement for spectroscopy) sequences Increased magnetic susceptibility (allows for T2* phasing) Evaluate new technologies such as BOLD contrast imaging Reduced B0 homogeneity Add shim Reduced B1 homogeneity Consider 3D rather than 2D imaging Increased SAR Decrease flip angle Longer T1 (potential increase in scan time) Increase TR SNR = signal-to-noise ratio; BOLD = blood-oxygen-level–dependent; 3D = 3-dim
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