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Breast MRI at 3T

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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.

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									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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