HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Figures Only Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kuhl, C. K. Articles by Schild, H. H. Search for Related Content PubMed PubMed Citation Articles by Kuhl, C. K. Articles by Schild, H. H.

Effect of B₁ Inhomogeneity on Breast MR Imaging at 3.0 T

Christiane K. Kuhl, MD ^* , Hendrik Kooijman, PhD , Juergen Gieseke, PhD ^* , and Hans H. Schild, MD ^*

^* Department of Radiology, University of Bonn, Sigmund Freud Str 25, Bonn 53105, Germany
Philips Medical Systems, Hamburg, Germany
e-mail: kuhl{at}uni-bonn.de

Editor:

In the June 2006 issue of Radiology, we reported on our results on dynamic contrast material–enhanced breast magnetic resonance (MR) imaging at 3.0 T, with 1.5 T comparison in the same patients (1). Our results suggest that the higher spatial resolution afforded by the higher magnetic field strength could be translated into a higher diagnostic confidence with which enhancing lesions were categorized. However, we would like to alert the radiologic community regarding an important pitfall that may be associated with breast MR imaging at 3.0 T. It seems that, due to spatial B₁ inhomogeneities across the field of view, enhancement of lesions may be reduced to a variable degree. The loss of enhancement will be more important with two-dimensional (2D) than with three-dimensional (3D) gradient-echo protocols. Accordingly, and until more data are available, we recommend to only use 3D gradient-echo for breast MR imaging at 3.0 T. But even then, 3.0-T breast MR images must be interpreted with great care. Enhancement thresholds must not be used for differential diagnosis. B₁ maps should be obtained in every patient in order to identify areas with reduced B₁ and, thus, reduced enhancement.

In our article we already reported that, against our expectation, the enhancement rates of lesions at 3.0 T had been lower than the respective enhancement rates obtained for the same lesions at 1.5 T. We already mentioned in the discussion that one reason for this was probably the fact that in order to comply with specific absorption rate limitations at 3.0 T, the flip angle had to be reduced from 90° (at 1.5 T) down to 60°–73° (at 3.0 T).

Meanwhile, we further investigated the reason for the lower-than-expected enhancement at 3.0 T and performed a systematic analysis of the radiofrequency field (B₁ field) obtained with the setup described in our article. We obtained B₁ maps for bilateral breast MR imaging at 3.0 T with a four- and a seven-channel breast coil (Figure). Our results show that the B₁ field across the field of view varies substantially, irrespective of the coil type. We observed consistently lower B₁ fields in the area of the right breast compared with the left breast, with a left-to-right difference by a factor of around two. The regionally variable B₁ fields will translate into regionally variable flip angles. Regionally variable flip angles will translate into a regionally variable T1-weighting and thus to variable enhancement of lesions, with reduced enhancement of lesions located in "low B₁ areas" of the right breast.

View larger version (140K): [in this window] [in a new window] [Download PPT slide]   Precontrast 3D T1-weighted gradient-echo breast MR image obtained with a four-element breast coil with a clinical 3.0-T system (repetition time, 6.0 msec; echo time, 2.3 msec; pulse angle set to 20°). Regions of interest (ROIs) are placed in the tissue of the left and the right breast; respective numbers give the local relative B1 in the ROIs as calculated by means of a B1 map (not shown). Regions with an ideal pulse angle would have the value 1.00. The actual pulse angles on this image vary between 22° and 12.5° over the field of view. Note that the area of low B1 field in the right breast, in particular close to the lateral chest wall, appears normal (or even with high signal-to-noise ratio) on the anatomic image.

The same will also apply for 3D gradient-echo protocols. However, due to the inherently higher baseline T1 contrast of 3D versus 2D imaging (which, in turn, is due to the substantially shorter repetition time of 3D protocols), the enhancement obtained even with lower flip angles will be consistently higher than that obtained with 2D protocols. Accordingly, nonenhancing breast cancers are not to be expected for 3D protocols at 3.0 T. However, diagnostic errors due to seemingly mild (ie, "benign") enhancement, are conceivable. This holds especially true because the predictable left-to-right differences in enhancement may be confusing.

It is important to note that the regionally variable flip angle may not be identifiable on the actual (non–fat-suppressed or fat-suppressed) gradient-echo images—that is, the pre- and postcontrast images will look fairly normal (Figure)—which means that a radiologist or a technologist will not realize that there is a B₁ inhomogeneity.

Currently, we investigate whether the observed B₁ inhomogeneities are caused by dielectric effects, and if so, which factors (patient size, patient position with respect to the body coil, breast size, breast composition) determine the B₁ homogeneity in high-field-strength breast MR imaging.

	REFERENCE

TOP REFERENCE

 

1. Kuhl CK, Jost P, Morakkabati N, Zivanovic O, Schild HH, Gieseke J. Contrast-enhanced MR imaging of the breast at 3.0 and 1.5 T in the same patients: initial experience. Radiology 2006;239:666–676.[Abstract/Free Full Text]

This article has been cited by other articles:

				C. E. Mountford, P. Stanwell, S. Ramadan, C. K. Kuhl, H. Kooijman, J. Gieseke, and H. H. Schild Breast MR Imaging at 3.0 T Radiology, July 1, 2008; 248(1): 319 - 320. [Full Text] [PDF]

				C. K. Kuhl, F. Traber, J. Gieseke, W. Drahanowsky, N. Morakkabati-Spitz, W. Willinek, M. von Falkenhausen, C. Manka, and H. H. Schild Whole-Body High-Field-Strength (3.0-T) MR Imaging in Clinical Practice * Part II. Technical Considerations and Clinical Applications Radiology, April 1, 2008; 247(1): 16 - 35. [Abstract] [Full Text] [PDF]

				C. K. Kuhl, F. Traber, and H. H. Schild Whole-Body High-Field-Strength (3.0-T) MR Imaging in Clinical Practice * Part I. Technical Considerations and Clinical Applications Radiology, March 1, 2008; 246(3): 675 - 696. [Abstract] [Full Text] [PDF]

This Article Figures Only Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kuhl, C. K. Articles by Schild, H. H. Search for Related Content PubMed PubMed Citation Articles by Kuhl, C. K. Articles by Schild, H. H.