Measuring magnetic ﬁelds generated by DC currents in receive-only coils
James N. Leea,*, J. Rock Hadleya, Michael C. Stecknerb
University of Utah, School of Medicine, Department of Radiology, Salt Lake City, Utah 84132, USA b Philips Medical Systems, 595 Miner Road, Cleveland, Ohio 44143, USA Received 17 September 2001; accepted 17 June 2002Abstract DC decoupling currents applied to receive-only coils during radiofrequency transmission can create stray magnetic ﬁelds capable of changing the resonant frequency of nearby nuclei. It is difﬁcult to measure these ﬁelds with conventional ﬁeld-mapping techniques because the ﬁelds are not present when the signal is acquired. The stray ﬁelds can be measured empirically with cardiac tags.© 2002 Elsevier Science Inc. All rights reserved.
Keywords: Receive-only; Coil; Cardiac tag; Phased-array
1. Introduction Some MR imaging systems use one radiofrequency (RF) coil for transmission and another for signal reception. During RF transmission the receive coil can be decoupled or detuned with DC current. This current activates a pin diode which completes a parallel resonance circuit inthe coil and disables the receive coil’s sensitivity to the transmitted frequency. When RF transmission is complete, the DC current is turned off, the parallel resonance circuit is eliminated and the receive coil returns to its tuned and matched condition for optimal signal reception. The DC decoupling current generates a stray magnetic ﬁeld which can shift the resonant frequency of nearbynuclei. This ﬁeld shift can change the resonant frequency of water nuclei sufﬁciently that they experience fat saturation pulses. However, the amplitude of this stray magnetic ﬁeld is difﬁcult to measure with conventional ﬁeld mapping techniques [1,2] because it is not present during signal acquisition. This note describes a technique for empirically measuring stray magnetic ﬁelds with cardiac tags[3–5]. By comparing tag patterns acquired in receive-only coils with those acquired in a transmit-receive coil, which requires no DC current for decoupling, one can determine how far the tags
were shifted by the stray magnetic ﬁeld. One can also compare tag symmetry near a coil element to tag symmetry far from the element to see how much the stray ﬁeld has shifted the tag. 2. Materials and methodsAll images were acquired on a Marconi Eclipse 1.5 T scanner. Informed consent was obtained from one human volunteer. Tagged images of a phantom were acquired using the transmit-receive body coil as the receive coil, and were compared to images acquired using receive-only coils. The receive-only coils included the head coil on the Marconi scanner and a four element phased-array (PA) coil designed andbuilt in our lab. A schematic of the DC current path in two elements of the PA coil is shown in Fig. 1. The inset to the right shows the circuit used in each coil loop for decoupling. The inductor and capacitor form a parallel resonance circuit which is activated when DC current ﬂows through the pin diode. In practice one pair of coil elements is placed on each side of the object to be imaged. DCcurrent in coil elements parallel to the main magnetic ﬁeld generates stray ﬁelds that do not have noticeable effect on the resonant frequency of nearby nuclei. Such stray ﬁelds are perpendicular to the main ﬁeld, and the vector addition of the small stray ﬁeld with the large main ﬁeld causes a negligible change in ﬁeld amplitude and therefore resonant frequency. For example, the net ﬁeld createdby a 100 Gauss
* Corresponding author. Tel.: 1-801-4228; fax: 1-801-585-3592. E-mail address: email@example.com (J.N. Lee).
0730-725X/02/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 2 ) 0 0 5 3 2 - 5
J.N. Lee et al. / Magnetic Resonance Imaging 20 (2002) 607– 610
Fig. 1. Schematic of the DC current path in the...