Page 259 - Biomedical Engineering and Design Handbook Volume 2, Applications

238 DIAGNOSTIC EQUIPMENT DESIGN

High quality factors improve signal-to-noise ratios for receive coils. Note that extremely high Q may
result in making precise coil tuning more critical.

8.4.3 Receive Coils
RF receive coils receive signal and noise from the matter in the coil. If only a small region were to
be imaged, then signal may be generated only from the region of interest while noise is received from
the entire sample in the coil. 86,87,91 To reduce noise in the image, it is sensible to receive with the
smallest coil capable of spanning the region of interest. This concept is referred to as fill factor.
Transmit coils may also double as receive coils. Frequently, a larger, relatively homogeneous coil
such as a birdcage body coil will be used to transmit the excitation pulses. Then, a smaller, less
homogeneous receive-only coil called a surface coil 86,87,91 will be used to receive the signal. The
smaller coil generally has a better fill factor and so produces higher signal-to-noise ratios (SNR) than
would have been possible with the larger coil.
Large surface coil currents may result if receive-only surface coils were resonant while RF trans-
mit pulses are generated on the body coil. Such currents can produce opposing B fields which may
1
destroy transmit homogeneity. In addition, these large currents could result in locally high RF depo-
85
sition near the coils. Boesiger has shown conditions where the surface coil amplified normal body
coil heating 47-fold. To prevent such problems, blocking networks 86,87 are used (see Fig. 8.6). These
blocking networks present high impedances to surface coil currents during body coil transmit. The
required blocking impedance depends on coil area, magnet frequency, and how large an opposing
field is to be allowed. Typical blocking impedances are a few hundred ohms. Poorly designed sur-
face coil blocking networks may become warm. IEC 60601-1 sets a surface temperature limit of
41°C for objects that may touch people. 92
The high blocking impedance is switched out during receive. During receive, the surface coil is
made resonant. The transmit body coil normally would couple noise from the rest of the body into
surface coils during receive, degrading images. To prevent this sort of coupling, body coils are
detuned during the receive phase.
Further increases in SNR might be obtained using phased-array surface coils. 93 Phased-array
coils are designed to be orthogonal (received noise is uncorrelated). If data from each coil are sepa-
rately received and reconstructed, then SNR can be significantly increased over levels possible with
individual coils that are not orthogonal. The coils are made orthogonal by slightly overlapping them
until their mutual inductance approaches zero.

Surface coil: Must prevent high local SAR
I dB /dt
1
Limit RF current with blocking network
Transmit: Z is large
Z Receive: Z is small

FIGURE 8.6 A receive-only surface coil with a blocking network is shown. During
body coil transmit the blocking network, Z, becomes a high impedance to prevent high
induced currents from flowing on the coil. Such currents could lead to very high local
SAR levels. During surface coil receive, the blocking network becomes a very low
impedance to improve the image signal-to-noise ratio.