CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 1. (a) Lumped-element “LC” loop, surface coil. (b) Divided lumped-element surface coil.
Figure 2. Gastrocnemeous muscle imaged at 170 MHz (4 T): (a) showing modeled magnetic vector potential (Webers); (b) conduction current density (A/mm2); (c) displacement current density
(A/mm2); (d) magnetic flux density (Webers/mm2); (e) power loss density (W/kg); (f) temperature
(ºC) with perfusion (g) and temperature (ºC) without perfusion (h).
Figure 3. (a) Phased-array coil. (b) Parallel array.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 4. (a) Inductively decoupled loops in a two-element array. (b) Single-loop surface
coil or array element with dedicated receiver circuit.
Figure 5. (a) Series RLC circuit & TEM analog. (b) Parallel RLC circuit & TEM analog.
Figure 6. (a) Coaxial transmission line, loop surface coil. (b) Strip line or microstrip line
surface coil.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 7. (a) B1 field generated by a single-loop coil. (b) B1 field generated by single line
element.
Figure 8. (a) High-pass birdcage; (b) bandpass birdcage; (c) birdcage shield.
Figure 9. (a) TEM volume coil [19]. (b) TEM volume coil element.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 10. Parallel transceiver [64].
Figure 11. (a) Linear-element TEM volume coil; (b) loop-element TEM volume coil.
Figure 12. (a) Linear-element volume coil image; (b) Loop-element volume coil image.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 13. Simple FEM model of (a) a three-layer head inside a linear-element TEM volume coil, (b) an RF magnetic flux density “B1 field” (Webers/mm2) in a head model with
linear coil excitation, (c) resultant power loss density (W/kg) in head model, and (d) consequential temperature contours (ºC) in the perfused-head model.
Figure 14. (a) An FDTD model of a head inside a uniform, circularly polarized TEM volume coil resonant at 300 MHz; (b) a 7T gradient echo head image.
Figure 15. (a) Model of TEM head coil with B1 gradient; (b) TEM head coil with B1 gradient and multi-element phase and magnitude control.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 16. (a) Model of central sensitivity profile ΣB1– calculated to partially compensate for the B1
field peak, ΣB1+; (b) in the center of the head coil of Fig. 15.
Figure 17. Model of head with B1 transmit magnitude optimization: (a) combined with phase angle
optimization; (b) to render a more uniform head image at 300 MHz.
Figure 18. B1 field control: (a) B1 gain (magnitude) control is shown; (b) B1 phase control is
demonstrated.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 19. (a) RF-shimmed 7T IR-turbo FLASH: (b) RF-shimmed 9.4T IR-gradient echo
image; (c) RF-shimmed 9.4T FLASH image.
Figure 20. Double-tuned TEM head coil: (a) proton, (b) phosphorus, and (c) sodium at 4 T.
Figure 21. (a) Actively detunable TEM transmit head coil with local receiver coil. (b)
Signal-to-noise contours acquired with transmit-receive surface coil only. (c) Signal-tonoise contour of transmit volume coil combined with surface coil receiver. (d) A local fourloop receiver array used with a homogeneous transmit coil improves both SNR and homogeneity at 4 T and above.
CHAPTER 6: Ultra High Field MRI: High-Frequency Coils
Figure 22. (a) Body coil with active detuning and multi-port drive. In this drawing a simple network of 180 and 90º splitters establishes circular polarization for the transmit field. Actively
switched shunt-detuning diodes are shown for each transmission line element of the coil. The segmented cavity serves as both a transmission line component and an eddy current-free shield for
the coil. (b) RF body imaging subsystem including body coil (a), surface array receiver (b), power
supply and detuning control unit (c), detuning diode driver unit (d), multichannel receiver preamp and bias tees (e), fiberoptic diode control lines (f), data lines (g), and RF signal cables (h).
Figure 23. (a) 4T cardiac image acquired with a homogeneous TEM body coil transmitter and a
four-loop phased-array receiver with two loops on the anterior and posterior thorax, respectively.
EKG cardiac gating was employed in these breath-held acquisitions. (b) The right atrium artifact
was corrected by interactive impedance adjustments coil elements (RF shimming). No intensity
correction was applied to these images.
Figure 24. (a) B1 field calculated for body-loaded, homogeneous, circularly polarized TEM volume
coil at 300 MHz (7 T). (b) Sagittal images of human body at 7 T showing RF artifact predicted.
Image acquisition parameters were: GE , TR/TE = 50/4 ms, matrix = 2562, thk = 3 mm, FOV = 35
cm2, NT = 2, 55-sec scan.