(E-189) Continuous Arterial Spin Labelling In Vivo Imaging at 21.1-T

Introduction::

Arterial Spin Labeling (ASL) is a non-invasive MRI technique that measures tissue blood perfusion using magnetically labeled water achieved by inflowing blood. Continuous ASL (CASL) uses continuous, low-amplitude, radio frequency pulses to produce inversion of flowing blood adjacent to the imaged slice. Within brain tissue, CASL measures perfusion in a complex network of capillaries, but with optimization, blood flowing into the choroid plexus of the brain’s ventricular system can be used to evaluate the production of CSF.

The use of single loop surface coils for CASL imaging has some advantages like having a high signal to noise ratio (SNR) proximal to the coil, as well as reduced magnetization transfer (MT) effects.  Smaller surface coils further enhance localization and SNR. The current approach seeks to implement a small ASL RF coil within the existing confines of an animal cradle and RF volume coil for operation in a vertical 21.1-T magnet. The goal of this effort is to enable CASL experiments to measure cerebral blood flow (CBF) and CSF perfusion in rats.

Materials and Methods::

Materials

The ASL coil as shown in Figure 2 was built as a single loop surface coil measuring 30-mm OD using 0.062” G-10/FR-4 copper clad laminate board with 1-oz copper on both sides.  The single loop coil is roughly tuned using ATC 100B/100A series fixed chip capacitors and fine-tuned/matched using variable trimmer capacitors. Multiple versions of the ASL coil were made to fine tune the performance for in vivo imaging at 21.1 T as described in Methods Table 1. For the initial versions (1-3), an SMA RF connector was used, while in the latest version a MCX RF connector was implemented to reduce the wavelength effects and achieve a smaller coil footprint. 

Methods

Version	Methods
1&2	Modified an existing 200 MHz loop design to impedance match at 900 MHz. The coil was tuned on bench using PEG (polyethylene glycol) sample. With an in vivo load, the range shifted down by 100 MHz. Tuning was adjusted to accommodate this shift.
3&4	When placed in the magnet, the frequency shifted by 50- 80 MHz. To minimize the effects of parasitic capacitance at high frequency, the impedance matching circuit was modified to a smaller footprint and stabilize for 900 MHz in vivo imaging.

Table 1: Differences between Coil Versions

 

Results, Conclusions, and Discussions::

Results

The bench measurements were done using CMT 804U Vector network analyzer as shown in Figure 4. The coil was tuned using fixed capacitors C_F1, C_F2 and balancing capacitor C_B as shown in the circuit Figure 1. The coil characteristics such as the tuning range and Quality factor Q(-3dB) recorded without sample are shown in Table 2. The image shown in Figure 3 is acquired at 21.1T using ASL coil for rat in vivo imaging.

CF1 (pF)	CF2 (pF)	CB (pF)	Tuning Range (MHz)	Q (dB)
0.7	0.5	n/a	898-930	24.1
0.2	0.2	n/a	900-920	25
0.3	0.3	3.3	974-1004	26
0.3	0.3	12	906-980	n/a

 

Table 2: ASL Coil Characteristics

Acknowledgements (Optional): : This work was sponsored by the National Science Foundation Undergraduate Research Experience Program. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement Nos. DMR-1644779 and DMR-2128556* and the State of Florida.

References (Optional): :

[1] S. Petcharunpaisan, J. Ramalho, and M. Castillo, “Arterial spin labeling in neuroimaging,” World Journal of Radiology, vol. 2, no. 10, p. 384, 2010.

[2] E. R. Muir, “Preclinical arterial spin labeling measurement of cerebral blood flow,” Preclinical MRI, pp. 59–70, 2018.