Magnetic resonance imaging biomarkers for pulsed focused ultrasound treatment of pancreatic ductal adenocarcinoma

Table 1 14T magnetic resonance imaging parameter acquisition methods

Method	Sequence type	TR/TE (ms)	Comments
T1	RARE	5500, 3000, 1500, 1000, 385.8/9.66	NA = 1; FOV = 30 mm × 30 mm; rare factor = 2, matrix size = 256 × 128 (reconstructed phase encoding steps = 128; acquisition phase encoding steps = 96); yielding spatial resolution of 0.117 × 0.234 mm/pixel. Approximately 9 min acquisition time.
T2	MSME, fat suppressed	4000/twelve echoes equally spaced from 6.28 to 75.4	NA = 1; FOV = 30 mm × 30 mm; matrix size = 256 × 128 (reconstructed phase encoding steps = 128; acquisition phase encoding steps = 91); spatial resolution of 0.117 × 0.234 mm/pixel. 10 contiguous slices were acquired with respiration gating to cover the entire abdomen. Approximately 6 min acquisition time.
ADC	EPI	2500/17.7	Echo train length = 16; Pulse duration = 3.0 ms; Diffusion time = 7.46 ms; NA = 1; FOV = 30 × 30 mm2; matrix size = 128 × 128; spatial resolution of 0.234 × 0.234 mm/pixel; 8 b values (7, 47, 81, 126, 180, 234, 340, 549) s/mm2. 10 contiguous slices were acquired to cover the entire abdomen. Approximately 2 min 40 s acquisition time.
CEST	(1) RARE	(1) 2200 / 7	(1) Center frequency estimate: Continuous-wave block saturation pulse with B1 = 3 μT and duration = 1 s; 25 frequency offsets from -360 Hz to 360 Hz with an interval of 0.5 ppm (WASSR approach). FOV = 30 mm x 30 mm; Matrix size = 128 x 128; Flip angle = 180o; NA = 1. A single, 1 mm slice delineating the tumor was acquired.
	(2) RARE	(2) 5000 / 7	(2) Frequency shift saturation: 14 frequency offsets at ± 0.5, ± 1.0. ± 1.5, ± 2.0, ± 2.5, ± 3.0, ± 3.5 ppm were acquired through the same single slice using respiration gating with an off-resonance radiofrequency pulse applied for 1 s at a power of 3 μT. Matrix = 128 × 128 (reconstructed phase encoding steps = 128; acquisition phase encoding steps = 96); FOV = 30 mm × 30 mm; rare factor = 8.
	(3) RARE	(3) 5000 / 7	(3) Control image: A control image was acquired through the same slice using the same settings as #2, except with saturation offset at 300 ppm. Approximately 30 min total acquisition time.
MTR	GRE	625 / 2	Flip angle = 30°; off-resonance frequency 7000 Hz; saturation pulse block pulse shape = 50 ms width and 10 µT amplitude; FOV = 30 mm × 30 mm; matrix size = 256 × 256; spatial resolution of 0.117 x 0.117 mm/pixel. 10 contiguous images were acquired to cover the entire abdomen. Approximately 3 min acquisition time.

ADC: Apparent diffusion coefficient; CEST: Chemical exchange saturation transfer; MTR: Magnetization transfer ratio.

Table 2 Magnetic resonance imaging quantitative map results pre and post focused ultrasound treatment for three mouse models of pancreatic ductal adenocarcinoma

	Pre-pFUS	Post-pFUS	Difference	P value
High-b ADC (10-3 mm2/s)
KPC	1.0 ± 0.33 (n = 6)	3.32 ± 0.61 (n = 6)	2.32 ± 0.60	0.01
Ortho	0.72 ± 0.07 (n = 6)	1.15 ± 0.15 (n = 6)	0.43 ± 0.13	0.02
SC	0.97 ± 0.21 (n = 6)	1.80 ± 0.14 (n = 6)	0.83 ± 0.27	0.03
GagCEST (%)
KPC1	25.89 ± 3.10 (n = 6)	10.61 ± 3.09 (n = 6)	-15.28 ± 2.67	2.2 × 10-3
Ortho	20.83 ± 4.02 (n = 5)	20.04 ± 5.34 (n = 5)	-0.79 ± 6.41	0.91
Amide CEST (%)
KPC	10.37 ± 2.76 (n = 4)	24.89 ± 5.17 (n = 4)	14.52 ± 2.89	0.01
Ortho	4.11 ± 1.25(n = 5)	4.79 ± 1.64 (n = 5)	0.68 ± 1.99	0.29
MTR (%)
KPC	38.15 ± 4.17 (n = 6)	59.80 ± 1.46 (n = 6)	21.65 ± 2.96	7.5 × 10-4
Ortho	53.92 ± 7.20 (n = 6)	60.91 ± 3.39 (n = 6)	6.99 ± 6.33	0.32
SC	58.1 ± 7.01 (n = 6)	60.50 ± 7.85 (n = 6)	2.40 ± 7.22	0.75
T2 (ms)
KPC	39.38 ± 2.32 (n = 6)	34.41 ± 1.93 (n = 6)	-4.97 ± 1.37	0.02
Ortho	32.74 ± 1.12 (n = 6)	31.91 ± 0.89 (n = 6)	-0.83 ± 1.54	0.62
SC	44.52 ± 8.97 (n = 6)	32.77 ± 1.67 (n = 6)	-11.75 ± 10.08	0.30
T1 (ms)
KPC	986.95 ± 236.99 (n = 6)	847.69 ± 255.94 (n = 6)	-139.26 ± 314.92	0.68
Ortho	453.62 ± 116.72 (n = 6)	618.10 ± 183.85 (n = 6)	164.48 ± 194.29	0.44
SC	508.82 ± 192.64 (n = 6)	1032.44 ± 293.46 (n = 6)	523.62 ± 255.40	0.10

Pre and post treatment quantitative values are presented as mean ± SE from the mean. Paired t-tests were used to generate P-values, and values < 0.05, considered significant, are in bold.

¹Post-hoc secondary analysis, performed to compare the pre and post-treatment KPC groups with gagCEST maps, that employed within-tumor regions of interest (ROIs) rather than whole tumor ROIs, as detailed in the text, generated mean gagCEST values (%) of 25.20 ± 4.63 pre-treatment vs 7.71 ± 2.22 post-treatment, for a difference of -17.49 ± 3.07, P = 6.7 x 10^-3. pFUS: Pulsed focused ultrasound treatment; ADC: Apparent diffusion coefficient; KPC: Genetic mouse model; Ortho: Orthotopic; SC: Subcutaneous; gagCEST: Glycosaminoglycan chemical exchange saturation transfer; MTR: Magnetization transfer ratio.