QUIXOTIC was used to evaluate Yv, OEF, and CMRO2 in healthy volunteers. The protocol was approved by the University’s Committee of Using Humans as Experimental Subjects. Ten young (22–32 years) nonsmoking, healthy subjects (six females, four male) were scanned at 3 T (Siemens Tim Trio, Erlangen, Germany) with the following MRI protocol:
- T1-weighted magnetization prepared rapid acquisition gradient recalled echo (MP-RAGE) image for anatomical localization: voxel size = 1 mm3, matrix size = 256 × 256 × 68, acquisition time = 4 min 32 s.
- VT-VSSL blood-weighted imaging: VCUTOFF = 2.0 cm/s (G = 1.6 G/cm, Δ = 17 ms, δ = 2 ms, duration between 180 pulses (τ180) = 10 ms, x-directed), TO1 = 400 ms (assuming T1,blood = 1664 ms at 3 T; Ref. 3), TO = 725 ms, τCPMG of T2-preparation module = 10 ms. A gradient recalled echo planar imaging (EPI) readout was used for both tag and control image acquisitions: echo time = 12 ms, phase partial Fourier 6/8, bandwidth = 2232 Hz/pixel, matrix size = 64 × 64, single slice, voxel size = 3.9 × 3.9 × 10 mm3, TR = 4 s. Eighty measurements were acquired (40 control, 40 tag images), for an imaging time of 5 min 30 s per TEeff. Either five or six TEeffs (depending on specific absorption rate constraints) were acquired with a ΔTEeff = 18.4 ms.
- Pulsed arterial spin label (ASL) CBF imaging: PICORE/Q2tips (25), TI1 = 700 ms, TI1 stop time = 1400 ms, TI2 = 1600 ms, pulsed ASL gap = 10 mm, Tag thickness = 160 mm, TR = 2000 ms, one slice, 60 measurements, acquisition time = 2 min. EPI parameters are as listed for 3). M0 calibration scan had identical imaging parameters, except ASL-specific pulses were disabled, and only one measurement was acquired.
- Double inversion recovery (DIR) for GM-only images: TI1 = 3700 ms, TI2 = 4280 ms, one slice, one measurement, with EPI parameters as listed in 3.
- TRUST sagittal sinus blood imaging: tag thickness = 50 mm, gap = 10 mm, TI = 800 ms, TR = 8 s, eight measurements per TEeff were acquired, ΔTEeff = 18.0 ms, one slice. EPI parameters and T2 preparation are as listed for 2. Hyperbolic secant pulses identical to those in QUIXOTIC were used for TRUST T2 preparation.
Automatic alignment routines (26
) were used to ensure similar slice placement among all subjects. A slightly oblique-axial slice-of-interest was prescribed immediately superior to the corpus callosum for acquisitions two to four and contained a significant fraction of GM. A different oblique-axial slice intersecting the sagittal sinus was positioned for acquisition five (TRUST).
Venular blood-weighted imaging comprised the bulk of the scan session and the imaging time for the full protocol was less than an hour. Data were acquired at six TEeffs if possible, but in four subjects, specific absorption rate constraints imposed a limitation of only five TEeffs. To test the reproducibility, we performed the VT-VSSL experiment twice on two of the volunteers. These volunteers remained in the scanner for an additional 30 min for the second VT-VSSL acquisition. The two VT-VSSL trials were spaced approximately half an hour apart and used identical scan parameters.
After MRI scanning, hematocrit was measured via finger prick blood sample using the Ultracrit device (Separation Technologies, Altamonte Springs, FL). O2 saturation was measured with a pulse oximeter (8600FO Pulse Oximeter, Magmedix, Fitchburg, MA).
The data from each TEeff acquisition were corrected for bulk motion and then subtracted, control minus tag, in a pairwise fashion. The subtraction series was averaged to produce mean PCV-weighted images. The DIR image was used as a GM mask, within which venular blood SI from cortical GM tissue was measured. PCV blood signal intensities from the entire cortical GM region were plotted versus TEeff to provide whole-slice cortical GM measurements. The plots were then exponentially fit using a Levenberg–Marquardt least squares optimization method to measure the T2 relaxation parameter for whole-slice cortical GM, and the standard error of the estimated parameter (SEE) assuming a t-distribution was computed.
The TRUST acquisitions were similarly subtracted in a pairwise fraction and averaged to produce maps containing sagittal-sinus only blood at the five TEeffs. The mean SI from the six brightest sagittal sinus voxels blood was plotted versus TEeff and fit to estimate sagittal-sinus blood T2. The SEE assuming a t-distribution was computed.
values from both VT-VSSL and TRUST analyses were then calibrated to Yv
using curves generated with Eq. 7
, incorporating the hematocrit measured from the volunteer. As VT-VSSL data derive mostly from blood in small vessels, the T2
calibration was calculated using the microvascular hematocrit (i.e., by multiplying the measured hematocrit by 0.85 to correct for hematocrit differences between small and large vessels; Ref. 27
). This correction was not needed for the TRUST calibration curves, as the source of venous blood was exclusively from the large sagittal sinus. Ya
was taken as the oxygen saturation measured with the pulse oximeter; Eq. 2
was then used to calculate OEF.
To obtain CMRO2
from Eq. 1
, CBF was estimated by ASL MRI, with a VT-VSSL matching slice prescription. The ASL data were similarly analyzed to generate blood flow-weighted images, via subtraction, motion correction, and signal averaging. These images were calibrated to absolute CBF maps by using the local tissue proton density provided by the M0
). GM CBF was estimated from the region segmented by the DIR GM mask. White matter (WM) CBF was estimated from the remaining, nonsegmented brain region. The WM and GM CBF values were used to calculate whole brain CBF, by assuming a whole-brain WM:GM ratio of 0.675:1 (29
). With these additional CBF measurements, and by using hematocrit to estimate [Hbtotal
], both QUIXOTIC (GM) and TRUST (whole brain) CMRO2
were, respectively, calculated.
To demonstrate feasibility of using QUIXOTIC to create quantitative Yv
, OEF, and CMRO2
maps, raw data from a representative subject were smoothed with a 10 mm full width at half maximum (FWHM) Gaussian kernel and fit for T2
, but this time on a voxel-by-voxel basis (as opposed to using a GM mask). The resultant T2
map was subsequently calibrated to generate quantitative Yv
and OEF maps, using aforementioned calibration curves. Finally a CMRO2
map was generated by multiplying the OEF map with the absolute ASL-CBF map on a voxel-by-voxel basis and incorporating [Hbtotal
] and Ya
(as described in Eq. 1
Finally, to briefly investigate effects of the two key QUIXOTIC parameters, TO and VCUTOFF, on the measured Yv, three of the original 10 subjects returned for a separate, additional imaging session and were scanned at five different TOs (525, 625, 725, 825, and 925 ms) with a fixed VCUTOFF of 2.0 cm/s, and four different VCUTOFFs (1.5, 2.0, 3.0, and 4.0 cm/s) with a fixed TO of 725 ms. As using the standard QUIXOTIC approach to explore this parameter space would have resulted in impractical scan times (approximately 5 h), a faster variant (a so-called “turbo QUIXOTIC” approach) was developed and incorporated. Turbo QUIXOTIC uses a turbo spin echo EPI readout to generate images at multiple echo times per TR; this contrasts the standard T2 preparation approach, which acquires a single TEeff image per TR. Turbo QUIXOTIC thus obtains the full set of images for a particular TO-VCUTOFF combination in a single scan, allowing data for the above parameter exploration to be acquired in less than an hour. Specific imaging parameters were: TEeff = 22.6 ms, bandwidth = 3256 Hz/pixel, matrix size = 32 × 32, single slice, voxel size = 7.8 × 7.8 × 10 mm3, TR = 4 s, generalized autocalibrating partially parallel acquisition with 3× acceleration. Eighty measurements were acquired at four TEeffs, resulting in 40 control/40 tag per TEeff, and an imaging time of 5 min 30 s per TO/VCUTOFF combination. A resolution-matched double IR image was acquired for GM segmentation; subsequent data processing to calculate Yv was performed as described for the standard approach.
Data processing was done in Neurolens (www.neurolens.org
) software and with custom Matlab (MathWorks, Natick, MA) routines.