MTR asymmetry analysis and APTW image contrast mechanism
When the MTR asymmetry analysis is used [1, 2], the quantified APT-weighted (APTW) MRI signal in
brain tumor is influenced by both the downfield APT and upfield nuclear Overhauser enhancement
(NOE) effects [3]. Based on our recent work [4, 5], however, the APT effect is larger than the NOE effect
at relatively higher saturation powers (such as 2 µT used in this study). Although the absolute APTW
signal intensity quantified by MTRasym(3.5ppm) is reduced by the NOE effect, the APTW image contrast
between tumor and contralateral brain tissue is actually increased by the presence of the NOE. Notably, it
is concluded that the APT effect is the major contributor to the APTW image contrast between the tumor
and the normal brain tissue at all saturation powers.
According to the theory [6], in addition to the mobile protein and peptide concentration, some
other tissue factors (pH, tissue water content, T1 of water) may affect the measured APTW signal.
Theoretically, the APT effect in tissue is proportional to water T 1 and inversely proportional to tissue
water content. Indeed, the water content is usually higher, and water T1 is enhanced in tumor. However, it
is very important to realize that these two changes are mostly compensated for in tumor and many other
diseases, as discussed in our early papers [1, 2]. Further, due to the fact that only a small intracellular pH
increase (<0.1 unit) is often detected in the tumor [7, 8], increased APTW intensities in the tumor, with
respect to the CNAWM, should be attributed primarily to increased protein and peptide content, in line
with expectations based on the increased content of cellular proteins in the tumor, as revealed by MRIguided proteomics [9] and in vivo MR spectroscopy [10].
Comparison of MTRasym Spectra for PCNSLs and HGGs
To reveal the specific origin of the APTW signal, we compared the average MTRasym spectra of the
PCNSLs and the HGGs (Supplementary Fig. S1). The multiple CEST effects were clearly visible in the
offset range of 14 ppm in the MTRasym spectra. Both the PCNSLs and HGGs had larger CEST effects,
compared to the CNAWM. Notably, there was a relatively stronger CEST effect at an offset of ~3.5 ppm
downfield from water, where the endogenous cellular backbone amide protons of mobile proteins and
peptides resonate [11]. This APT effect was significantly larger in both the PCNSLs and HGGs than in
the CNAWM (3.47% ± 1.14% vs. 0.64% ± 0.55%; 4.38% ± 1.26% vs. 0.23% ± 0.35%; both P < 0.01).
The other larger CEST effects that peaked at around 2.5 ppm downfield from water may be associated
with a more complicated origin and possible mechanisms (Supplementary Fig. S2) [12], and thus, were
not further assessed in this study. The CEST signal intensities (including the protein-based APTW signals
at 3.5 ppm downfield from water) were relatively lower in PCNSLs than in HGGs.
Fig. S1 Average MTRasym spectra of PCNSLs (n = 11) and HGGs (n = 21), corresponding
to the ROI with APTWmax from the lesions and CNAWM. The relatively stronger CEST
effects that peaked at roughly 3.5 ppm downfield from water (namely, the APT signals)
were contributed by the endogenous cellular backbone amide protons of mobile proteins
and peptides.
Fig. S2 Simulated MTRasym spectra using a five-pool model [bulk water protons, semisolid macromolecular protons, amide protons at 3.5 ppm, amine protons at 2 ppm, NOErelated protons at -3.5 ppm (NOE)] at the RF powers of 2 and 3 µT and duration of 0.8
sec. Five-pool parameters used in the simulation were as follows [5]:
Water
Exchange rate (ksw)
Concentration
T1
T2
Chemical shift
M0w
1 sec
0.1 sec
0 ppm
MT
20 Hz
1/25 M0w
1 sec
20 µsec
0 ppm
Amide
30 Hz
1/250 M0w
1 sec
40 msec
3.5 ppm
Amine
1 kHz
1/1000 M0w
1 sec
40 msec
2 ppm
NOE
20 Hz
1/125 M0w
1 sec
0.4 msec
-3.5 ppm
It can be seen clearly that the amine CEST effect at 2 ppm downfield from water moved
left (e.g., to ~2.4 ppm at 2 µT) as increased RF power. Therefore, the larger CEST effect
peaked at around 2.5 ppm (as shown in Supplementary Fig. S1) may be associated with a
more complicated chemical origin (various amine protons) and physical mechanism
(shifting, as shown above).
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