Saturation power dependence of amide proton transfer image contrasts in human brain tumors and strokes at 3 T

Abstract

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma, one with lung cancer metastasis, and three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3 T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization transfer ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional magnetization transfer ratio asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3 T with the existing clinical hardware.

Fig. 1

Measured z-spectra and MTR_asym spectra of white matter from healthy subjects (n = 4). a: z-spectra. b: Corresponding MTR_asym spectra. c: Plot of MTR_asym(3.5ppm) as a function of RF saturation power levels. The error bars are too small to see clearly. MTR_asym(3.5ppm) is roughly zero for the RF saturation power level of 2 μT.

Fig. 2

z-spectra, MTR_asym spectra, and ΔMTR_asym spectra for three RF saturation power levels (a–c: 1 μT; d–f: 2 μT; g–i: 3 μT) measured from brain tumor patients (n = 8). The MTR_asym(3.5ppm) value for the CNAWM is approximately zero for 2 μT. The ΔMTR_asym spectra are maximized at the offsets of about 3–4 ppm with respect to the water resonance. The ΔMTR_asym(3.5ppm) difference is small across the three RF power levels.

Fig. 3

z-spectra, MTR_asym spectra, and ΔMTR_asym spectra for three RF saturation power levels (a–c: 1 μT; d–f: 2 μT; g–i: 3 μT) measured from stroke patients (n = 4). The magnitude of ΔMTR_asym(3.5ppm) increases with applied RF saturation power levels.

Fig. 4

MTR_asym(3.5ppm) (a) and ΔMTR_asym(3.5ppm) (b) for tumors and strokes as a function of RF irradiation power levels. MTR_asym(3.5ppm) increases significantly with RF saturation power levels for the tumors (P < 0.001, both from 1 to 2 μT and from 2 to 3 μT) and the strokes (P < 0.001, from 1 to 2 μT; P < 0.01, from 2 to 3 μT). The changes in ΔMTR_asym(3.5ppm) are significant for the strokes as the RF irradiation power increases from 1 to 2 μT (P < 0.05), but insignificant for the others (all P > 0.2). * P < 0.05; ** P < 0.001; *** P < 0.001; n.s.: nonsignificant.

Fig. 5

Conventional and APT_w MR images of a patient with lung cancer metastasis (a–d) and a patient with stroke at 5 days post-onset (e–h). The RF saturation power used for APT imaging was 2 μT. The tumor (solid arrow) is hyperintense, while the stroke (open arrow) is hypointense on the APT_w images. The hyperintense signal in the basal ganglia region (black arrow) contralateral to ischemia on the APT_w image (h) is an artifact.

Fig. 6

APT_w MR images (a–d, f) at different saturation power levels and for different display windows and Gd-T₁w MR image (e) for a patient with cerebral metastasis (the same patient as Fig. 5a–d). The tumor (red bold arrow) shows hyperintense on all of the APT_w images. However, the lesion appears diffuse on the APT_w image at 1 μT (a,d) due to the presence of CSF artifacts (white thin arrow).