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. 2008 Oct;60(4):842-9.
doi: 10.1002/mrm.21712.

Practical data acquisition method for human brain tumor amide proton transfer (APT) imaging

Jinyuan Zhou  1 , Jaishri O Blakeley, Jun Hua, Mina Kim, John Laterra, Martin G Pomper, Peter C M van Zijl
Affiliations

Practical data acquisition method for human brain tumor amide proton transfer (APT) imaging

Jinyuan Zhou et al. Magn Reson Med. 2008 Oct.

Abstract

Amide proton transfer (APT) imaging is a type of chemical exchange-dependent saturation transfer (CEST) magnetic resonance imaging (MRI) in which amide protons of endogenous mobile proteins and peptides in tissue are detected. Initial studies have shown promising results for distinguishing tumor from surrounding brain in patients, but these data were hampered by magnetic field inhomogeneity and a low signal-to-noise ratio (SNR). Here a practical six-offset APT data acquisition scheme is presented that, together with a separately acquired CEST spectrum, can provide B(0)-inhomogeneity corrected human brain APT images of sufficient SNR within a clinically relevant time frame. Data from nine brain tumor patients at 3T shows that APT intensities were significantly higher in the tumor core, as assigned by gadolinium-enhancement, than in contralateral normal-appearing white matter (CNAWM) in patients with high-grade tumors. Conversely, APT intensities in tumor were indistinguishable from CNAWM in patients with low-grade tumors. In high-grade tumors, regions of increased APT extended outside of the core into peripheral zones, indicating the potential of this technique for more accurate delineation of the heterogeneous areas of brain cancers.

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Figures

Fig. 1
Fig. 1. Schemes for APTw image acquisition
(a) Standard two-offset APT scan (+3.5 ppm for label, −3.5 ppm for reference). (b) Six-offset APT scan (±3, ±3.5, ±4 ppm) used in this study. The effects of conventional MT and direct water saturation reduce the water signal intensities at all offsets (±3, ±3.5, ±4 ppm), and the existence of APT causes an extra reduction around the offset of 3.5 ppm.
Fig. 2
Fig. 2. MR images of a patient with WHO grade III oligodendroglioma
a: Uncorrected z-spectra for tumor core (red), tumor periphery (blue), and CNAWM (dark green). b: Corresponding MTRasym spectra. c and d: B0-inhomogeneity corrected data (corresponding to a and b). After correction, the resulting MTRasym curve for the tumor is maximized at 3.5 ppm, where the amide protons of mobile proteins and peptides resonate. e: T2w image. f: T1w image. g: FLAIR image. h: Gd-T1w image. i: MTR map. j: Water center-frequency offset map. k: Uncorrected APTw image. l: Corrected APTw image. The water center-frequency map (j) as derived from z-spectrum shows field inhomogeneity near air-tissue interfaces (sinus, ear), leading to large artifacts (white stars) in the uncorrected APTw image (k), which disappear in the frequency-corrected APTw image (l). Elevated APT signal can be seen in Gd-enhanced tumor core (red arrow). In the tumor periphery (FLAIR abnormality minus Gd-enhanced portion), both areas of APT hyperintensity and normal APT intensity (orange arrow) can be seen. The ROIs used for z-spectrum analysis (a-d, about 20 pixels each ROI) and for APTw quantitative analysis (Table 1) are shown in the FLAIR (g) and Gd-T1w (h) images.
Fig. 3
Fig. 3. MR images of a patient with an anaplastic astrocytoma
a: T2w. b: T1w. c: FLAIR. d: Gd-T1w. e: MTR. f: z-spectrum center frequency map. g: Uncorrected APTw. g: Center-frequency-corrected APTw. The ROIs for quantitative analysis (Table 1) are shown in the FLAIR (c) and Gd-T1w (d) images. B0 field inhomogeneity (f) causes some large artifacts (white stars) in the uncorrected APTw image (g). The hyperintense APTw area (h) is comparable in size to the lesion identified on T2w (a), T1w (b), FLAIR (c), and MTR (e), but larger than that on the Gd-T1w image.
Fig. 4
Fig. 4. MR images of a patient with a low-grade oligodendroglioma
a: T2w. b: T1w. c: FLAIR. d: Gd-T1w. e: MTR. f: z-spectrum center frequency map. g: Uncorrected APTw. g: Corrected APTw. The B0 field inhomogeneity is obvious (f), causing a large susceptibility artifact (white star) near the ethmoid sinus area in the uncorrected APTw image (g). Despite clear abnormalities (red arrow) on T2w (a), T1w (b), FLAIR (c), and MTR (e), there is no visible signal enhancement in tumor on both the Gd-T1w (d) and APTw (h) images. The ROIs for quantitative analysis (Table 1) are shown in the FLAIR image (c).
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