Amide Proton Transfer Imaging of Diffuse Gliomas: Effect of Saturation Pulse Length in Parallel Transmission-Based Technique

<div><p>In this study, we evaluated the dependence of saturation pulse length on APT imaging of diffuse gliomas using a parallel transmission-based technique. Twenty-two patients with diffuse gliomas (9 low-grade gliomas, LGGs, and 13 high-grade gliomas, HGGs) were included in the study. APT imaging was conducted at 3T with a 2-channel parallel transmission scheme using three different saturation pulse lengths (0.5 s, 1.0 s, 2.0 s). The 2D fast spin-echo sequence was used for imaging. Z-spectrum was obtained at 25 frequency offsets from -6 to +6 ppm (step 0.5 ppm). A point-by-point B0 correction was performed with a B0 map. Magnetization transfer ratio (MTR_asym) and ΔMTR_asym (contrast between tumor and normal white matter) at 3.5 ppm were compared among different saturation lengths. A significant increase in MTR_asym (3.5 ppm) of HGG was found when the length of saturation pulse became longer (3.09 ± 0.54% at 0.5 s, 3.83 ± 0.67% at 1 s, 4.12 ± 0.97% at 2 s), but MTR_asym (3.5 ppm) was not different among the saturation lengths in LGG. ΔMTR_asym (3.5 ppm) increased with the length of saturation pulse in both LGG (0.48 ± 0.56% at 0.5 s, 1.28 ± 0.56% at 1 s, 1.88 ± 0.56% at 2 s and HGG (1.72 ± 0.54% at 0.5 s, 2.90 ± 0.49% at 1 s, 3.83 ± 0.88% at 2 s). In both LGG and HGG, APT-weighted contrast was enhanced with the use of longer saturation pulses.</p></div

Amide Proton Transfer Imaging of Diffuse Gliomas: Effect of Saturation Pulse Length in Parallel Transmission-Based Technique - Fig 2

<p>MTR_asym of tumor <b>(A)</b> and NAWM <b>(B)</b> and ΔMTR_asym <b>(C)</b> in LGG. MTR_asym (<b>A</b>) of tumor was decreased with the saturation length in lower frequency range (1–2 ppm), but equivalent at 3.5 ppm. MTR_asym of NAWM (<b>B</b>) is decreased with the saturation length in the entire frequency range. ΔMTR_asym (<b>C</b>) was increased with the saturation length at higher frequency offsets (>2 ppm).</p

A case of diffuse astrocytoma (Grade II, LGG).

<p>The APT-weighted signal of the tumor in the left frontal lobe is almost comparable in all the saturation pulse lengths, but the contrast between tumor and normal brain tissue is slightly increased at longer saturation pulses due to decreased signal in NAWM.</p

A case of glioblastoma multiforme (Grade IV, HGG).

<p>The APT-weighted signal of the tumor in the left temporal lobe is increased with the saturation length, and the contrast between tumor and normal brain tissue becomes larger at longer saturation pulses.</p

Amide Proton Transfer Imaging of Diffuse Gliomas: Effect of Saturation Pulse Length in Parallel Transmission-Based Technique - Fig 1

<p>Z-spectra of LGG <b>(A)</b>, HGG <b>(C)</b>, and corresponding NAWM <b>(B, D)</b>. Z-spectra of tumor was steeper than that of NAWM, presumably because of less MT effect in tumor compared with NAWM. Prolongation of saturation pulses results in larger MT effect and thus wider Z-spectra in both tumor and NAWM.</p

MTR_asym of tumor and NAWM and ΔMTR_asym in HGG.

<p>MTR_asym of tumor <b>(A)</b> was decreased with the saturation length in lower frequency range (<2 ppm), but was increased at 3.5 ppm. MTR_asym of NAWM <b>(B)</b> was decreased with the saturation length in entire frequency range. ΔMTR_asym <b>(C)</b> was increased with the saturation length at higher frequency (>2 ppm). ΔMTR_asym (3.5 ppm) with the 2 s saturation reached maximum at around 3.5 ppm (specific frequency of amide protons).</p

MTR_asym (3.5 ppm) and ΔMTR_asym (3.5 ppm) of LGG and HGG.

<p>No significant differences were observed in MTR_asym (3.5 ppm) among the three saturation lengths in LGG, while MTR_asym (3.5 ppm) with the 1 s and 2 s saturation was significantly higher than that with the 0.5 s saturation in HGG (<b>A</b>). ΔMTR_asym (3.5 ppm) with the 1 s and 2 s saturation length was significantly higher than that with the 0.5 s saturation in LGG, and ΔMTR_asym (3.5ppm) of HGG significantly increased with the saturation length (<b>B</b>). Both MTR_asym (3.5ppm) and ΔMTR_asym (3.5ppm) were significantly higher in HGG than in LGG at any saturation pulse lengths.</p