Amide Proton Transfer (APT) weighted imaging
White Paper
MRI NetForum Team
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Philips Healthcare
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Netherlands
In APT weighted imaging and other CEST methods, the MRI signal is generated by a mechanism different from that of basic MRI. These CEST techniques are based on the chemical exchange of hydrogen atoms. The signal of amide protons of peptide bonds in proteins is too low to be measured in normal MRI. The hydrogen (proton) exchange between protein amide groups and surrounding water allows a different way to measure these amide protons.
Studies have shown that the APT signal correlates with the concentration of a protein that is related to cell proliferation. The concentration of this protein, and thus the APT signal strength responds to the grade of malignant tumors*. APT contrast can potentially highlight tumors that wouldn’t be seen otherwise.
In APT a narrow RF prepulse (saturation pulse) at the amide hydrogen’s frequency is given to attenuate its MR signal. Because the amide group and water continually exchange hydrogen atoms, the number of saturated protons will build up in water, so that the measured water signal will become lower. The change of the MRI signal of water provides an indirect way to measure the presence of amide. APT images are usually presented as color maps, created by using an asymmetry calculation so that presence of APT is shown as a positive colored signal.
Click any of the pictures below to view a larger version
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Proteins with amide protons are surrounded by water molecules that are moving around.
The MR signal of water is high.
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A saturation prepulse on the protein’s amide proton frequency nulls the MR signal of these protons.
The MR signal of water is high
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As a result of proton exchange the nulled protons move from the protein to water molecules.
MR signal of water is reduced due to the proton exchange. In APT, this signal reduction is measured to create an APT map that is sensitive to the concentration of the protein.
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Orange = protein
Dark blue = hydrogen (proton)
Clear blue+2 dark blue = H2O (water molecule)
Pink = saturated proton
ExamCards and other information on NetForum related to APT
*References:
- Togao et al. Amide proton transfer imaging of adult diffuse gliomas: correlation with histopathological grades. Neuro-Oncology 2014; 16(3), 441–448.
- Jiang et al. Amide proton transfer-weighted magnetic resonance image-guided stereotactic biopsy in patients with newly diagnosed gliomas. Eur J Cancer 2017 83:9-18
- Zhou, J. Amide Proton Transfer Imaging of the Human Brain. Magnetic Resonance Neuroimaging. Methods in Molecular Biology (Clifton, N.J., 2011) 711, 227–237.
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