Amide proton transfer imaging

Amide proton transfer (APT) imaging is the most common type of chemical exchange saturation transfer technique that generates image contrast due to proton exchange between labeled protons in solute and free water protons¹. APT imaging does not require exogenous gadolinium-based contrast and thus can safely be used in patients having contraindications to contrast media.

Methodology

Amide proton transfer imaging is a pH-weighted imaging technique and its effect is measured as a reduction of bulk water intensity owing to the exchange of protons between water protons and labeled amide protons of mobile proteins and peptides in tissues ². Due to this exchange of protons, specific molecular information is obtained through the reduction of bulk water signal.

A specific radiofrequency pulse is applied and assuming complete amide proton saturation, the proton transfer ratio for amide protons (APTR) in water can be mathematically calculated ³.

Clinical applications

Amide proton transfer imaging could be potentially used for imaging in the following clinical settings ^4,5:

• intracranial neoplasms

• differentiating tumor recurrence from radiation necrosis

• leukoencephalopathies

• hippocampal sclerosis

• stroke

• neurodegenerative disorders

APT MRI pulse sequences and signal characteristics

Fast volumetric imaging methods are used, either multislice or three-dimensional (3D). The latter is preferred to minimize the differences in saturation losses due to T1 relaxation ⁶.

Using sufficient saturation power and saturation time, images acquired by APT are homogenous for most of the normal brain tissue including the ventricles. This enables easy detection of the hyperintense signal from neoplasms and the hypointense signal from infarcted brain tissues.