fMRI contrast at high and ultrahigh magnetic fields: insight from complementary methods.

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging (MRI)
• Functional MRI (fMRI)

Background:

• Functional MRI (fMRI) is crucial for understanding brain activity.
• Ultrahigh magnetic fields offer enhanced signal-to-noise ratio but present technical challenges.
• Investigating novel encoding methods is vital for optimizing fMRI at high fields.

Purpose of the Study:

• To examine the origins and nature of function-derived activation detected by fMRI at ultrahigh fields.
• To compare different encoding methods, including gradient echo EPI, spin echo EPI, and spatio-temporally encoded (SPEN) strategies.
• To investigate the dependencies of fMRI signal changes on magnetic field strength and acquisition parameters.

Main Methods:

• Preclinical fMRI experiments were conducted at high (7 T) and ultra-high (17.2 T) magnetic fields.
• Gradient echo EPI, spin echo EPI, and SPEN sequences were employed.
• Artifact-free rat brain images and localized activation maps were acquired using fully refocused SPEN sequences.

Main Results:

• SPEN sequences provided artifact-free rat brain images with good resolution.
• Significant localized activation maps were obtained upon forepaw stimulation in a single scan.
• fMRI SPEN signals exhibited a strong component related to apparent T1 effects, in addition to T2*-weighted BOLD contributions.

Conclusions:

• Fully refocused SPEN sequences are effective for fMRI at ultrahigh fields, yielding high-resolution images and activation maps.
• SPEN fMRI signals incorporate both T2-weighted BOLD and apparent T1-related effects.
• This technique demonstrates significant potential for exploring functional brain activation in rodents and humans at ultrahigh magnetic field strengths.