MRI

The lecture begins with a congratulations to Freya, a graduate student who studied experimental psychology at Oxford before pursuing a PhD in the presenter’s department.
Freya’s background includes researching brain plasticity in children with epilepsy and working as an assistant psychologist at Great Ormond Street Children’s Hospital.

The lecture is structured into two main parts:
1. Introduction to MRI - Mechanics of MRI
2. Functional MRI (fMRI) - Overview and applications in psychology and clinical settings.
Freya expresses gratitude for the opportunity to teach despite having a busy schedule at the hospital.
Questions are encouraged throughout the lecture, and Freya reassures the audience to not worry if they find the subject complex, as she also continually learns while teaching.

Magnetic Resonance Imaging (MRI) is a type of scan that utilizes strong magnets and radio waves to generate detailed images of the internal structures of the body, primarily focusing on the human brain.
MRI can capture images of various tissues and organs across different species.

Physics of MRI: The body contains atoms, particularly protons in hydrogen atoms, that spin and behave like tiny magnets.
- Protons have a positive electrical charge and produce a small magnetic field when they spin.
- When placed in a larger magnetic field, protons align with this field, which can be manipulated using radio waves.

Resonance occurs when protons are exposed to radio waves that match their precession frequency (the frequency at which they spin).
- Energy can be transferred to protons, causing them to move and affect the strength of the magnetic field.
The analogy presented compares this to two vehicles moving at the same speed being able to exchange items easily (e.g., hamburgers).

After the radio frequency pulse, protons eventually relax back to their original state:
- T1 Relaxation Time: The time it takes for the longitudinal magnetization to recover.
- T2 Relaxation Time: The time it takes for the transverse magnetization to lose phase coherence.
The differences in these times depend on tissue type and characteristics in the brain (e.g., gray matter, white matter).

Different tissue types affect the speed at which protons realign, which may result in differences in MRI images.
- Tissues with smooth magnetic fields tend to stay in phase longer, while those with obstacles (such as certain tissue properties) get out of phase quicker.

MRI images are derived from the relaxation processes of protons, measured by a receiver coil.
T1 and T2 weighted images differ visually:
- T1 images show different contrasts between gray and white matter vs. cerebrospinal fluid (CSF).
- T2 images display inversed contrasts.
- These variations offer insights into changes over development, particularly in young infants with myelination processes.

MRI is deemed safe when used correctly, although it poses risks primarily due to its powerful magnetic fields.
- Metal Safety: The presence of certain metals in the body can be hazardous, prompting thorough screening for patients prior to MRI.
- Common risks involve heating or burns due to metal, and tattoos with certain inks may also present risks of heating.

fMRI measures the blood oxygenation level dependent (BOLD) signal, reflecting brain activity based on blood flow and oxygen availability.
- Active brain regions require increased oxygenated blood flow, which is measured in fMRI.
Oxygenated vs. Deoxygenated Blood:
- Oxygenated blood is non-magnetic, while deoxygenated blood is magnetic.
- The changes in the levels of these blood types affect the magnetic resonance imaging results.

Baseline tasks are critical for distinguishing specific brain activity related to a task versus general, ongoing brain activity.
- Commonly used baselines involve engaging the individual in other simple activities to control for default mode activity.

fMRI is particularly strong in identifying spatial activations in the brain, making it useful for:
- Examining the neural correlates of specific cognitive tasks (e.g., language processing).
- Analyzing developmental questions by comparing brain activation patterns between children and adults.

The lecture critiques common inaccuracies portrayed in films and TV shows about how fMRI operates, emphasizing the importance of proper representation in the field.