L11 - Bioimaging II

Magnetic Resonance Imaging (MRI)

• Magnetic field $\sim 1.5\mathrm{T}$
• Relies on the magnetic properties of a hydrogen atom to produce images

Working Principle

1. Randomly oriented hydrogen atoms
2. Aligns parallel to the magnetic field $B_0$ $\to$ net upward magnetic field
3. RF Pulse is applied to change the net magnetic field towards the transverse plane
4. Measure relaxation when RF pulse is gone

Relaxation

• T1 (longitudinal, parallel to $B_0$) $\to$ increase over time
• T2 (transverse, perpendicular to $B_0$) $\to$ decrease over time

Images

• Bone and air are invisible
• Fat and blood vessels are bright
• Cerebrospinal fluid (CSF) and muscle are dark

Skin can also be seen

Magnetic Resonance Angiography (MRA)

By using contrast agent, generate pictures of arteries.

• Improve visibility of internal body structures
• Gadolinium-based agents are most common
• Works by altering the relaxation times of atoms

Neurological Functional Imaging (fMRI)

1. Active specific region of the brain with specific task
2. Active region demand more oxygen
3. Oxygenated blood generate a small signal change (~1%)
4. Signal intensity change can be seen on the MRI images

Pros and Cons

Pros

• Programmable, can be tuned for good contrast
• Excellent soft tissue contrast

Cons

• Cannot image bone (no water content)
• Expensive (4 - 5x of CT)

Positron Emission Tomography (PET)

• Detect cancerous tumors, how much cancer has spread, how well cancer treatment is working
• Evaluate neurological illnesses (e.g. epilepsy, Alzheimer’s disease)
• Evaluate how well the heart muscle is functioning (for patients with coronary artery disease or cardiomyopathy)

Working Principle

1. Inject positron emitting tracer
2. Positron combines with electron, emitting two gamma rays traveling in $180^{\circ }$
3. Gamma rays is detected by opposing detectors
4. Location is measured by time difference

Positron is radioactive

Ultrasound (US)

Sound wave with frequency $\ge 20\mathrm{kHz}$ is considered as ultrasound, but typical diagnostics sonography scanners operate at $2-13\mathrm{MHz}$.

Working Principle

1. Piezoelectric crystal as sound transmitter (mechanical $\to$ electrical)
2. Piezoelectric crystal as sound receiver (electrical $\to$ mechanical)

Piezoelectric Effect

Develop a voltage across opposite surface in response to applied mechanical stress, the opposite is the converse piezoelectric effect. (e.g. crystals, ceramics, bone DNA, proteins)

• Apply voltage $\to$ thicker crystal
• Apply reversed voltage $\to$ thinner crystal
• Apply alternating voltage $\to$ oscillate at the same frequency as the voltage, producing a sound wave

Ultrasonic transducer

Operating frequency at $2-20\mathrm{MHz}$, $\text{resolution}\propto \frac{1}{\text{penetration depth}}$.

Better/Higher resolution $\to$ lower resolution number Resolution

Imaging Techniques

• A-mode
  • time $\propto$ depth
  • Reflected waves are recorded as a function of time
• M-mode
  • Repeated A-mode measurement
  • For moving/time varying object
• B-mode
  • Obtain image by two A-mode acquisition (translating or tilting)

There are also 3D, 4D (time) ultrasound, the higher the dimension, the longer it takes.

Doppler effect

Presence, direction, velocity of blood flow can be detected.

• Moving towards $\to$ frequency increase, wavelength decrease
• Moving away $\to$ frequency decrease, wavelength increase

Colors can be applied according to the flow direction

There are two piezoelectric crystals acting as transmitter and receiver, they can detect doppler signals in the overlapping region.