MRI

Physics

Nuclear Spin and Magnetic Moments

Many atomic nuclei (notably hydrogen, abundant in the body as water and fat) have an intrinsic property called spin.
This spin gives each nucleus a tiny magnetic dipole moment.
In the absence of an external magnetic field, these dipoles are randomly oriented.

Alignment in a Magnetic Field (B₀)

When a strong external magnetic field (B₀) is applied, the magnetic moments of nuclei align either:

Parallel (low energy state) with B₀
Antiparallel (high energy state) to B₀

Larmor Precession

The nuclei don’t just sit aligned; they precess around the B₀ direction at a specific frequency For clinical MRI (1.5–3 Tesla), this falls in the radiofrequency (RF) range.

Excitation with RF Pulses

If an RF pulse matching the Larmor frequency is applied, nuclei absorb energy and “tip” away from alignment with B₀.
This creates a transverse magnetization (in the x-y plane) that rotates at the Larmor frequency.

Relaxtion

Once the RF pulse stops, nuclei relax back toward equilibrium:

T1 relaxation (longitudinal recovery):
- Magnetization along B₀ regrows.
- Time constant of T1 depends on tissue properties.
T2 relaxation (transverse decay):
- Coherence of transverse magnetization is lost due to spin–spin interactions.
- Time constant of T2 also tissue dependent and generally shorter than T1.

These differences in relaxation times allow MRI to distinguish tissues (e.g. gray vs white matter, healthy vs diseased tissue).

Signal detection

The changing transverse magnetization induces a current in receiver coils via Faraday’s law of induction.
This produces the measurable MR signal, which decays over time.

Spatial Encoding

To form images, spatial information must be encoded:

Magnetic field gradients are applied along x, y, and z axes.
These gradients make the Larmor frequency vary with position, enabling localization of signal sources via Fourier transforms.

Image Contrast

By choosing timing parameters (e.g., repetition time TR, echo time TE) and pulse sequences, MRI can emphasize:

T1 differences (good for anatomy)
T2 differences (good for detecting pathology like edema)
Proton density (water vs fat content)
More advanced contrasts (diffusion, perfusion, functional MRI).

MRI Sequences

There are a plethora of MRI sequences being used clinically to assist with diagnosis.

Sequence	Description	TR (msec)	TE (msec)
T1-Weighted	Short TR and TE	500	14
T2-Weighted	Long TR and TE	4000	90
T2-FLAIR	Very Long TR and TE	9000	114
T2*	T2* decay makes detection of magnetic susceptibility effects such as calcium and blood easier
Steady State Sequences	- Steady-state free procession (SSFP) - Fast imaging employing steady-state acquisition (FIESTA)/Constructive interference in steady-state (CISS)
Dixon	Can generate four sequences. Can be used for fat suppression.
Turbo Spin Echo
Gradient Echo

Gradient Echo vs Spin Echo Sequences

Gradient Echo	Spin Echo
Faster	Slower
Flip angle <90 degrees (10-80)	Flip angle 90 at or close to degrees

Dixon Method

In original method, two sets of spin echo images were acquired with slightly different echo times and four sets of images are generated

water only
fat only
in-phase (IP)
out-of-phase (OOP)

Pros	Cons
- Generally better than CHESS/Fat-sat sequences	- Fat-water swap - Limitations around highly inhomogeneous areas such as neck and around metal

MRI