3.7 Advanced Sequence Families

3.7 Advanced Sequence Families

Advanced MRI sequence families expand the capabilities of magnetic resonance beyond static anatomical mapping. These techniques utilize highly specialized gradient waveforms, ultrafast k-space filling strategies, and motion-encoding mechanisms to assess microstructural integrity (diffusion), microvascular perfusion, and vascular anatomy (angiography) in real time.

Echo Planar Imaging (EPI)

Echo Planar Imaging (EPI) is an ultra-fast acquisition technique that allows an entire slice to be imaged in a single repetition time (TR), often in under 100 milliseconds.

Physics of EPI

In standard spin echo or gradient echo sequences, only one line of k-space is filled per TR cycle. EPI achieves its speed by filling the entire k-space matrix after a single RF excitation pulse (known as Single-Shot EPI).

• Frequency Encoding: A rapidly oscillating frequency encoding gradient (readout gradient) alternates between positive and negative polarities. This creates a train of gradient echoes.
• Phase Encoding: A small gradient pulse (called a phase blip) is applied at each transition of the readout gradient to shift the acquisition to the next line in k-space.

Because of this unique strategy, k-space is filled in a zig-zag trajectory. Alternatively, Multi-Shot EPI divides k-space into multiple segments acquired over a few TRs, which reduces distortion but increases scan time.

Artifacts and Limitations

EPI's speed comes with a high susceptibility to artifacts:

• Magnetic Susceptibility: EPI has a very low effective receiver bandwidth in the phase encoding direction. This makes it highly sensitive to local field inhomogeneities, causing severe geometric distortion and signal dropout at air-tissue interfaces (e.g., near the sinuses and auditory canals).
• Chemical Shift: Because of the low bandwidth in the phase encoding direction, fat and water signals are significantly displaced. Fat suppression (e.g., spectral fat saturation or spatial inversion) is mandatory.
• N/2 (Nyquist) Ghosting: Slight timing mismatches or eddy currents between the positive and negative readout gradients cause a duplicate "ghost" image shifted by half the field of view.

Diffusion-Weighted Imaging (DWI)

Diffusion-Weighted Imaging (DWI) measures the random, thermal motion of water molecules (Brownian motion) within tissues, providing a window into cellular density and microstructural boundaries.

Physics of DWI

DWI is typically based on a single-shot spin-echo EPI sequence. It incorporates a pair of strong, symmetric diffusion-sensitizing gradients (the Stejskal-Tanner gradient pair) placed on either side of the 180° RF refocusing pulse:

1. Dephasing Gradient: The first gradient dephases the spins of all water molecules.
2. Rephasing Gradient: The second gradient has the same amplitude and duration. For stationary water molecules, this gradient perfectly rephases the spins, resulting in high signal intensity. For moving (diffusing) water molecules, their spatial position has changed during the delay between gradients. They do not experience the same rephasing magnetic field, leading to incomplete rephasing and signal attenuation.

The b-Value

The degree of diffusion sensitivity is determined by the b-value (measured in s/mm²). It depends on the gradient amplitude (G), duration (δ), and time spacing (Δ):

b = γ² * G² * δ² * (Δ - δ/3)

• b = 0 s/mm²: No diffusion weighting; the image is a standard T2-weighted image.
• b = 1000 s/mm²: High diffusion sensitivity. Tissues with restricted diffusion (e.g., acute stroke, hypercellular tumors) appear bright because water molecules cannot move far enough to lose signal.

ADC Maps and T2 Shine-Through

Because DWI sequences are based on T2-weighted spin-echo designs, a hyperintense area on a DWI image can sometimes be caused by a very long T2 relaxation time rather than restricted diffusion. This phenomenon is called T2 shine-through. To differentiate, an Apparent Diffusion Coefficient (ADC) map is calculated using images acquired at different b-values (typically b = 0 and b = 1000).

• True Restricted Diffusion: Bright on DWI, Dark (hypointense) on the ADC map.
• T2 Shine-Through: Bright on DWI, Bright (hyperintense) on the ADC map.

Perfusion-Weighted Imaging (PWI)

Perfusion-Weighted Imaging (PWI) measures the microvascular blood supply (perfusion) to tissues. There are two primary techniques:

1. Dynamic Susceptibility Contrast (DSC): An exogenous contrast agent (gadolinium) is injected as a rapid bolus. A series of fast T2*-weighted EPI images are acquired. As the gadolinium passes through the capillary bed, it induces local magnetic susceptibility changes, causing a transient drop in signal intensity. The signal-loss curve is used to calculate:
   • Cerebral Blood Volume (CBV): Total volume of blood in a tissue area.
   • Cerebral Blood Flow (CBF): Volume of blood flow per unit time.
   • Mean Transit Time (MTT): Average time for blood to pass through the tissue.
2. Arterial Spin Labeling (ASL): A non-contrast technique. RF pulses are used to magnetically label (invert) the water protons in the arterial blood of the neck before it flows into the brain. After a delay, an image is acquired. A control image (without labeling) is subtracted to isolate the signal from the perfusing blood, yielding a quantitative map of blood flow.

Magnetic Resonance Angiography (MRA) and Venography

MRA encompasses several techniques designed to visualize blood vessels:

Time-of-Flight (TOF) MRA

TOF MRA relies on inflow enhancement. A rapid train of RF pulses with a short TR is applied to a thin slice of tissue, causing stationary background tissue to become magnetically saturated (dark). Unexcited (fresh) blood flowing into the slice carries fully recovered longitudinal magnetization, producing a bright signal when it experiences the RF pulse.

• 2D TOF: Acquired slice-by-slice. Excellent for slow flow (e.g., venography, carotid arteries).
• 3D TOF: Acquired in a volume slab. Offers high spatial resolution and SNR. Ideal for fast flow (e.g., intracranial Circle of Willis).

Phase Contrast (PC) MRA

PC MRA utilizes velocity-induced phase shifts to image flow. It applies a bipolar gradient pair: the first gradient dephases spins, and the second rephases them.

• Stationary spins experience zero net phase shift.
• Moving spins acquire a phase shift that is directly proportional to their velocity.
• Velocity Encoding (VENC): The operator must select a VENC value (in cm/s) matching the expected blood velocity. If the VENC is too low, aliasing occurs (flow direction/velocity is mismapped). PC MRA allows for quantitative flow measurement and directionality.

Contrast-Enhanced MRA (CE-MRA)

CE-MRA utilizes the T1-shortening properties of gadolinium contrast. It does not rely on inflow or velocity effects, making it independent of flow direction or turbulence. It uses an ultra-fast 3D spoiled GRE sequence. Accurate bolus timing (via bolus tracking or real-time fluoroscopic triggering) is critical to ensure that the data acquisition window coincides with peak arterial enhancement, avoiding venous contamination.