Week 4 physics study guide

Intrinsic Contrast Parameters

Tissue-dependent properties that exist regardless of scanner settings

T1 relaxation time

T2 Relaxation time

Proton density

Flow diffusion (ADC)

Extrinsic Contrast Parameters

Scan parameters that the technologist adjusts at the console

• TR (repetition time)

• TE (echo time)

• Flip angle

• TI, turbo factor, b-value

What Creates Image Contrast in MRI?

visible differences in signal intensity between tissues.

How contrast changes when T1, T2, or proton density is emphasized?

• T1: differences in longitudinal recovery times

• T2: differences in transverse decay times

• PD: differences in number of protons

Why some tissues appear bright in one weighting and dark in another

Same tissues can flip bright ↔ dark depending on whether T1, T2, or PD is being weighted

What T1 relaxation represents physically

Time for longitudinal magnetization (Mz) to recover to 63% of its final value after excitation.

Why tissues with faster longitudinal recovery behave differently

Tissues with long T1 (e.g., water/CSF) are still “energy-saturated,” haven’t recovered → less Mz → less transverse signal → dark

Short T1

• Tissues with short T1 (e.g., fat) recover quickly → have more Mz ready → produce more transverse magnetization after the next 90° → appear bright.

How does TR influence T1 contrast in spin echo sequences?

• Short TR (~350–800 ms) → tissues with long T1 stay saturated and dark, tissues with short T1 recover and are bright → strong T1 contrast [Week 4 lecture].

• Long TR (≥ ~2500 ms) → all tissues fully recover Mz → T1 differences minimized

Why fat behaves differently than water on T1-weighted images

Fat has a molecular tumbling rate close to the Larmor frequency, giving efficient energy exchange → short T1, so it recovers quickly. Water’s tumbling rate is less favorable → long T1, recovers slowly.

What happens if the next RF pulse arrives before full longitudinal recovery?

Tissues with long T1 remain energy‑saturated (little longitudinal magnetization), so they produce low transverse signal and appear dark. Tissues with short T1 can still recover between pulses and appear bright → this incomplete recovery is exactly what creates T1 contrast

What does T2 decay represent physically?

T2 is true transverse (spin–spin) relaxation: time for Mxy to decay to 37% of its original value due to loss of phase coherence

Why tissues with long transverse relaxation appear bright

Tissues with long T2 (e.g., water, CSF, edema) stay phase‑coherent longer, so at a long TE they still have substantial transverse magnetization → stronger signal → bright

The role of TE in emphasizing or minimizing T2 contrast

• Long TE (~80–120 ms) → lots of transverse decay has occurred; tissues with short T2 have lost most Mxy, long‑T2 tissues still have Mxy → strong T2 contrast [Physics week 4 lecture part 2].

• Short TE (≈ min–25 ms) → little decay in any tissue; T2 differences don’t have time to express → T2 contrast minimized

Why water-containing tissues stand out on T2-weighted images

Water‑rich tissues have long T2, so with long TE they maintain transverse magnetization while short‑T2 tissues (like fat) have decayed → water‑containing structures appear bright relative to surrounding tissue

How do long TR and short TE suppress T1 and T2 contrast for PD imaging?

• Long TR (≥ ~2500 ms) → all tissues fully recover Mz → minimize T1 contrast.

• Short TE → little transverse decay → minimize T2 contrast.
  What remains is mainly differences in proton density

Why proton density reflects the number of hydrogen nuclei in tissue

Proton density is literally the number of hydrogen protons per unit volume

Which tissues typically have higher or lower proton density

Fat has more hydrogen protons than water, so fat has higher PD

What RF pulses are used in a spin echo sequence?

• 90° RF excitation pulse

• Followed by a 180° RF refocusing pulse (or pulses)

What RF pulses are used in a gradient echo sequence?

• Small flip angle excitation only (e.g., 5–90°, usually much less than 90° in practice)

• No 180° RF refocusing pulse; echo is formed using gradients

What role does the 180° RF pulse play in signal formation?

The 180° pulse refocuses dephasing caused by B₀ inhomogeneities and susceptibility differences

Why does spin echo correct for field inhomogeneities but GRE does not?

Spin echo uses that 180° RF pulse, which reverses dephasing from static field inhomogeneity and susceptibility. GRE lacks this RF refocusing;

Why are GRE sequences generally faster

• No 180° RF pulse → shorter TR and TE possible.

• Echo is formed quickly within the FID using gradients

How does the presence or absence of the 180° RF pulse fundamentally change contrast behavior

• With 180° (SE): removes T2* effects → can get true T1, T2, PD weighting.

• Without 180° (GRE): T2* remains → can get T1, T2*, and PD, but never true T2; susceptibility and inhomogeneities strongly influence contrast

What causes T2 decay?

T2 decay is caused by spin–spin interactions—magnetic moments of neighboring pro

What additional factors contribute to T2* decay beyond T2?

• True T2 (spin–spin) decay, plus

• B₀ field inhomogeneities, and Magnetic susceptibility

Why are GRE images more sensitive to magnetic field inhomogeneities?

GRE has no 180° RF refocusing to correct those inhomogeneity/susceptibility

Which tissues/materials exaggerate susceptibility effects?

• Metal (implants, surgical clips, metallic eyeliner/mascara)

• Air–tissue interfaces (paranasal sinuses, lungs)

T1‑weighted SE:

• TR: short (~350–800 ms)

• TE: short (~min–25 ms)

• Flip Angle: 90

• 180 degree refocus

T2‑weighted SE:

• TR: long (≥ ~2500 ms)

• TE: long (~80–120 ms)

• Flip Angle: 90

• 180 degree refocus

PD‑weighted SE:

• TR: long (≥ ~2500 ms)

• TE: short (~min–25 ms)

• Flip Angle: 90

• 180 degree refocus

How does flip angle affect saturation and T1 contrast in GRE?

• Large flip angle (~70–90°) with short TR → tissues with long T1 get saturated → strong T1 contrast (T1‑weighted GRE) 2].

• Small flip angle (~5–25°) with short TR → less saturation, more longitudinal magnetization survives → T1 effects reduced, enabling PD or T2* weighting

How does TE in GRE influence T2* sensitivity?

• Very short TE (~0–5 ms) → little dephasing → minimal T2* contrast.

• Longer TE (~20–40 ms) → more dephasing due to inhomogeneities/susceptibility → stronger T2* contrast

Why does GRE rely more on flip angle than TR for contrast control?

GRE TR is always short, so there’s limited room to modulate contrast via TR. Instead, you change flip angle to control how much longitudinal magnetization is consumed per TR (degree of saturation), which controls how much T1 weighting you get

How does changing flip angle alter steady‑state magnetization in GRE?

Changing flip angle changes how much Mz is tippped into Mxy each TR. Over many TRs, this sets a steady‑state level of longitudinal and transverse magnetization. Larger flip → more saturation → lower steady‑state Mz but higher T1 contrast; smaller flip → less saturation → higher steady‑state Mz, reduced T1 emphasis

What does TR represent in a pulse sequence?

TR (repetition time) is the time between successive excitation pulses (90° to 90°) for the same slice. It controls how much longitudinal recovery (T1) occurs between excitations

What does TE represent, and where is the echo measured?

TE (echo time) is the time from the excitation pulse (90°) to the peak of the echo (180), i.e., when the induced current in the receive coil is maximal (center of the echo/K‑space)

When does the 180° RF pulse occur relative to TE in spin echo?

The 180° refocusing pulse occurs at TE/2 after the 90° pulse (e.g., TE = 10 ms → 180° at 5 ms)

hy does timing (TR, TE, 180°) matter for contrast and signal formation?

• TR determines how much T1 recovery we allow before re‑excitation.

• TE determines how much T2/T2* decay we allow before measuring.

• 180° timing determines how effectively T2* effects are refocused in SE

Why is transverse magnetization required for signal?

transverse magnetization (Mxy) precesses in a way that crosses the receive coil’s field lines and induces a changing magnetic flux. This changing flux generates an induced current—the actual MR signal

How does signal induce current in the receive coil?

cuts through the coil’s magnetic field, changing the magnetic flux through the coil and inducing an alternating current via Faraday’s law

Why does longitudinal magnetization alone produce no signal?

Mz is aligned with B₀ and does not produce a changing magnetic field through the receive coil; no changing flux → no induced current → no measurable signal

How does loss of phase coherence affect signal strength?

As spins dephase (T2/T2*), their transverse components cancel each other out; net Mxy shrinks, so the induced current falls → signal decays

Why do tissues with long T1 appear dark on T1‑weighted images?

With a short TR, long‑T1 tissues don’t fully recover Mz before the next RF pulse; they remain energy‑saturated with small longitudinal components. Less Mz → less Mxy → low signal → dark

Why do tissues with long T2 appear bright on T2‑weighted images?

On long‑TE images, long‑T2 tissues maintain transverse coherence longer, so at the echo time they still have substantial Mxy → high signal → bright

Why is GRE more sensitive to metallic implants and susceptibility

GRE has no 180° RF refocusing, so dephasing from susceptibility and inhomogeneities is not corrected

Why do field inhomogeneities matter clinically?

They can:

• Distort anatomy

• Create signal voids or pile‑ups

• Mimic or hide pathology, especially on GRE where T2* effects are strong

How does gradient behavior differ from RF behavior (safety‑wise)?

• RF pulses deposit energy into tissue and are a major source of heating (SAR).

• Gradients change the magnetic field linearly in space and are switched rapidly, leading to peripheral nerve stimulation and acoustic noise, but they do not cause the same type of tissue heating as RF