3.6 Inversion Recovery Sequences

3.6 Inversion Recovery Sequences

Inversion recovery (IR) sequences are a highly adaptable class of spin echo sequences designed to manipulate tissue contrast by selectively suppressing signal from specific tissues based on their longitudinal (T1) relaxation times. By initiating the pulse sequence with a 180° inversion pulse, IR sequences can generate dramatic contrast enhancement or complete signal nulling of tissues such as fat (STIR) or cerebrospinal fluid (FLAIR).

Physics of Inversion Recovery

An inversion recovery sequence differs from a standard spin echo sequence by the addition of a 180° RF pulse at the very beginning of the cycle. This initial pulse is called the inversion pulse.

The sequence of events is as follows:

1. Inversion Pulse (180° RF): This pulse inverts the net magnetization vector (NMV) from the positive longitudinal axis (+Mz) to the negative longitudinal axis (-Mz). No transverse magnetization is created at this step.
2. Inversion Time (TI) Delay: The NMV is allowed to relax back towards the positive longitudinal axis via spin-lattice (T1) relaxation. During this time, different tissues recover at different rates depending on their characteristic T1 relaxation times.
3. Excitation Pulse (90° RF): After a specific delay known as the Inversion Time (TI), a 90° excitation RF pulse is applied. This pulse flips the recovered longitudinal magnetization into the transverse plane.
4. Refocusing Pulse (180° RF): A subsequent 180° refocusing pulse is applied after a time interval of TE / 2 to produce a spin echo at time TE, identical to a standard spin echo sequence.

The critical parameter is the Inversion Time (TI), which is the time between the initial 180° inversion pulse and the 90° excitation pulse.

Inversion Time (TI) Selection and the Null Point

As the inverted longitudinal magnetization recovers from -Mz to +Mz, it must pass through the zero point (where the magnetization vector is exactly zero). This point of transition is known as the null point.

If the 90° excitation pulse is applied at the exact instant a specific tissue reaches its null point, there is no longitudinal magnetization available to be flipped into the transverse plane. As a result, that tissue yields no signal and appears black (suppressed) on the final image.

The relationship between a tissue's T1 relaxation time and the inversion time required to null it (TI_null) is given by the formula:

TI_null ≈ T1 * ln(2) ≈ 0.693 * T1

Because a tissue's T1 relaxation time increases with magnetic field strength (B0), the TI required to null a specific tissue must also be adjusted upward when transitioning from a 1.5T to a 3.0T scanner.

Short TI Inversion Recovery (STIR)

Short TI Inversion Recovery (STIR) is an inversion recovery sequence designed to suppress the signal from fat. Fat has a very short T1 relaxation time (approximately 220 to 250 ms at 1.5T). To null the signal from fat, a short inversion time is selected.

• Typical TI for STIR:
  • At 1.5T: 140 - 160 ms (typically 150 ms)
  • At 3.0T: 170 - 180 ms
• Mechanism: The 90° excitation pulse is applied when the fat magnetization is passing through its null point. Consequently, fat appears completely dark, while water (which has a longer T1 and has not yet reached its null point) appears bright.

Clinical Rule: The Post-Contrast Contraindication

A critical clinical rule for STIR sequences is that STIR must not be used after the administration of gadolinium contrast. Gadolinium shortens the T1 relaxation time of enhancing tissues (such as tumors or areas of inflammation) so that their T1 becomes similar to that of fat. If a STIR sequence is run post-contrast, the signal from the contrast-enhancing pathology will be nulled along with the fat, masking the pathology. For post-contrast fat suppression, chemically selective fat saturation (Fat-Sat) should be used instead.

Advantages of STIR

STIR is highly robust and insensitive to magnetic field inhomogeneities. This makes it the preferred fat-suppression method for large fields of view (e.g., spine imaging), areas with significant susceptibility differences (e.g., joints with surgical hardware), and off-center imaging.

Fluid Attenuated Inversion Recovery (FLAIR)

Fluid Attenuated Inversion Recovery (FLAIR) is an inversion recovery sequence designed to suppress the signal from free fluid, primarily cerebrospinal fluid (CSF). CSF has a very long T1 relaxation time (approximately 3000 to 4500 ms). To null the CSF signal, a very long inversion time is required.

• Typical TI for FLAIR:
  • At 1.5T: ≈ 2000 ms
  • At 3.0T: 2200 - 2500 ms
• Mechanism: The 90° excitation pulse is applied when the CSF magnetization reaches its null point. This nulls the bright CSF signal, while brain parenchyma (which has shorter T1 times and has already recovered past the null point) generates a strong signal.

Clinical Applications

FLAIR is a staple of neuroimaging. In standard T2-weighted brain images, the high signal from CSF is bright, which can obscure lesions located near the ventricles or subarachnoid space. By nulling the CSF, FLAIR allows periventricular pathology, such as multiple sclerosis (MS) plaques, infarctions, and subarachnoid hemorrhage, to stand out as hyperintense lesions against a dark fluid background.

Advantages and Disadvantages of Inversion Recovery

Advantages:

• High Contrast specificity: Enables highly targeted suppression of specific tissues.
• Additivity of T1 and T2 contrast: The recovery from -Mz to +Mz adds to the T2 contrast, generating higher contrast-to-noise ratio (CNR) than conventional spin echo.

Disadvantages:

• Increased Scan Time: The extra time required for the TI delay during every TR cycle significantly increases the overall acquisition time. To mitigate this, clinical IR sequences are almost always combined with Fast Spin Echo (FSE) readouts (e.g., Fast STIR, Fast FLAIR).
• Reduced SNR: Nulling a tissue reduces the total signal contribution to the image.
• Higher SAR: The use of multiple 180° RF pulses (inversion and refocusing) increases the Specific Absorption Rate (SAR), leading to RF deposition and patient heating.