Inflowrelated Signal Enhancement Inflow Effect

Although the signal of blood flowing quickly out of the measured slice is reduced with SE sequences, under certain circumstances the opposite effect may occur: spins flowing into the slice may generate a higher signal than the surrounding tissue. This effect is referred to as inflow enhancement.

On T1-weighted imaging, contrast is generated by repeated RF pulses that are applied with a time interval (repetition time, TR) that is shorter than the T1 relaxation time of the tissue (typically TR < 700 msec). As a result, the tissue components are saturated unequally, depending on their individual T1 times. This is the basis of T1-weighted image contrast (Fig. 4). Irrespective of flow effects, blood in the vessels would appear hypointense on a normal T1-weighted image due to its relatively long T1 time.

The signal emitted by the tissue diminishes when the TR is reduced. With GRE sequences, repetition times shorter than 50 msec can be achieved. This allows the majority of non-moving spins to become saturated, thus minimizing the background signal.

Spins outside the excited slice (or volume) are not influenced by the RF pulses. Consequently, blood entering into the slice being imaged is fully relaxed, experiencing not more than a few excitations on its way through the slice. As a result, flowing blood gives rise to considerably higher signal intensity relative to that of the saturated spins in the stationary tissue. This effect is called "inflow enhancement" or "flow-related enhancement" (Fig. 5).

The signal intensity of flowing blood increases with:

• decreasing slice thickness s,

• increasing flow velo city v.

If the blood flow velocity is so high that all vessel spins are replaced by unsaturated spins in the time interval TR (i.e. v > s/TR), flow enhancement is maximal and the vessel appears bright on a gray or black background (Fig. 6).

Although the inflow effect occurs both with SE and GRE sequences, SE sequences are not practical for the TOF method because the competing washout effect (see above) tends to overbalance the inflow effect at higher flow velocities, leading to decreased flow signal.

Consequently, flow-related enhancement using GRE sequences to produce bright-blood images is the basis of time-of-flight angiography.

Flow Related Enhancement
Fig. 4. With a very long repetition time TR, the magnetization fully relaxes yielding maximum signal strength M0. Shortening of the TR leads to partially saturated magnetization and therefore decreased signal
Time Flight Inflow Saturation
Fig. 5. Inflow effect: In the interval between two RF excitations, the blood in the vessel is replaced by "fresh", unsaturated blood, while the stationary tissue in the slice is saturated due to the short TR

Fig. 6. Brain vessels, axial slice. Due to the inflow effect, there is high contrast between the vessels and the surrounding tissue

Î

flowing spins, velocity v

/A J Ms

M , f

\ -A / time I (J> = y • À • t « Y

stationary/spins

Fig. 7. Spins moving along bipolar magnetic field gradients experience a phase shift O of their transverse magnetization proportional to the gradient A, the time interval t between the pulses, and the velocity v of the spins along the gradient direction. In contrast, the phase shift for stationary spins is zero

Fig. 6. Brain vessels, axial slice. Due to the inflow effect, there is high contrast between the vessels and the surrounding tissue

Fig. 7. Spins moving along bipolar magnetic field gradients experience a phase shift O of their transverse magnetization proportional to the gradient A, the time interval t between the pulses, and the velocity v of the spins along the gradient direction. In contrast, the phase shift for stationary spins is zero

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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