Contrast Agents

Contrast Agents

• Paramagnetic agents (with unpaired electrons)

• Modify relaxation times in areas where they are present

1 T₁ = 1

T_1,0 +R₁n_a 1

T₂ = 1

T_2,0 +R₂n_a

T₁and T₂are observed relaxation times with paramagnetic agent present.

T_1,0and T_2,0are observed without any agent, n_ais the concentration and R₁ and R₂are the agent’s relaxivities.

For Gd-DTPA, R₁= 4.5 kg•mmol•s^-1 R₂= 6.0 kg•mmol•s^-1 between 0.5 and 1.5 T

Typical contrast agent administration is 0.1-0.2 mmol/kg

Contrast agent theory

• Gadolinium must be strongly bound in chelates - free Gd is very toxic!

• Gd-DTPA is a large molecule and therefore cannot cross many membranes in the body.

• Tumors and other lesions (MS) often cause a disruption in the blood-brain- barrier, the membrane that preserves the sensitive brain tissues

• The disrupted BBB allows the Gd chelate into the area of the tumor, causing an local decrease in T1

Other contrast mechanisms

• Flow and motion

• Diffusion

• Contrast agents

– Paramagnetics

• Oxygenation level

– Blood Oxygenation Level Dependent imaging (BOLD)

How does blood flow affect contrast?

• A) entrance of “fresh” spins into the image plane - exit of saturated spins out of the image plane

• B) for slow flow, phase change because of gradients is dependent upon flow rate, as spins change location within the gradient field

• C) turbulent flow causes loss of signal

coherence, thereby reducing signal intensity

MR Angiography

• MRA pulse sequences and processing

techniques take advantage of these inherent contrast modifying properties to distinguish flowing blood from “static” tissues.

• Two types of sequences

– Time-of-flight - in/out of plane spin motion

• White blood

• Black blood - uses presaturation pulse

– Phase contrast - phase change measurement

Image slice

Motion of spins

Image slice

Motion of spins presat volume

TOF MRA

TR•1

TR•2 TR•3

If flow direction is perpendicular to the slice plane, a portion of the blood in the slice plane is saturated during the next TR - some more of the previously saturated blood exits before 2•TR and all of it is gone by 3•TR. The degree of blood saturation depends upon slice thickness, TR, αand flow velocity

Diffusion

• The migration of water in extracellular tissue space can be visualized…

• In the presence of a gradient field the diffusion can be expressed as

∂ ^M

∂ ^{t = D∇}

^M

where D is the diffusion coefficient

Diffusion

Re-writing the Bloch equations to include diffusion terms…

∂ ^M

∂ ^{t = −} M

T

+ γ ^GrM

+ D ∇

M

∂ ^M

∂ ^{t =}

− M

T

+ γ ^GrM

+ D∇

M

∂ ^M

∂ ^{t =}

−M

T

+ i γ ^GrM

+ D ∇

M

Diffusion

Solving for the transverse magnetization, with integrals of the gradient over time

M

= M

( ) 0 ^{e (}

⁾

Diffusion

Stejskal-Tanner sequence

S TE ( ) ^{= S 0} ( ) ^{e (}

⁾

90° 180°

δ δ

∆Can be solved for D by varying Gδor ∆

DWI-EPI Sequence

rf G_f

G_ph

90°

180°

What do diffusion images mean?

• Diffusion represents small translational motion (i.e.

across a cell membrane)

• In MRI, diffusion is complicated by the perfusion of blood through microscopic blood vessels, therefore, the

measurement of diffusion in MRI is known as the apparent diffusion coefficient (ADC)

90° 180°

δ ∆ δ

b = γ

^G

^t

( ∆ −

)

S TE ( ) ^{∝ exp} [ ^{− TE T}

] ^{∗ exp} [ ^{− b D ′} ]

Moving spins experience unequal effects from the gradient pulses and therefore do not rephase at the echo time TE. Thus, there is a signal loss for “diffusing”

spins. The larger the b-value, the larger the signal loss. Spins that are impeded from diffusion by lack of blood flow, cellular exchange, etc. do not lose as much signal. These areas look brighter on the diffusion coefficient images.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 S(100) S(500) S(1000) S(3000)

Diffusion coefficient value

Relative signal

Varying b-values

Diffusion Imaging Example

Figure 12 : Multiple Sclerosis: isotropic diffusion-weighted image with (a) b = 1000 s/mm²and (b) b = 3000 s/mm². The high

Acute Stroke: (c) b = 1000 s/mm²and (d) b = 3000 s/mm². The high b-value DWI demonstrates the