The Common Contrast Agent use in MRI
The use of Contrast enhancement agents in MRI has an
extremely important value in diagnostic studies of diseases processes like tumors,
inflammations and infections. However these pathology contains a high water
contents are often visualized in T2 weighted images, but sometimes the image is
insufficient in contrast between the lesion and surrounding tissue.
Furthermore, T1 weighted images is added and has the advantage of demonstrating
a high SNR, but water and pathology are commonly isointens in these sequences.
Therefore it is sometimes necessary to selectively enhance pathology by administering
MRI contrast agents such as Gadolinium.
MRI contrast Agent Routes
MRI contrast agent can be administered either indirectly via
IV intravenous, oral and rectal routes or can be directly injected into a
structures like joints. MRI Contrast agent has two types these are positive
contrast and negative contrast.
Types MRI Positive Contrast Agents
Gadolinium
The most common positive contrast agent used in
MRI is the Gadolinium is a paramagnetic substance that has a relatively large
magnetic moment. Gadolinium is known as a T1 enhancement contrast agent. It is
a heavy metal and binds to certain element in the body such as membranes and
the osseous matrix. Hence, gadolinium cannot be excreted unless it is attached
to a chelate. The fucntion of chelates, surround the gadolinium ion and enable its exretion
mainly through the kidneys. The most common chelate use is Diethylene Triaminepentaacetic Acid (DTPA), This binds to eight of the nine sites in the
gadolinium ion and leaves the last free to facilitate the close approach to
water molecules. Other examples of chelates used with gadolinium are:- Gadopentetate Dimeglumine
- HP-DO3A
- DTPA-BMA
- DOTA
Manganese Enhancement Agent
an Intravenous contrast agent used in MRI for enhancement of image of the liver
Hyperpolarized Helium Enhancement Agent
A T1 ventilation agent used for the
evaluation of the Lungs.
Why Gadolinium is use as MRI enhancement Agents
Gadolinium produces in increased of fluctuations in the
magnetic fields of the water protons so that they tumble near to the Larmor
frequency. As a result there is a transfer of energy to the surrounding lattice
and both T1 and T2 relaxation times are reduced. In T2 images a higher
concentration of contrast agents to produce a significant shortening of T2.
However, much smaller doses are effective at shortening the T1 relaxation time
of water protons thereby increasing their signal intensity on T1 weighted
images. and T1 weighted images are used to study these type of pathology because
in these pathologies contains a high water content and are often visualized in
T2 weighted images, on some images the contrast between lesion and surrounding
tissue is insufficient.
How Gadolinium MRI contrast Agent is Use?
Gadolinium maybe given through IV (intravenously), orally or
rectally or also injected directly into the joint. The recommended dose of
Gd-DTPA and Gd-DTPA-BMA is 0.1 mmol/kg and 0.3 mmol/kg for Gd-HP-DO3a.
Oral Gadolinium – it is use to provide positive contrast of
the gastrointestinal tract to label the bowel thereby increasing the
visualization of abdominal organs such as the pancreas. Oral gadolinium has a
neutral taste and is easily mixed with water prior to ingestion. Gadolinium may
also be injected directly into the cavity of the joints. Procedure like
arthrography is an important technique, especially in the hip, shoulder and
ankle.
Symptoms of Oral Gadolium when taken by patient
Whiting out of the bowel – this can be minimized by
carefully adjusting of the dose and optimum timing of the scan sequence or post
ingestion.
MRI Negative Contrast Agent
Negative contrast agents is also known as the
Superparamagnetic agents an example of these are the Iron Oxides and manganese
and these are termed as the T2 enhancement agents. This is because their
presence causes a shortening of T2 decay times and reduced signal intensity.
These agents are taken up by the reticulo-endothelial system and transported to
the Kupffer cells of the liver parenchyma. Just like gadolinium,
superparamagnetic agent are dangerous in their pure form. However, unlike
gadolinium, chemical barriers[u1]
are not used. Instead the iron oxide particles are coated with either
hydrophilic polymer or arabinagalactin to provide a physical barrier. The function
of the this agent is to shorten the T2 relaxation times of normal liver so that
it appears dark and lesions appear bright on T2 weighted images.
Use of Negative Contrast Agent
Negative Contrasr agent is use specifically used for liver
imaging and are given via intravenous (IV). 0.56 mg of iron per kg of the body
weight is diluted in 100ml of 50% dextrose and given over a 30 min period at a
rate of 2 – 4 mm/min through a 5 micron filter. For bowel examination other
negative contrast agent such as Gastromark, is mixed with blueberry juice and
air, to delinaeate the large bowel in pelvic examinations.
MRI Blood Pool Agents
Blood Pool Agents is an Intravascular (blood pool) a agents
differ standards gadolinium agents in several areas. These agents are use
because of their persistence in the vessels for an extended period of time
rather than diffusing out of the blood stream as occurs with standard
extra-cellular fluid space agents. Using a standard agents, maximum
concentration, and therefore maximum signal, persist only for several seconds
resulting in a small window of opportunity for obtaining high-resolution
images. While intravascular agents, not only can data be acquired in the first
pass, as with standard agents, but high resolution images can be acquired in
the equilibrium phase as well with high signal persisting well over 30 mins.
The 2nd major difference is in relaxivity.
Relaxivity is an expression of the amount of T1 and T2 shortening provided by
the contrast agent. Increasing relaxivity can be obtained by several means.
However in the case of intravascular agent it is accomplished by the reversible
blinding of the agent to human albumin in plasma. This result in slowed
molecular tumbling of the hydrogen protons in albumin and a markedly increased
relaxivity and therefore increased signal.
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