MRI CONTRAST MEDIA
Introduction
Contrast enhancement is extremely valuable in many disease processes including tumours, inflammation and infection. Although these pathologies contain a high water content and are often visualized in T2 weighted images, sometimes there is insufficient contrast between the lesion and surrounding tissue. In addition, T1 weighted images demonstrate a higher SNR and are therefore advantageous, but water and pathology are commonly isointense in these sequences. Therefore it is sometimes necessary to selectively enhance pathology by administering
contrast agents. This can be done either indirectly via the intravenous (IV), oral or rectal routes or directly into a structure such as a joint. Only a brief overview is provided here. There are
two types of contrast agents: those that produce positive contrast and those that result in negative contrast.
Positive contrast agents
The most common positive contrast agent used in MRI is gadolinium (Gd). Gadolinium is a paramagnetic substance that has a relatively large magnetic moment. When introduced into the body, its presence causes increased fluctuations in the magnetic fields of 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. Since T2 relaxation is much shorter than T1, a high concentration of agent is required to produce 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. Gadolinium is therefore known as a T1 enhancement
agent. Gadolinium is a heavy metal and binds to certain elements in the body such as membranes and the osseous matrix. Therefore gadolinium cannot be excreted unless it is attached to a chelate. This chelate surrounds the gadolinium ion and enables its excretion, mainly through the kidneys. The most common chelate in use is diethylene triaminepentaacetic acid
(DTPA), which binds to eight of the nine binding 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 and DOTA. Gadolinium may be given intravenously (IV), orally or rectally or injected directly into a joint. The recommended IV dose of Gd-DTPA and Gd-DTPA-BMA is 0.1 mmol/kg and 0.3 mmol/kg for Gd-HP-DO3A. Oral gadolinium provides 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. Problems may arise from the bowel ‘whiting out’, although this can be minimized by careful adjustment of the dose and optimum timing of the scan sequence post-ingestion. Gadolinium may also be injected directly into a cavity such as a joint. Magnetic resonance arthrography is an important technique, especially in
the hip, shoulder and ankle. Other positive agents include manganese, an IV agent used in liver imaging and hyperpolarized helium a T1 ventilation agent used for the evaluation of the lungs.
Negative contrast agents
Superparamagnetic agents such as iron oxides and manganese are termed T2 enhancement agents. This is because their presence causes a shortening of T2 decay times and reduced signal intensity. They are taken up by the reticulo-endothelial system and transported to the Kupffer cells of the liver parenchyma. Like gadolinium, superparamagnetic agents are dangerous in their pure form. However, unlike gadolinium, chemical barriers are not used. Instead the iron oxide particles are coated with either hydrophilic polymer or arabinagalactin to provide a physical barrier.
These agents dramatically shorten the T2 relaxation times of normal liver so that it appears dark and lesions appear bright on T2 weighted images. They are, therefore, specifically used for liver imaging and are given IV. A recommended dose is 0.56 mg of iron per kg of body weight. This is diluted in 100 ml of 50% dextrose and given over a 30 min period at a rate of 2–4 mm/min through a 5 micron filter. Other negative contrast agents include oral agents known as Gastromark ™, blueberry juice and air, which are used for bowel imaging to delineate the large bowel in pelvic examinations.
Blood pool agents
Intravascular (blood pool) agents differ from standard gadolinium agentsin several areas. First and foremost is 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. When imaging with standard agents, maximum concentration, and therefore maximum signal, persists only for several seconds resulting in a small window of opportunity for obtaining high-resolution images. With 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 minutes. The second 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 agents it is accomplished by the reversible binding of the agent to human albumin in plasma. This results in slowed molecular tumbling of the hydrogen protons in albumin and a markedly increased
relaxivity and therefore increased signal.
Introduction
Contrast enhancement is extremely valuable in many disease processes including tumours, inflammation and infection. Although these pathologies contain a high water content and are often visualized in T2 weighted images, sometimes there is insufficient contrast between the lesion and surrounding tissue. In addition, T1 weighted images demonstrate a higher SNR and are therefore advantageous, but water and pathology are commonly isointense in these sequences. Therefore it is sometimes necessary to selectively enhance pathology by administering
contrast agents. This can be done either indirectly via the intravenous (IV), oral or rectal routes or directly into a structure such as a joint. Only a brief overview is provided here. There are
two types of contrast agents: those that produce positive contrast and those that result in negative contrast.
Positive contrast agents
The most common positive contrast agent used in MRI is gadolinium (Gd). Gadolinium is a paramagnetic substance that has a relatively large magnetic moment. When introduced into the body, its presence causes increased fluctuations in the magnetic fields of 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. Since T2 relaxation is much shorter than T1, a high concentration of agent is required to produce 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. Gadolinium is therefore known as a T1 enhancement
agent. Gadolinium is a heavy metal and binds to certain elements in the body such as membranes and the osseous matrix. Therefore gadolinium cannot be excreted unless it is attached to a chelate. This chelate surrounds the gadolinium ion and enables its excretion, mainly through the kidneys. The most common chelate in use is diethylene triaminepentaacetic acid
(DTPA), which binds to eight of the nine binding 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 and DOTA. Gadolinium may be given intravenously (IV), orally or rectally or injected directly into a joint. The recommended IV dose of Gd-DTPA and Gd-DTPA-BMA is 0.1 mmol/kg and 0.3 mmol/kg for Gd-HP-DO3A. Oral gadolinium provides 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. Problems may arise from the bowel ‘whiting out’, although this can be minimized by careful adjustment of the dose and optimum timing of the scan sequence post-ingestion. Gadolinium may also be injected directly into a cavity such as a joint. Magnetic resonance arthrography is an important technique, especially in
the hip, shoulder and ankle. Other positive agents include manganese, an IV agent used in liver imaging and hyperpolarized helium a T1 ventilation agent used for the evaluation of the lungs.
Negative contrast agents
Superparamagnetic agents such as iron oxides and manganese are termed T2 enhancement agents. This is because their presence causes a shortening of T2 decay times and reduced signal intensity. They are taken up by the reticulo-endothelial system and transported to the Kupffer cells of the liver parenchyma. Like gadolinium, superparamagnetic agents are dangerous in their pure form. However, unlike gadolinium, chemical barriers are not used. Instead the iron oxide particles are coated with either hydrophilic polymer or arabinagalactin to provide a physical barrier.
These agents dramatically shorten the T2 relaxation times of normal liver so that it appears dark and lesions appear bright on T2 weighted images. They are, therefore, specifically used for liver imaging and are given IV. A recommended dose is 0.56 mg of iron per kg of body weight. This is diluted in 100 ml of 50% dextrose and given over a 30 min period at a rate of 2–4 mm/min through a 5 micron filter. Other negative contrast agents include oral agents known as Gastromark ™, blueberry juice and air, which are used for bowel imaging to delineate the large bowel in pelvic examinations.
Blood pool agents
Intravascular (blood pool) agents differ from standard gadolinium agentsin several areas. First and foremost is 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. When imaging with standard agents, maximum concentration, and therefore maximum signal, persists only for several seconds resulting in a small window of opportunity for obtaining high-resolution images. With 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 minutes. The second 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 agents it is accomplished by the reversible binding of the agent to human albumin in plasma. This results in slowed molecular tumbling of the hydrogen protons in albumin and a markedly increased
relaxivity and therefore increased signal.
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