Basic contrast media used in MRI.pptxsarita gaire

contrast used in mri

1. Basics of contrast media used in MRI. Presented by:-Sarita gaire Roll no :- 160 BSc. MIT 2 nd year Maharajgunj medical campus,iom

2. Introduction Contrast media  are a group of chemical agents introduced to the anatomical or functional region being imaged, in order to produce the sufficient differences between the adjacent organs & to aid in the characterization of pathology by improving the  contrast resolution between the pathology & normal structure

3. Evolution of MR contrast agents Initially, MRI was believed to provide sufficient soft tissue contrast without needing contrast agents, making it non-invasive. Contrast in MRI arises mainly from heterogeneous distribution of tissue relaxation times , and the lack of need for contrast materials is one of the major advantages of MR over other imaging techniques. However, many pathological conditions didn't show distinct changes in relaxation times. To address this, scientists introduced contrast agents (CAs) to improve image clarity and differentiation between healthy and diseased tissues

4. Exogenous MR contrast agents were developed in the 1980s, soon after the first commercial MR systems. Today, they are commonly used to: Differentiate similar structures Provide more detail in abnormal areas Highlight specific spaces Show tissue vascularity and perfusion

5. CONTRAST IN X-RAY &CT X-ray and CT contrast agents create contrast based on electron density, directly affecting anatomical parts. In MRI, contrast enhancement is more complex, involving interactions between contrast agents and water protons, influenced by factors like proton density and MRI pulse sequences.

6. BASIC PRINCIPLE OF MR CONTRAST AGENTS MRI uses NMR to visualize tissues via hydrogen proton relaxation in a magnetic field. T1 and T2 relaxation times determine image contrast. MR contrast agents alter relaxation times, impacting image contrast (measured as relaxivity ). Relaxivity depends on magnetic moment, tumbling frequency, and spin relaxation time. Contrast agents are classified as T1 or T2 based on their relaxation processes. Paramagnetic, superparamagnetic, or ferromagnetic ions create a magnetic moment. Gadolinium compounds are common MR contrast agents

7. CLASSIFICATION OF MR CONTRAST AGENTS

8. Based on magnetic property most common method used to classify the contrast agents into 2 main groups, paramagnetic and super paramagnetic

9. Paramagnetic contrast agents Paramagnetic materials have their own magnetic field, which can reduce the T1 and T2 relaxation times of surrounding hydrogen protons. This shortening of relaxation times leads to increased signal intensity on T1-weighted MRI images. Gadolinium (Gd3+) is the most commonly used paramagnetic contrast agent due to its seven unpaired electrons and long electron spin relaxation time, making it highly effective at enhancing relaxation. Manganese has similar relaxation-enhancing properties and has also been used as a contrast agent. Paramagnetic contrast agents are chelated ( bound) to a ligand to minimize the toxicity of the free metallic ion in the body

10. Superparamagnetic contrast agents Superparamagnetic iron oxide (SPIO) particles are used as contrast agents in clinical MRI. SPIO particles have a large magnetic moment when in an external magnetic field, but no residual magnetism when the field is removed. ( superparamagnetism ). SPIO agents can enhance both T1 and T2/T2* relaxation, but their primary effect is on T2/T2* relaxation. The degree of T1 or T2/T2* relaxation enhancement depends on the size and composition of the SPIO particles. Compared to paramagnetic agents, SPIO particles have a much larger net magnetic moment, leading to more pronounced changes in MRI signal.

11. Based on Effect on signal intensity Contrast agents can have either a positive effect (increased signal) on T1-weighted images or a negative effect ( decreased signal) on T2-weighted images. Most contrast agents affect both T1 and T2 relaxation to some degree, but the dominant effect depends on the specific agent. Gadolinium-based agents primarily act as positive T1 enhancers, but also have a weaker negative T2 effect. Iron oxide-based agents mainly have a negative T2/T2* shortening effect, resulting in signal reduction on T2-weighted images. The overall effect depends on the contrast agent dose and various MRI-specific parameters.

12. Based on bio-distribution of agents The biodistribution of a contrast agent describes how the agent is distributed in vivo after intravenous administration. Extracellular fluid agents Intravascular agents Tissue specific agents

13. Extracellular fluid agents Small paramagnetic contrast agents can easily move from the bloodstream into the space between cells (the interstitium ) and distribute throughout the extracellular fluid. These agents are called extracellular fluid (ECF) agents. ECF agents are not absorbed by cells and are eliminated from the body through the kidneys, with their clearance rate depending on the glomerular filtration rate. Gadolinium-based contrast agents are examples of small ECF paramagnetic agents.

14. Intravascular agents Intravascular agents are large enough to stay confined within the blood vessels and not leak out into the surrounding tissues. Iron oxide nanoparticles are all intravascular agents, with time in the bloodstream ranging from minutes to hours. In the blood, certain iron oxide agents can significantly shorten T1 relaxation, making them useful for MR angiography (blood vessel imaging). Another type of intravascular agent is macromolecular gadolinium compounds . These are designed to either bind to gadolinium during synthesis or attach to blood proteins after injection, creating larger molecules that stay in the bloodstream. The key difference from extracellular agents is that intravascular agents are restricted to the blood vessels and do not distribute into the tissues.

15. Tissue specific agents Tissue-specific contrast agents are designed to accumulate in particular organs or tissues. Intravascular iron oxide agents are taken up by Kupffer cells in the liver, spleen, and lymph system. Once in the liver, the iron oxide particles cluster together, leading to strong T2/T2* effects. So iron oxides can act as both intravascular agents and liver-specific agents. Some gadolinium-based agents and mangafodipir also target the liver, being absorbed by liver cells (hepatocytes). New contrast agents are also being developed to target atherosclerotic plaques and different types of tumors

16. GADOLINIUM BASED CONTRAST AGENTS Gadolinium is a lanthanide metal with atomic number 64, named after Finnish chemist Johan Gadolin . Gadolinium is paramagnetic due to its 7 unpaired electrons. This strong paramagnetism affects the relaxation of nearby water protons, decreasing both T1 and T2 relaxation times. At low gadolinium concentrations, the T1 reduction is more dominant, leading to increased signal intensity on T1-weighted MRI images.

17. History In the 1980s, copper, manganese, and gadolinium were identified as paramagnetic ions that could shorten the T1 relaxation of water. After testing many paramagnetic chelates, gadolinium-DTPA (gadolinium diethylene triamine pentaacetic acid) was selected due to high tolerability and good relaxation properties. Gadolinium-DTPA was first used in humans in 1983, and in 1988 the gadopentetate dimeglumine formulation was launched as the first MRI contrast agent

18. Classification of GBCA BASED ON THEIR BINDING TO SERUM PROTEINS: Non-specific extracellular gadolinium agents The high relaxivity agents.

19. Non-specific extracellular GBCAS Gadolinium-based contrast agents do not bind to proteins, so their distribution and behavior is similar to water-soluble iodinated contrast media. After injection, they quickly move into the space between cells (the extravascular/extracellular space). There is also a back-and-forth diffusion between the blood and this extravascular space, reaching an equilibrium within about 2 hours. These agents are eliminated unchanged primarily by the kidneys, with over 95% excreted in the urine within 24 hours in people with normal renal function. Only a very small amount, less than 0.1%, is eliminated through the stool. The biological half-life of these agents is around 1.5 hours in patients with normal kidney function.

20. some nonspecific extracellular GBCAs Gadopentetate, Gadoterate, Gadodiamide, Gadoteridol, Gadobutrol, Gadoversatamide.

21. Basic phases of vascular &tissue enhancement Three basic phases of vascular and tissue enhancement occur following administration of GBCAs Arterial phase Blood pool Extracellular phases

22. Arterial phase Arterial phase imaging is best done within the first 20 seconds after contrast injection, during the initial pass through the arteries. Early arterial phase images are ideal for clearly depicting arteries. Later arterial phase images are better for showing hypervascular tissues

23. Blood pool phase The blood pool or portal venous phase occurs 60-90 seconds after contrast injection, when the agent has distributed through the body's blood vessels. This phase also sees maximum enhancement of the liver parenchyma. Dynamic imaging during this phase, similar to CT, can be used to detect and characterize lesions in the liver and other organs when using non-specific extracellular gadolinium-based contrast agents. .

24. Extracellular phase Post-contrast images are taken about 2 minutes after injection when contrast reaches the extracellular space. Fast scanning isn't necessary because extracellular enhancement remains stable for several minutes. Contrast is particularly noticeable in: Neoplasms (tumors) Inflammation areas Fibrous tissue has large extracellular spaces, making it appear enhanced even if it's usually low in blood vessels. Many metastases also have large extracellular spaces and appear bright on extracellular phase images

25. Extracellular phase Brain and testicular capillaries don't let contrast agents through, so brain enhancement is minimal, mostly showing blood pool enhancement. Tumors and injured brain tissue enhance more due to their interstitial space, making brain lesions very visible in post-contrast images. Gadolinium-based contrast agents (GBCAs) also move into renal tubules, getting concentrated as water is reabsorbed. This makes GBCAs very effective for enhancing the kidneys and urinary tract.

26. High relaxivity agents high relaxivity agents includes Gadobenate , Gadoxetate and Gadofosveset. These agents have higher T1 relaxivity than non-specific extracellular agents, meaning they provide better contrast in MRI images. Immediately after injection, gadobenate and gadoxetate act as non-specific extracellular agents. Over time, they are taken up by liver cells, making them act as liver-specific agents during the delayed phase. These agents are eliminated through both the kidneys and the liver/bile duct system.

27. Gadobenate dimeglumine can weakly and temporarily bind to proteins, and only 2-4% is taken up by liver cells, resulting in a delayed imaging time of 90-120 minutes. Gadoxetate disodium has greater biliary excretion (similar to renal excretion) and a shorter delayed imaging time of 10-20 minutes. It also provides more intense liver enhancement compared to gadobenate . Both agents behave like other gadolinium-based contrast agents initially, allowing assessment of tumor enhancement and characterization. In the delayed phase, they are selectively taken up by liver cells, helping detect small liver tumors. The biliary excretion of these agents during the delayed phase allows good visualization of the bile ducts on T1-weighted MRI

28. Gadofosveset: 80-90% binds to serum albumin. High protein binding has 2 effects: Increases relaxivity, requiring lower doses and concentration. Confinement to the vascular space. Used as a blood pool agent for MR angiography due to prolonged intravascular retention. Protein binding is reversible. Excreted unchanged by the kidneys through glomerular filtration.

29. Gadolinium chelation There are two basic structure for chelating agents: Linear Macrocyclic. Linear agents have an elongated organic molecular ligand that wraps around the ion.  Macrocyclic agents form a cage-like ligand structure with the ion trapped in a preformed central cavity. Also, both linear and macrocyclic agents can either be ionic or non-ionic.

30. Chemical structure of DTPA MOLECULE

31. Osmolality& viscosity contrast agents (GBCAs) vary in osmolality (0.63 to 1.97 osmol/kg) and viscosity. The osmotic load of GBCAs is much lower than iodinated contrast media, as only a small amount is needed for MRI. Nonionic GBCAs tend to have lower osmolality and are less viscous. Nonionic, low-osmolar GBCAs also have fewer negative effects on the heart, which is important for cardiac patients. Low viscosity GBCAs can be injected with less pressure and smaller needles/catheters

32. Stability The stability of a gadolinium-based contrast agent (GBCA) depends on the affinity of the gadolinium atom for its compound, described by thermodynamic and conditional stability. For linear GBCAs, the dissociation constant indicates stability - higher is more stable. Macrocyclic GBCAs are more stable, requiring more energy to dissociate the gadolinium complex, described by dissociation rate and half-life. Ionic GBCAs are generally more stable than non-ionic ones. In terms of stability, it increases from non-ionic linear to ionic linear to macrocyclic GBCAs

33. Stability Gadolinium-based contrast agents are bound to excess chelate due to the toxicity of free gadolinium ions. The excess chelate helps prevent trans- metallation , where the gadolinium can be replaced by other metal ions like zinc in the blood. Agents with weaker thermodynamic stability have more excess chelate to compensate for their higher risk of gadolinium release. The amount of excess chelate can indicate the relative stability of the contrast agent - more excess means lower stability.

34. Adverse effect of GBCAs Gadolinium-based contrast agents (GBCAs) are generally well-tolerated, with a low incidence of adverse effects. Mild reactions like nausea, vomiting, and skin rash can occasionally occur after GBCA administration. Other uncommon effects include headache, taste changes, dizziness, and tingling. Adverse reactions are more common in patients with asthma, allergies, or when injected too quickly, but the overall incidence is less than 5%. Serious anaphylactoid reactions are very rare, around 1 in 100,000 cases. Life-threatening reactions and conditions like nephrogenic systemic fibrosis (NSF) are extremely rare with GBCAs.

35. Nephrogenic systemic fibrosis(NSF) Gadolinium-enhanced MRI was previously considered a very safe imaging procedure. However, the discovery of a link between gadolinium and nephrogenic systemic fibrosis (NSF) has led the radiology community to reevaluate its use. NSF was first described in 1997 as a skin disorder, later identified as a systemic fibrotic condition. In 2006, the association between gadolinium exposure and NSF development was first reported. Multiple studies have since confirmed the presence of gadolinium in the tissues of patients with NSF, supporting its causative role.

36. Mechanism of NSF NSF is characterized by scleroderma-like skin changes, mainly affecting the limbs, and can also impact internal organs. NSF can be an aggressive disease, leading to severe deformities or even death. NSF occurs exclusively in patients with poor kidney function (GFR less than 30 mL/min/1.73 m²). The leading hypothesis is that in these patients, gadolinium can dissociate from its chelate, forming insoluble precipitates that deposit in tissues and trigger a fibrotic reaction. Evidence for this includes the presence of gadolinium in affected tissues of NSF patients. Symptoms typically appear within days to months after GBCA exposure, but in rare cases, can occur years later.

37. Cases of nephrogenic systemic fibrosis

38. Treatment of NSF Currently, there is no effective treatment for NSF. Trials with corticosteroid, photopheresis, plasmapheresis, thalidomide, methotrexate, etc. have not had much success. There is also no evidence that immediate hemodialysis protects against the development of NSF. Improving renal function seems to slow or arrest progression of this condition.

39. Risk factors for NSF The risk factors for NSF can be mainly described as: Patient factors & Contrast medium factors.

40. Patient factors Patients with end-stage kidney disease (CKD 5, eGFR less than 15 mL/min/1.73m²) and advanced CKD (stage 4, eGFR 15-29 mL/min/1.73m²) have a 1-7% risk of developing NSF after GBCA exposure. However, it's not fully understood why some high-risk patients develop NSF while others do not. One study found a 3% incidence of NSF in patients with eGFR less than 30 mL/min/1.73m² who received high doses of gadodiamide. Various cofactors have been proposed, such as metabolic acidosis, elevated iron/calcium/phosphate, high-dose erythropoietin, and inflammation, but none have been consistently confirmed. Routine screening for these potential cofactors before GBCA administration is not universally recommended, though some clinicians may choose to do so. Liver disease alone, without kidney dysfunction, is not considered a risk factor for NSF

41. Contrast medium factors The stability of gadolinium binding within chelates is crucial in NSF pathogenesis; macrocyclic chelates are more stable than linear ones. High doses of GBCAs, single or cumulative, increase NSF risk. No approved GBCAs are proven safe for at-risk patients, though all are safe for patients without renal disease.

42. Current guidelines on GBCAs usage: Categories Guidelines 1. Patients with end-stage renal disease on chronic dialysis: If a CE examination is essential, it would be reasonable performing a CECT rather than an MRI. If CE-MRI is to be performed, use of lowest possible dose is recommended & Group I agents are contraindicated. ACR also recommends that GBCA enhanced MRI examinations be performed as closely before hemodialysis, as it may reduce the likelihood that NSF will develop but it still unproved till date. 1. In patients with Renal dysfunction

43. Categories Guidelines 2. Chronic kidney disease stage IV or V (eGFR less than 30 mL/ min/1.73 m2 ) not on dialysis: The correct course of action in this patient group is problematic, as administration of iodinated contrast media for CT may also lead to further deterioration of renal function, while administration of GBCA for MRI could result in NSF. It is recommended that any GBCA be avoided in this patient group. However, if GBCA enhanced MRI is deemed essential, use of the lowest possible dose is recommended & Group I agents are contraindicated.

44. Categories Guidelines 3. Chronic kidney disease stage III with eGFR between 30 and 59 mL/min/1.73 m2 NSF developing after GBCA administration to patients with eGFR greater than 30 mL/min/1.73 m2 is exceedingly rare. However, eGFR determinations may fluctuate from one day to the next (with an eGFR level just above 30 on one day changing to an eGFR below 30 on another day) For this reason that the precautions described above for CKD4 and CKD5 patients are also recommended for inpatients with an eGFR less than 40 mL/ min/1.73 m 2 . In comparison, no special precautions are required in patients with an eGFR of 40 to 59 mL/min/1.73m 2 . 4. Chronic kidney disease stage I or stage II with eGFR greater than 60 mL/min/1.73 m2 : There is no evidence of increased risk of NSF and All GBCAs can be safely administered using a dose of less than or equal to 0.1 mmol /kg.

45. categories Guidelines 5. Patients on hemodialysis: There is no evidence supporting the use of hemodialysis in preventing or treating NSF in patients not already undergoing hemodialysis. But it may be useful to remove GBCAs in patients already on hemodialysis according to the EMA . The ACR recommends that enhanced MRI be done shortly before hemodialysis & MRI should be schedule as per dialysis appointment

46. In children Few pediatric cases of NSF have been reported, with no cases in children under 6 years old. However, there is insufficient data to conclude that NSF is less likely to occur in children compared to adults with similarly severe kidney disease. Therefore, the same guidelines for avoiding GBCA exposure should be followed for both adult and pediatric patients with impaired renal function.

47. During pregnancy & lactation Pregnancy GBCAs can cross the placenta and reach the fetus when given to pregnant women. No well-controlled studies have been done on the effects of GBCAs in pregnant women. One small study of 26 women exposed in the first trimester showed no evidence of fetal harm. However, the potential effects on the fetus are unclear, so GBCAs should be used with extreme caution in pregnancy. Each case should be carefully evaluated, weighing the potential risks and benefits to both the mother and fetus before administering a GBCA.

48. Lactation: Only tiny amounts of GBCAs given to a lactating mother reach the milk and only a minute proportion in absorbed from the baby’s gut . . Therefore the ACR recommends that they can be safely administered to a lactating mother. However, the EMA recommends discontinuation of breast feeding for at least 24 hours for all patients receiving high NSF risk GBCAs.

49. The ACR recommends calculating eGFR prior to GBCA administration for the following risk factor groups : Age>60 years. History of renal disease, including dialysis, renal transplant, history of renal surgery, history of renal cancer or a single kidney patient. Hypertension requiring medical therapy. Diabetes mellitus.

50. Manganese based contrast agents Manganese (Mn2+) is also a paramagnetic substance like gadolinium (Gd3+), shortening T1 and T2 relaxation times. However, manganese-based contrast agents (MBCAs) readily dissociate in the body, releasing free manganese ions. This raised concerns about potential toxicity, as free manganese can accumulate in the brain, causing Parkinson-like symptoms, and depress heart function.

51. The instability and toxicity of MBCAs have led to their withdrawal from the market as of 2021. Manganese-based nanoparticles are currently being researched as potentially safer contrast agents. Natural products high in manganese, like pineapple juice, are being used as negative oral contrast agents to null GI tract signals in procedures like MRCP.

52. Iron oxide-based contrast agents Iron oxide nanoparticles (magnetite or maghemite) are super-paramagnetic agents that shorten T2 and T2* relaxation times, creating decreased signal intensity on T2- and T2*-weighted MRI. After IV administration, the nanoparticles are removed from the blood by the reticuloendothelial system in the liver, spleen, and lymph nodes, which is useful for imaging metastatic disease. These iron-based agents have an extremely long intravascular half-life, making them an attractive choice for MR angiography (MRA).

53. Iron oxide nanoparticles are categorized into two groups based on their size distribution: Superparamagnetic iron oxide particles (SPIOs): These have aggregated iron oxide cores and a mean diameter greater than 50 nm. Ultrasmall superparamagnetic iron oxide particles (USPIOs): These have non-aggregated iron oxide cores with a mean diameter less than 50 nm. The commercially available MR contrast agent is Ferrotran (formerly Combidex , ferumoxtran-10), approved in the Netherlands for detecting metastatic disease in lymph nodes

54. Oral contrast agents Oral contrast agents are essential for delineating bowel pathology and distinguishing the bowel from other organs or mass lesions. The ideal oral contrast agent should have the following properties: Minimal absorption by the stomach or intestines Complete excretion No motion or susceptibility artifacts Uniform marking of the GI tract Good patient acceptance (taste, volume, timing) Safety Low cost

55. Oral contrast agents for MR imaging are categorized as: Low signal intensity on all pulse sequences (low-low intensity, proton displacement or T2 shortening) High signal intensity on all pulse sequences (high-high intensity, T1 shortening) Low signal intensity on T1 and high signal intensity on T2 (low-high intensity, water-based) High signal intensity on T1 and low signal intensity on T2 (high-low intensity, T1 and T2 shortening) The last two categories are known as biphasic agents.

56. Low-low intensity(dark lumen) The signal intensity in the bowel can be reduced by using materials without protons, such as air. However, air is not used as an oral contrast agent due to patient discomfort and artifacts in T2* weighted gradient echo images and echoplanar images. Perfluorocarbons: Contain no protons, similar susceptibility to water, and do not cause artifacts. Perflubron was the first approved oral MR contrast agent but was withdrawn due to high cost.

57. SPIOs (Superparamagnetic Iron Oxide): High concentrations can eliminate signal from the bowel, even on T1-weighted images. Can distort the local magnetic field and cause artifacts, affecting fat-suppression. Ferumoxsil (commercially GastroMARK or Lumirem ) is available but can cause side effects like nausea, vomiting, flatulence, and a feeling of fullness in 15% of patients. Clays such as kaolin, attapulgite and helonite restrict the motion of water, thus shortening the T1 and T2 relaxation times. Appear as high signal intensity in T1 weighted gradient echo images with a TE of less than 3 msec due to their short T1.

58. High-high or bright lumen Positive contrast agents reduce T1 relaxation time, increasing the signal intensity (bright lumen) on T1 weighted images. These agents include paramagnetic materials like gadolinium chelate solutions, ferrous, and manganese ions. Key points: Gadolinium Chelate : Among the first proposed oral MRI contrast agents. Dissolved in water (≈1 mmol/L) increases signal intensity on T1 weighted images without affecting T2. One of the first commercially available positive oral contrasts was gadopentetate dimeglumine solution ( Magnevist Enteral

59. Blueberry juice can be used as a positive agent due to its high content of manganese, a paramagnetic ion, which shortens the T1 relaxation time. Fat containing food materials like milk, vegetable oil and ice-cream have an intrinsically short T1, thus providing high signal on T1 weighted MRI..

60. High-low intensity Materials that shorten both T1 and T2 relaxation times, making bowel contents have High signal intensity on T1 weighted images , Low signal intensity on T2 weighted images. Advantages : Differentiates bowel signal intensity from tumors and fluid collections (tumors and fluid collections: low T1, high T2).

61. Limitations: Some T2 weighted sequences (e.g., fast spin echo) may depict bowel contents more clearly with high signal intensity, making this contrast not ideal for all applications. Examples: Clay,High concentration gadolinium chelates, Manganese compounds (found in bananas and blueberries)

62. Low-high intensity The least expensive, most widely available, and best understood oral contrast agent used for MRI is water which makes bowel as low signal intensity on T1 weighted and high signal intensity on T2 weighted images. It provides good small bowel distension, but its major limitation is intestinal absorption, which does not allow good distention of distal small bowel. For this reason, water is used in conjunction with different additives that reduce intestinal absorption, e.g. mannitol, sorbitrate, etc. Dilute suspensions of BaSO4 or dilute solutions of iodinated contrast material such as those used in opacification of bowel for CT are very useful as low-high MR oral contrast agents.

63. Polyethylene glycol (PEG) is a strong hydrophilic molecule, which simulates the properties of water with regard to signal intensity with the added advantage of no absorbability. Using 600 mL of PEG, a good distension of bowel loops can be obtained with no significant side effects. Low-high agents are also useful for delineating the pancreas; however, bowel may be difficult to distinguish from simple fluid collections and tumors; high bowel signal intensity can intensify motion artefacts. The choice of a single agent presents advantages and disadvantages; thus, the radiologist should choose the appropriate contrast agent according to clinical setting, MR experience, availability of agent and patient tolerance.

64. Ongoing research and future trends Current MRI contrast agents are useful for improving visibility of pathology, but have limitations for extended intravascular, tissue/organ-specific, and molecular imaging. Iron oxide agents are organ-specific, but give a signal-decreasing effect, producing lower contrast images compared to T1 agents.

65. Researchers are exploring new contrast agents to improve specificity and accuracy, approaching that of biopsy or histopathology: T1 nanoparticles, target-specific, tumor-specific, and thrombus-specific agents "Smart" agents like enzyme-activated and chemical exchange saturation transfer (CEST, PARACEST) agents Fluorinated agents Many of these advanced contrast agents are still in preclinical research stages, but successful in vivo imaging has been reported in various preclinical studies .

66. MR contrast media used in TUTH Gadopentate Meglumine Gadobutrol

67. SUMMARY At present gadolinium chelates are the most widely used contrast agents in MR imaging and the only MR contrast agents available in Nepal & India. They are easily available, have good tolerability and provide excellent lesion detection and characterization. The main concern with GBCAs is NSF, which is seen to occur only in patients with severe renal disease. Macrocyclic agents ( Dotarem , Prohance & Gadovist ) are safer to use in this regard as they are difficult to dechelate & cause less deposition in brain as compared to linear agents. The tissue-specific agents can be used for specific indications, depending on their availability.

68. References