Porphyrins occur naturally in plants and animals. All porphyrin molecules feature an aromatic macrocycle ring with a central binding site. This site accommodates transition metals, which are held in place by inward-facing nitrogen atoms. Metalloporphyrins have usually a low toxicity and a potential of a selective uptake in tumors or necrosis. These properties are advantageous for a use as MRI tumor specific agents with positive enhancement. These contrast agents enhance tumors on T1 weighted sequences, which are isointense to surrounding tissues. Porphyrin-based compounds have also necrosis avid properties; they can depict the extent of myocardial infarction as defined by histopathology.
Metalloporphyrins are also used in photodynamic therapy of tumors. The compounds contain a 'lone star' metal atom at the center of the ring and are 'bigger than the average porphyrin'. They contain five N atoms in the central chelating core and this allows them to form complexes with large trivalent lanthanide metals, which have useful cancer therapy properties.

See also Classifications, Characteristics, etc., Gadophrin, MnIIITPPS4, Necrosis Avid Contrast Agent.

Short name: MION, central moiety: Fe3+/Fe2+, relaxivity: r1=3.7, r2=6.5, B0=0.47T
MIONs are under development (preclin.) as contrast agents (taken up by the RES) for MR angiography, MR lymphography, tumor detection and detection of infarctioned tissue. Also called monodisperse iron oxide nanoparticles or nanocompounds.

MultiHance® is a paramagnetic contrast agent for use in diagnostic magnetic resonance imaging (MRI) of the liver and central nervous system. MultiHance® is a small molecular weight chelate, which tightly binds the Gd atom. The substance is excreted partly by the kidneys, partly by the biliary system, which is especially unique.
MultiHance® is indicated, for the detection of focal liver lesions in patients with known or suspected primary liver cancer (e.g. hepatocellular carcinoma) or metastatic disease.
MultiHance® is also indicated in brain MRI and spine MRI where it improves the detection of lesions and provides diagnostic information additional to that obtained with unenhanced MRI.
Gd-BOPTA-enhanced MRA can provide superior vascular signal intensity and SNR, as compared with Gd-DTPA, due to its higher relaxivity, even at lower doses.
1 ml of solution MultiHance® contains: (0.5M) gadobenate dimeglumine 529 mg = gadobenic acid 334 mg + meglumine 195 mg. Viscosity at 37°C: 5.3 mPa

WARNING: NEPHROGENIC SYSTEMIC FIBROSIS Gadolinium-based contrast agents increase the risk for nephrogenic systemic fibrosis (NSF) in patients with acute or chronic severe renal insufficiency (glomerular filtration rate less than 30 mL/min/1.73m²), or acute renal insufficiency of any severity due to the hepato-renal syndrome or in the perioperative liver transplantation period.

(NACA) Some porphyrin (e.g. Gadophrin-2) and non-porphyrin (e.g. CEIII-60) paramagnetic chelates are able to specifically accumulate in nonviable tissues and can be applied as a MRI contrast agent for acute myocardial infarction and therapeutic necrosis. This function of necrosis avid contrast agents is a unique pathological targetability. These agents can also be exploited for multipurpose applications, because NACAs also bear other common properties including early systemic distribution, albumin binding, hepatocellular uptake and renal elimination.

See also the related poll result: 'The development of contrast agents in MRI is'

Nitroxide radicals (or nitroxyl spin labels) are stable organic compounds with theoretical potential for use as a paramagnetic MRI contrast agent. Similar to gadolinium they have an unpaired electron, a property that provides enhancement in T1 based MRI, and a comparable pharmacokinetic. Depending on their structure and chemical bonding, different nitroxides formula may have the potential for use as cardiovascular imaging agents, to enhance the MR imaging on joints (e.g., dendrimer-linked nitroxides have a strong affinity for cartilage), to evaluate brain tumors and infarction, and as a contrast enhancement agent of body/abdominal NMR imaging. Nitroxides are rapidly enzymatically reduced in tissues to products that do not enhance the NMR signal, which can be a problem for MR imaging. In animal experiments with EPRI (electron paramagnetic resonance imaging), tissue redox studies show differences between tumors and normal tissues, which reflect their respective redox status consistent with the reduction/clearance of nitroxides.