By: Assoc. Prof Dr. Mohammed Ahmed Ali

MR Contrast Media MRI contrast agents contain paramagnetic or super paramagnetic metal ions which affect the MR signal properties of the surrounding tissues. They are used to enhance contrast, to characterize lesions and to evaluate perfusion and flow-related abnormalities. They can also provide functional and morphological information.

Paramagnetic Contrast Agents Paramagnetic contrast agents are mainly positive enhancers which reduce T1 and T2 relaxation times and increase tissue signal intensity on T1-weighted MR images.

The most widely used paramagnetic contrast agents are non-specific extracellular gadolinium chelates. Their active constituent is gadolinium, a paramagnetic metal in the lanthanide series, which is characterized by a high magnetic moment and a relatively slow electronic relaxation time.

Non-specific extracellular gadolinium chelates: can be classified by their chemical structure to: macrocyclic or Linear, and by whether they are ionic or nonionic form (Fig. 2) and they are excreted via the kidneys.

Paramagnetic contrast agents also include liver specific gadolinium based agents (gadobenate dimeglumine, Gd-BOPTA and gadoxetate, Gd-EOBDTPA) and manganese-based preparations [manganese chelate (mangafodipir trisodium)

These hepatobiliary contrast agents are taken up by hepatocytes and then there is variable biliary excretion. The gadolinium based liver specific contrast media are also excreted by the kidneys.

Advantage and Disadvantages of Contrast Media Both types of I.V contrasts i.e. ionic and non ionic; contain iodine and most of these agents are derivatives of tri-iodobenzoic acid. The idea of using iodine dyes is the simple fact that iodine molecule absorbs x-rays effectively in the energy range at which most clinical imaging systems operate.

There are many types of contrast dyes available in the market like the "Optison - is an ultrasound contrast agent used with an echocardiogram. The "omnipaque" used in the angiocardiography, CT enhanced studies of the head and neck, as well as many other uses. There is also the non- ionic "Vasipaque" which is used in peripheral arteriography, cerebral arteriography, as well as the angiocardiographic studies.

The complications of contrast dyes include both allergic reactions to the dye material, and the common serious complication as nephropathy, which is a variant of acute renal failure (due to the high-osmolality contrast agents).

The allergic, due to direct effect of the iodinated dye material that acts directly on mast cells releasing histamine and other biochemical mediators causing the clinical picture of an allergic reaction, and that is why the possibility of development of allergic reaction and its severity is directly proportional to the concentration of iodine in the specific dye used, as well as on the dose of the intravenous dye that is injected.

The allergic reactions range from mild allergic symptoms to severe life threatening anaphylaxis, and affects about 5-8 % of patients exposed to radiocontrastagents. The mild symptoms of allergic reactions include a feeling of warmth, nausea, and vomiting that develops shortly after injecting the dye, and requires no treatment other than observing the patient

The moderate reaction as (severe vomiting, hives and swelling) requires Treatment & close observation: The treatment involves I.V. anti-emetic agents as well as the H1 antagonist agent Diphenhydramine in a dose of 50 mgs I.V. or I.M., and sometimes a short acting steroid (hydrocortisone) in a dose of 100 mgs I.V may be used. The severe reaction include the life threatening anaphylactic shock, presenting as severe bronchospasm with cyanosis and a circulatory shock, and sometimes a cardiovascular arrest.

The risk for developing severe reaction appear to be higher in patients with conditions like food allergies, iodine skin allergy, bronchial asthma, kidney and thyroid diseases, as well as patients on metformin or beta blocker therapy, so special attention should be given to this patient population prior to applying a radiocontrast dye.

In case of severe shock: immediate infusion of intravenous fluids to support the circulation, I.M injection of 0.5 mgs of epinephrine, to be repeated every 2-5 minutes; if there is no response (I.V injection would be more appropriate in a setting of severe shock with compromised perfusion for a more optimum effect ), 200 mgs I.V injection of hydrocortisone, 50 mgs of diphenhydramine as well as respiratory support with oxygen therapy

Case of nephropathy: or impairment of the renal functions: It measured as a 25% increase in serum creatinine level, or 0.5 mgs absolute increase in serum creatinine level within hours of administration of radiocontrast dye. Contrast-induced nephropathy is associated with a higher morbidity and mortality

The iodine compounds are toxic to the kidneys, but an important etiological factor is the hyperosmolarity of the dye material injected, and that is why the development of this complication is more common with the ionic hyperosmolar dye preparations