Contrast Media & Their Applications

RAD 361By:

Assoc. Prof Dr. Mohammed Ahmed Ali

MR Contrast MediaMRI 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 AgentsParamagnetic 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

MediaBoth 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  48-72  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

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