Mar 27

Contrast in CT and MR Imaging: Applications and Contraindications.

By taqidoc Radiology Add comments

WHAT IS CONTRAST AGENT?

A substance placed in the body to increase image differentiation of anatomical structures.
It increases the differentiation between the areas containing contrast media and the areas not containing contrast media.

WHY NEED TO USE CONTRAST AGENT?

1.To better differentiate between anatomical structures.
2.To increase detection of pathology.

CT CONTRAST AGENTS

POSITIVE CONTRAST AGENT:
Iodinated Contrast Media

IODINATED CONTRAST MEDIA – Ionic

High Osmolar Contrast Media (HOCM)
-Composed of salts which dissociate in water into anions (radiopaque) and cations (osmotically active).
-Osmolality up to 5x serum ? toxicity.
-Non-intravascular route – oral / rectal.
-Not for use in subarachnoid space.
E.g. Gastrografin, Urografin, Isteropac

IODINATED CONTRAST MEDIA – Non-ionic

Low Osmolar Contrast Media (LOCM)
-Non-dissociating.
-Only about 2x serum osmolality.
-Less side effects, less nephrotoxic.
E.g. Ultravist, Omnipaque
Iso-osmolar Contrast Media
-Osmolality approximately serum osmolality.
-Preferred in renal impaired patients.
E.g. Visipaque
Oily Contrast Media
-Currently not widely used
-Used for – chemoembolisation
E.g. Lipiodol Ultra Fluid

METHODS OF ADMINISTRATION:

A)Intravenous injection
1.     Highlight blood vessels.

2.    Enhance structure of organs.

3.    Rapidly eliminated by kidneys.

B) Oral administration

C)Rectal administration

For Oral and rectal–>

1.  Highlight gastrointestinal tract in the abdomen and pelvis.

2.  Positive contrast – iodinated CM(Gastrografin)

3.  Negative contrast
– water
– air (CT colonoscopy)

D)Intrathecal injection

E)Intra-articular injection

CONTRAINDICATIONS

Recognizing patients at risk of contrast media reaction

1.Proven/suspected hypersensitivity to iodine

2.Previous severe reaction to contrast media

3.Asthma/significant allergy history

4.Heart disease

5.Infants/children/elderly

6.Liver failure

7.Renal impairment (moderate-severe) – NIDDM on Metformin

8.Myelomatosis

9.Poor hydration

10.Sickle cell anaemia

11.Thyrotoxicosis

12.Pregnancy

13.Phaeochromocytoma

IN THESE GROUPS OF PATIENTS, LOCM SHOULD ALWAYS BE THE CHOICE OVER HOCM!!

Overall incidence of contrast media reactions

HOCM: 5-12%

LOCM: 1-3%

Mortality rates of contrast media reactions

HOCM: approx 1:40,000

LOCM: approx 1: 200,000

Types of Contrast Media Reactions

1. Anaphylactoid (idiosyncratic)

Urticaria, facial & laryngeal oedema, bronchspasm, hypotension – life threatening

2. Non-idiosyncratic

Direct effect on organs (nephrotoxicity, vasovagal attacks, arrythmias, MI)

3. Local reaction

Extravasation, phlebitis

Severity of Contrast Media Reactions

Mild: Hives, flushed feeling, metallic taste, nausea, vomiting

Moderate: Bronchospasm, laryngospasm, hypotension

Severe: Life threatening anaphylaxis

!! REACTION MAY BE DELAYED UP TO 1 WEEK !!

PATIENT PREPARATION

  1. Fasting 4-6 hrs
  2. Steroid cover  – allergy, asthma
  • T. prednisolone 50 mg 13, 7, 1 hr before examination
  • IV hydrocortisone 200mg stat

3. Renal impairment

  • N-acetyl cysteine 600mg the day before and 1 day after examination

MR CONTRAST AGENTS

Paramagnetic / Superparamagnetic
-Causes changes in the local magnetic field
–>shortened T1 relaxation time – increases signal intensity on T1-weighted images
–>shortened T2 relaxation time – decreases signal intensity on T2-weighted images
e.g.Gadolinium-DTPA (Magnevist)


Newer MRI contrast media
1.Liver-specific MRI agents
-Accumulates in the liver
e.g. Resovist, Primovist, MultiHance
2.USPIO (ultrasmall paramagnetic iron peroxide particles)
-Accumulate in macrophages in lymph nodes
-Currently undergoing clinical trials, no FDA approval as yet

COMPLICATIONS

1.Hypersensitivity reaction
2.Nephrogenic systemic fibrosis
-Recently reported serious late adverse reaction of gadolinium-based contrast.
-Seen in patients with renal failure and on dialysis.
-Fibrosis of skin and organs.
-Clinical symptoms developed 2-8 weeks after exposure to gadodiamide.
-Avoid in patient with creatinine clearance <15mL/min.

Source :
Dept of Biomedical Imaging UMMC

CT and MRI for non-radiologist Course

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