Imaging Unzipped – Contrast

Noncontrast scans are best for:

• Bones (fractures- like cspine or tibial plateau), 
• stones, 
• masses or general outlines, 
• Strokes (ischemic or hemorrhagic)

Contrasted scans are best for: 

• showing inflammation & abscesses, 
• blood clots/ infarction/ dissection, 
• tumors/ bleeding/ fluid

 Types of Contrast

• Oral – must drink it, highlight inside of bowel. Thinks: bowel fistulas, mechanical obstructions like tumors, volvulus, etc. Takes 2-4 hrs to make it from stomach to the rectum. Not absorbed, so ok for CKD or ESRD pts.
• Rectal contrast, no wait time, but only for diagnoses of lower GI tract esp masses & fistulas, rectal is an option. Not done that commonly.
• Last IV is divided even further into 2.
  • Venous – this is the most common kind of contrast. Its what people think of when they hear the generic term “contrast” it highlights veins bright and then as heart circulates it, certain tissues show up. Remember anything that gets lotrs of blood flow is going to show up. Things like inflammation or abscess or tumors, these processes draw blood to them and IV contrast will help highlight them. 
    • Fun practical nursing facts–  Venous contrast is recommended to be in an IV 20 gauge or higher (in peds, it can be 22). Most radiology depts won’t allow contrast thru a hand IV since it can infiltrate. The hand is a very small compartment and any amount of pressure or fluid can cause necrosis (essentially its compartment syndrome)
  • Arterial –  the contrast is still put in the veins, and we wait until it makes it pas the heart. The contrast is administered fast!! (heart pumps fast) so preferred IV of 18 gauge or larger. And a larger load of contrast is typically used. Arterial contrast is important for blood flow or lack of blood flow. Most Scans done with  arterial contrast are called angiograms. This is how we make the diagnosis of a PE or dissection. We can also see mesenteric ischemia or severe peripheral vascular disease.

Kinds of Contrast

• Barium is the original contrast, was used rectal or oral only not IV. It is a heavy metal that is not absorbed but its also NOT water soluble and if gets outside of the GI tract, say due to perforation– big inflammatory reactions! This would also mean if someone is having a GI bleed should probably not give due to inflammation of open wound. SE of barium include severe abdominal cramping and either diarrhea or constipation. We still use this for barium enema or barium swallows but it has fallen out of favor. 
• Iodinated CT contrast is the most commonly used contrast and there are many brands Ex are (iohexol, gastrograffin). This highlights areas with blood flow, think things like inflammation, abscesses, or tumors. Biggest side effects is flushing, warmth and making pt feel like they peed themselves. This is a NORMAL reaction.  Headache, N/V are also considered normal side effects.
• IV MRI Contrast (gadolinium) which is an entirely different agent than CT contrast. It is also a heavy metal and works with your cells to essentially highlight the water in your tissues for the MRI. Most of the side effects are headache, dizziness or N/V. Allergies to this contrast are extremely rare. 

Contrast Allergy Myths

• Shellfish or seafood allergies. Shellfish allergy is a problem with a specific protein NOT iodine. We have known that shellfish and IV contrast allergies NOT related for AGES. So why is this myth still around? There is no crossover between shellfish allergy and IV contrast OR iodine.
• Iodine allergies. You can’t physically be allergic to iodine. Your thyroid and other body tissues require iodine for life. But you can have skin irritation or urticaria from iodine skin preps, due to preservatives and other chemicals in skin preps. If you’re still not convinced think about this: Bread and dairy are some of the largest dietary sources of iodine and all salt in the US is fortified with iodine. And I doubt anyone is having an allergic reaction to salt, bread and dairy. 

• IV contrast myths @ emdocs
• Shellfish & contrast & myths

Contrast Nephropathy

This is the reason why your radiology department has a cut off for who is allowed to get contrast. Functionally, most departments policies use GFR. Our cut off is GFR is 43 or lower. I have heard some department use 45 and some use 40. You will have to inquire about your own radiology department.

So what’s the big deal here? All forms of contrast are cleared through the kidney, and contrast has a chance to bump the kidney off and cause them to require dialysis. This is the very reason why so many people consider it standard of care to withhold metformin for 48 hours before a scan with contrast– they are worried that since metformin could cause lactic acidosis, it will push those kidneys over the edge. But you should know contrast nephropathy is controversial!

Medscape states the definition of contrast nephropathy is a 25% increase in Cr or a .5 increase in absolute Cr number appearing about 48 hours after getting IV contrast. Interestingly it has never been shown to be the cause of to start dialysis nor has ever lead to death in patients who were already headed down the road. And for most folks it is a transient process going back to normal after about 14 days.  However, There is very little evidence that this elevation in Cr actually does anything to the patient.

Honestly, I think it might be more a theoretic scare in most patients than a true worry. I am going to continue to follow the recommendations of radiology dept, but i’m interested to see if anything happens in the next decade to change all this. 

Other Links & Resources

• Different Contrast Phases
•  source on contrasted scans
• OB and imaging studies:  LINK
• Contrast nephropathy is a myth
• Guide on which scan to order with and without contrast