Tuesday, September 01, 2015

Which Picture Do I Want?

2 comments
If dispo is king in the ED, the plan is the prince. But imaging plans can be tricky. You want to look at someone's abdomen, sure. But CT or MRI? Or is this one of the rare instances plain films help? Do they need IV contrast? PO contrast?

By no means exhaustive, the guide below outlines common points of confusion with radiology.



X-ray

-AP vs PA Lateral: The "AP" or "PA" refers to the direction the beam is traveling to the receiver - either anterior to posterior (AP) or posterior to anterior (PA). Despite this being an x-ray this matters because is changes the observed size of images. Imagine your lamp is the x-ray beam and your desk is the receiver: putting your hand close to the desk results in a small hand shadow, moving it towards the lamp increases the size of the shadow. Why we care about this happening at all is the cardiac shadow on a chest x-ray. When the beam goes AP the heart, being more anterior, casts a bigger shadow that may overemphasize the heart size and suggest cardiomegaly when there actually is none. Why get an AP at all then? It's the portable version. So make sure when you're using your system you note which is which and don't comment on the cardiac silhouette if it's a portable film.

-Plain abdominal x-ray: quick test, but not useful for everything. Okay for perforated viscus (looking for abdominal free air and pneumoperitoneum), renal caliculi if they are large (>3 mm) and radiolucent, toxic megacolon, and bowel obstruction with high suspicion (looking for air-fluid levels - see film to the right). Great for foreign objects.

-Hand films: Remember scaphoid fractures may not show up until 2 weeks after injury. MRI would be diagnostic but you'll never get that in the ED, so follow-up is the plan if x-ray is non-diagnostic.

-Hip films: Cannot rule out a fracture with a plain film. CT is necessary if there is any suspicion and x-ray is negative.



CT

-Consider IV contrast carefully: you can only give contrast once every 24 hours, and the patient cannot get contrast if they have bad kidney disease (GFR of 30 or 40 in diabetics). Another consideration is that you will need reasonable access to give the contrast. So the patient must have an IV, and something around an 18 gauge if you are evaluating for PE or Aortic dissection (because of the speed at which contrast needs to flow into the patient).

-PO contrast takes 90-120 minutes to travel through most of the bowel. Use unless you're looking for kidney stones, a retroperitoneal bleed, or a CT cystogram.

-Always order CT abdomen AND pelvis. You will be very unhappy with the coverage if you just order abdomen.



MRI

-Contraindications - pacemaker, implanted metal (potentially), or shrapnel if near something important like a blood vessel or nerve.

-Use AFTER head CT (to rule out intracranial hemorrhage) in suspected stroke patients. This includes brain parenchymal imaging, MR angiography, and MR diffusion.

-There is also evidence the MRI can be a good modality to assess for occult fractures based not only on diagnosis, but also cost effective-ness.



Ultrasound
-Transvaginal ultrasound for pregnancy must be correlated with Beta-HcG - levels above 1,000 to 2,000 are needed to be able to visualize pregnancy.





Further Reading:

The University of Virginia School of Medicine's interactive radiology guide is fantastic and comprehensive. Definitely spend time looking through it.





References:

1. Schellinger PD, Jansen O, Fiebach JB, Hacke W, Sartor K. A standardized MRI stroke protocol: comparison with CT in hyperacute intracerebral hemorrhage. Stroke. 1999;30(4):765-8.
2. Available at: http://lifeinthefastlane.com/scaphoid-fractures-the-ed-perspective/. Accessed August 20, 2015.
3. Vinson EN. Images in clinical medicine. Occult hip fracture. N Engl J Med. 2008;359(26):e33.
4. Lubovsky O, Liebergall M, Mattan Y, Weil Y, Mosheiff R. Early diagnosis of occult hip fractures MRI versus CT scan. Injury. 2005;36(6):788-92.
5. Available at: http://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=178&seg_id=3758. Accessed August 20, 2015.
6. Available at: http://www.med-ed.virginia.edu/courses/rad/edus/index.html. Accessed August 20, 2015.

Read More...