What crayons look like on CT and MR images

By Staff News Brief

Some young children find crayons irresistible to chew on and, with concomitant trauma, crayons may become penetrating foreign bodies. Do radiologists know what they look like on imaging if ingested? A recently published article in Pediatric Radiology describes the appearance of crayons on CT and MR images.

(A) MRI images of a yellow Crayola crayon. From left to right: T2, T1, and susceptibility weighted images. (B) CT images of green crayons. From right to left: Cray-Z-Art, Crayola, C3. Note the central air tract, best visualized in the Crayola crayon on the displayed soft tissue window. (C) Broken crayons demonstrating central air tract (arrow) corresponding to airtract noted on CT.

Researchers from Cincinnati Children’s Hospital investigated the appearance of 22 crayons of different colors from three manufacturers. The radiologists characterized the imaging appearance of crayons after unexpectedly discovering that a crayon fragment was the source of a foreign body inflammatory process in the masticator space of a 15-month old child.

Lead author Aaron S. McAllister, MD, wrote that the 3-cm crayon fragment had three characteristics that surprised the interpreting radiologist. On CT, it contained an unexpected central linear tract of hypo-attenuation and was higher in attenuation than expected for a wax object. On MRI, a signal void was seen on both T1 and T2 images, with blooming on susceptibility weighted images.

The 22 commercial crayons, as well as three additional hand-cost non-pigmented paraffin crayons with and without wrappers were suspended in a water bath and imaged. All had a central longitudinal hypo-attenuating tract with attenuation values consistent with gas, but the “gas tract” varied in prominence by brand.

Crayon attenuation on CT varied by brand and color, based on the different formulations used in their manufacturers to make them.The authors explained that fillers and pigments are known to affect imaging characteristics. Pigments add color to a crayon and fillers enable crayons to lay down an even layer of wax, write smoothly, and last longer.

C3® brand crayons had the highest attenuation on CT images, ranging from 491-538 HU, that Crayola® crayons had attenuation ranging from 118-396 HU, and that Cray-Z-Art® crayons had the lowest, with a range from -114 to-16 HU.

Differing characteristics were also seen on MR images. MRI Signal voids were demonstrated on both T1- and T2-weighted images. A T-1 hyperintense rim was identified on crayons with paper wrappers. The different colors and different brands of pigmented crayons demonstrated a large variability in the size and heterogeneity of the susceptibility effect. The diameter of the signal void on susceptibility-weighted images ranged from 33 mm for Crayola brown to 9 mm for Cray-Z-Art purple. 

“Familiarity with the appearance of crayons on CT and MRI is of value both in recognizing a foreign body and in correctly identifying it as a crayon,” they said.

REFERENCE

  1. McAllister AS, Lall NU, Tawfik KO, et al. The imaging appearance of crayons. Pediatr Radiol. Published online March 7, 2017.
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What crayons look like on CT and MR images.  Appl Radiol. 

By Staff News Brief| June 23, 2017
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