How does the concentration of iodine in enhancing lesions relate to image quality in head CT?

A. M. Fischman, R. G. Dixon, W. Huda, K. M. Ogden, K. A. Lieberman, M. L. Roskopf

Research output: Contribution to journalConference articlepeer-review


Patients undergoing head CT examinations with iodinated contrast received 100 cc of Iohexol 240 injected intravenously by hand. We developed a software package to align non-contrast and contrast head CT images, and obtain the "difference image" consisting of the iodine enhancement within a given lesion. This "difference image" of the iodine enhancement was added to the non-contrast study at reduced intensities. The signal to noise ratio (SNR) for detecting the added iodine was taken to be directly proportional to the concentration of contrast in the lesion. The visibility of the lesion enhancement in this composite image was compared with the original contrast image using a six-point scale ranging from 5 (no observable difference) to 0 (unacceptable). Two radiologists evaluated head CT images of eleven metastatic lesions. The iodine concentration required to generate an image quality rank of 3, deemed to be satisfactory for diagnosis (S), was determined. We also performed a Receiver Operating Characteristic (ROC) study to identify the iodine concentration corresponding to an area under the ROC curve of 0.95, which corresponds to the detection threshold (D) for each lesion. Reducing the intensity to 50% resulted in an average image quality score S of 3, suggesting that it may be possible to reduce the administered iodine by a half in head CT examinations with no significant loss of diagnostic performance. The average iodine concentration at the detection threshold D was 16%. The average S:D ratio was 3.8 ± 1.7, and was similar for both readers. The value of S was independent of enhancement characteristics, whereas the detection threshold D correlated inversely with the size and intensity of the iodine enhancement. The resultant S: D ratio correlated with the lesion area (r 2 = 0.31), mean lesion intensity (r 2 = 0.44), and the product of the mean lesion intensity and the lesion area (r 2 = 0.37). Our results indicate that the SNR of enhancing lesions in head CT that is needed to satisfy radiologists is about a factor of four greater than the SNR required for iodine detection.

Original languageEnglish
Article number47
Pages (from-to)360-371
Number of pages12
JournalProgress in Biomedical Optics and Imaging - Proceedings of SPIE
Issue number26
StatePublished - 2004
Externally publishedYes
EventMedical Imaging 2004 - Image Perception, Observer Performance, and Technology Assessment - San Diego, CA, United States
Duration: 17 Feb 200419 Feb 2004


  • CT
  • Contrast agents
  • Detection threshold
  • Image quality
  • Neuroradiology
  • Receiver operating characteristics
  • Subtraction imaging


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