Color intensity projections provides a fast, simple and robust method of summarizing the grayscale images from a renal scan in a single color image

Abstract

To assess its usefulness, the peak version of color intensity projections (CIPs) was used to display a summary of the grayscale images composing a renogram as a single color image. Method For each pixel in a renogram, the time point with the maximum intensity was used to control the hue of the color of the corresponding pixel in the CIPs image. The hue ranged over red-yellow-green-light blue-blue with red representing the earliest time. Results For subjects with normal appearing kidneys, the injection site shows up in red, the kidneys in a red-yellow and the bladder in a green-blue. A late fill kidney typically appeared greener or bluer than a normal kidney indicating it reached its peak intensity at a later time point than normal. Conclusions Having the time and intensity information summarized in a single image promises to speed up the initial impression of patients by less experienced interpreters and should also provide a valuable training tool.

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Color intensity projections of renograms 1 of 4
Color intensity projections provides a fast, simple and robust method of summarizing the
grayscale images from a renal scan in a single color image
Keith S Cover1, Sandra A. Srbljin2, Arthur van Lingen2
1Department of Physics and Medical Technology, 2Department of Nuclear Medicine and PET Research,
VU University Medical Center, Amsterdam,
Corresponding author:
Keith S Cover, PhD
Department of Physics and Medical Technology
VU University Medical Center
Postbus 7057
1007 MB Amsterdam
The Netherlands
Email: Keith@kscover.ca
Tel: 31 20 444-0677
Fax: 31 20 444-4816
Abstract
To assess its usefulness, the peak version of color intensity projections (CIPs) was used to
display a summary of the grayscale images composing a renogram as a single color image.
Method For each pixel in a renogram, the time point with the maximum intensity was used to
control the hue of the color of the corresponding pixel in the CIPs image. The hue ranged over
red-yellow-green-light blue-blue with red representing the earliest time. Results For subjects
with normal appearing kidneys, the injection site shows up in red, the kidneys in a red-yellow
and the bladder in a green-blue. A late fill kidney typically appeared greener or bluer than a
normal kidney indicating it reached its peak intensity at a later time point than normal.
Conclusions Having the time and intensity information summarized in a single image promises
to speed up the initial impression of patients by less experienced interpreters and should also
provide a valuable training tool.
Keywords color intensity projections, renograms, nuclear medicine
INTRODUCTION
Radionuclide imaging has been used for many decades to measure the uptake and excretion of
the kidneys (TaplinGV1956, MettlerFA2006). Images are acquired over a 20 to 30 minute period
using a gamma camera after the injection of a radionuclide. Each image is the accumulation of
activity over a 1 second to 15 second period with shorter windows at earlier times.
The basic display and analysis of the renograms has changed little over the years although
modern computers has made the processing much quicker, simpler and more reliable. Typically,
a series of the images is reviewed repeatedly. In addition, ROI’s are often drawn around each of
the kidneys, and an average time curve for each of the kidneys plotted and examined in detail.
The ROI’s may be drawn on a single image or on an average of a few adjacent images.

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Color intensity projections of renograms 2 of 4
Color intensity projections (CIPs) are a simple technique for summarizing a series of grayscale
images in a single color image (CoverKS2006, CoverKS2007A, CoverKS2007B). It has been
applied to a wide variety of applications including 4DCT, angiography, perfusion MRI and
astronomy.
If all the grayscale images are identical then the resulting CIPs will also be a grayscale image
that is identical to the component grayscale images. However, if on a pixel by pixel basis there
are changes over the images, the amount and timing of the changes will introduce specific
colors into corresponding pixels the CIPs image. Depending on the complexity of the motion, a
CIPs can often provide most, if not all, of the important information that would be gained from
examining the individual grayscale images.
MATERIALS AND METHODS
A total of 80 images were acquired from each of four subjects after the injection of an 99mTc-
labeled agent. Each image was accumulated over a 15 second interval yielding a total of 20
minutes of images for each subject.
The peak version of the CIPs was found to be the most useful for renograms (CoverKS2007B).
For each pixel in a renogram, the time point with the maximum intensity was found. The
brightness and time point of this maximum intensity value was used to control the color of the
corresponding pixel in the CIPs image. The brightness of the pixel was set to be the brightness
of the maximum value. The timing of the maximum intensity value was used to select the hue
of the pixel in the CIPs. The hue values used ranged over red-yellow-green-light blue-blue with
the earliest times corresponding to red and the latest to blue.
Renograms of two subjects were used to demonstrate the application of CIPs. Both subjects
were scanned because they had shown indications of renal dysfunction. The first subject was a
12 year old female while the second was a 3 year old male.
RESULTS
Figure 1 shows the renogram of the first subject displayed as a CIPs. The injection site is clearly
visible in the right arm. The counts from the arm appear in the first image acquired. Thus all the
pixels are the same hue of red indicating very early time. However, as expected, the pixels do
vary in intensity. The isotopes rapid movement into the lungs is also clearly evidenced by the
red pixels in the lungs.
The display of the two kidneys in the CIPs are particularly informative. The first impression is
the two kidneys are largely symmetric with each other, an indication of normal kidney function.
More careful examination of the two kidneys shows redder pixels in the lateral regions of the
kidneys and some yellow and light green pixels in the medial regions indicating the isotope
reached the lateral regions 4 to 8 minutes earlier than the medial regions. This is consistent with
the normal passage of isotope through the kidneys.
Figure 1 also shows the renograms curves for ROI’s of the left kidney and of the bladder. The
curve for the left kidney shows a peak intensity at 3 to 4 minutes, again in line with a normal
kidney. The renogram curve for the bladder is included to demonstrate a late peaking curve. The
green-light blue-blue pixels of the bladder indicate a curve that plateaued between 10 and 20
minutes. Examination of the bladder curve shows this is the case.

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Color intensity projections of renograms 3 of 4

Figure 2 shows the CIPs of the renogram for the second subject. It also includes curves for the
right kidney and the lower region of the left kidney. Examination of the right kidney in the CIPs
shows isotope peaking at 2 to 3 minutes, consistent with a healthy kidney. Again most of the
lateral pixels are slightly redder than the medial pixels, indicating the expected motion of the
isotope through the kidney.
The upper region of the left kidney shows a similar pattern to the right kidney. However, the
lower region of the left kidney is primarily green, indicating a peak of intensity at about 10
minutes, abnormally late. The late peak is suggestive of a blockage.
CONCLUSIONS
The presentation of a renogram as CIPs provides a simple and intuitive way to summarize a
renogram in a single color image. Presenting a renogram as a CIPs should be helpful in
interpreting renograms for those with less experience especially for training purposes. The CIPs
may also be useful diagnostically but more studies are warranted, especially into the influence
of patient movement. While color intensity projections has only been applied to renograms in
this publication, it well may be a useful display technique for a range of other nuclear medicine
modalities.
ACKNOWLEDGEMENTS
Thanks to Prof. Otto S. Hoekstra assistance with this study. One of the authors (KSC) was
funded by the Department of Medical Technology and Physics, VU University Medical Center,
Amsterdam. The VU University Medical Center, where all the authors work, is pursuing a
patent on color intensity projections and one of the authors is listed as an inventor (KSC).
REFERENCES
CoverKS2007A Cover KS, Lagerwaard FJ, van den Ber R, et al. Color intensity projection of digitally subtracted
angiography for the visualization of brain arteriovenous malformations. Neurosurgery. 2007;60:511-514.
CoverKS2007B Cover KS, Lagerwaard FJ, Senan S. Color intensity projections: A simple way to display changes
in astronomical images. Pub Astron Soc Pac. 2007;119:523-526.
CoverKS2006 Cover KS, Lagerwaard FJ, Senan S. Color intensity projections: A rapid approach for evaluating
four-dimensional CT scans in treatment planning. Intern J Rad Oncol Bio Phys. 2006;64:954-961.
MettlerFA2006 Mettler FA, Guiberteau MJ. Essentials of nuclear medicine imaging 5th ed. Philadelphia, USA:
Saunders Elsevier; 2006.
TaplinGV1956 Taplin GV, Meredith GV, Kade H, et al. The radioisotope renogram – an external test for
individual kidney function and upper urinary tract patency. J Lab Clin Med. 1956;48:886-901.
Figure Captions
FIGURE 1. Renogram displayed as a CIPs for the first subject. The count curves for the left kidney (L) and the
bladder (B) are displayed below the CIPs.
FIGURE 2. Renogram displayed as a CIPs for the second subject. The count curves for the right kidney (R) and
the lower region of the left kidney (LL) are also displayed.

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Color intensity projections of renograms 4 of 4
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