DOI:10.2214/AJR.07.2175
AJR 2007; 189:W96-W99
© American Roentgen Ray Society
Anterior Layering of Excreted 18F-FDG in the Bladder on PET/CT: Frequency and Cause
Derk D. Purcell1,
Fergus V. Coakley,
Benjamin L. Franc,
Randall A. Hawkins,
Sophie E. Boddington and
Benjamin M. Yeh
1 All authors: Department of Radiology, University of California, San Francisco,
Box 0628, M-392, 505 Parnassus Ave., San Francisco, CA 94143-0628.
Received December 5, 2006;
accepted after revision March 28, 2007.
Address correspondence to B. M. Yeh.
WEB This is a Web exclusive article.
Abstract
OBJECTIVE. The objective of our study was to determine the frequency
and cause of anterior layering of excreted 18F-FDG in the bladder
on PET/CT.
CONCLUSION. Anterior layering of excreted FDG in the bladder is
commonly seen on PET/CT scans obtained with IV iodinated contrast material and
is due to displacement of FDG by excreted iodinated contrast material; this
phenomenon may unmask FDG-avid bladder disease.
Keywords: bladder cancer • bladder neoplasm • contrast media • FDG PET • image artifacts • oncologic imaging • PET/CT
Introduction
Combined PET/CT with 18F-FDG is increasingly used for the
diagnosis, staging, and surveillance of malignancy. The role of PET/CT in
urothelial malignancies is limited in part by the normal concentration and
accumulation of FDG in the renal collecting system that may mask
hypermetabolic lesions within and adjacent to the ureters and urinary bladder
[1-3].
Different protocols have been proposed to minimize bladder activity from
excreted FDG, including instructing the patient to void immediately before
imaging, scanning caudad to craniad, and performing bladder lavage
[4]. Conversely, excreted
iodinated contrast material may appear in the bladder as ureteral jets or as a
dependent high-attenuation layer, and the utility of IV contrast material for
whole-body PET/CT and the potential artifacts related to its use have been
reported [5,
6].
To our knowledge, the potential for any imaging interaction between
excreted FDG and excreted iodinated contrast material in the bladder has not
been described. At our institution, IV iodinated contrast material is
routinely administered for the CT component of the examination because we
believe that contrast-enhanced PET/CT provides diagnostic morphologic and
functional data in a single study, rendering additional diagnostic CT
examinations unnecessary. In our experience, attenuation-correction errors
related to IV contrast material are not clinically significant. In addition,
we have anecdotally noted anterior layering of FDG in the urinary bladder on
PET/CT examinations (Fig. 1).
We therefore undertook this study to determine the frequency and cause of
anterior layering of excreted FDG in the bladder on PET/CT performed with IV
iodinated contrast material and to investigate any potential effect on the
diagnosis of urinary tract disease.
Materials and Methods
Subjects
This retrospective single-institution study was approved by the Committee
on Human Research at the University of California, San Francisco, and was
compliant with the Health Insurance Portability and Accountability Act.
Informed consent was waived. We performed a computerized search of our
radiology information system (IDXrad software, version 9.7.1, IDX Systems) to
identify the first 100 consecutive patients who had undergone PET/CT at our
institution during March 2006. Each patient underwent one study, for a total
of 100 PET/CT examinations.
The study population consisted of 58 women and 42 men with a mean age of 55
years (range, 18-84 years). The indications for PET/CT were staging or
surveillance of breast cancer (n = 29), lymphoma (n = 15),
lung cancer (n = 14), melanoma (n =12), head and neck cancer
(n = 8), ovarian cancer (n =7), colorectal cancer
(n = 6), sarcoma (n = 2), pheochromocytoma (n = 3),
esophageal cancer (n =2), bladder cancer (n = 1), and nerve
sheath tumor (n =1).
PET/CT Technique
All studies were performed on a PET/CT scanner (Biograph 16, Siemens
Medical Solutions) that is a hybrid PET and 16-MDCT unit. CT was performed
first at 152 mAs, 120 kV, and 0.75-mm collimation. Images were obtained from
the top of the patient's head to the feet and were reconstructed as contiguous
5-mm slices. In 86 patients, 150 mL of IV iohexol (Omnipaque 350, Nycomed
Amersham) was administered 70-80 seconds before CT. The contrast material was
injected at a rate of 3 mL/s and followed by a 50-mL saline flush. PET was
performed immediately after CT without repositioning the patient, and imaging
commenced 60 ± 15 minutes (mean ± SD) after the injection of
12.5 ± 2.5 mCi (462.5 ± 92.5 MBq) of FDG. PET images were
obtained at 7-10 stations per patient, with an acquisition time of 4 minutes
per station, from the top of the patient's head to the feet. In other words,
the PET images of the pelvis were obtained approximately 25-40 minutes after
the injection of IV CT contrast material. CT data were used for attenuation
correction, and results were reconstructed into sagittal, coronal, and axial
planes. In the other 14 patients, no iodinated contrast material was given
because of renal insufficiency (n = 11) or suspected pheochromocytoma
(n = 3). Patients were instructed to remain still throughout the
entire examination.
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Fig. 2A —75-year-old man with melanoma imaged without IV iodinated contrast
material. CT (A) and PET (B) images of urinary bladder show
homogeneous appearance of bladder contents without anterior layering of
18F-FDG.
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Fig. 2B —75-year-old man with melanoma imaged without IV iodinated contrast
material. CT (A) and PET (B) images of urinary bladder show
homogeneous appearance of bladder contents without anterior layering of
18F-FDG.
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Image Interpretation
A single observer who was unaware of whether IV contrast material had been
administered reviewed only the PET images on a dedicated image-processing
workstation (Leonardo, Siemens Medical Solutions) and recorded the presence or
absence of anterior layering of excreted FDG in the bladder. All studies were
viewed with and without attenuation correction. Anterior layering of FDG in
the urinary bladder was considered to be present when a fluid-fluid level was
seen in the expected position of the urinary bladder with relative photopenia
in the posterior bladder. Similar assessment was made for the renal pelvises.
Any abnormal focal uptake in the urinary tract was also noted. Each patient's
medical records were reviewed for the presence or absence of bladder
abnormalities, and these findings were later correlated with the PET
findings.
PET/CT Phantom
To further elucidate the cause of anterior layering of excreted FDG in the
bladder, we performed in vitro PET/CT of specially constructed phantoms
designed to mimic the content of the bladder during a PET/CT examination. The
first phantom consisted of a 1-L radiolucent bottle containing an agitated
mixture of 400 mL of diluted iodinated contrast material and 200 mL of diluted
FDG. The diluted solution of FDG consisted of 250-µCi (9,250 kBq) FDG in
200 mL of isotonic saline. The diluted solution of iodinated contrast material
consisted of 360 mL of isotonic saline mixed with 40 mL of iohexol (Omnipaque
350).
For the second phantom, a separate 1,000-mL bottle was filled with 100 mL
of 125-µCi (4,625 kBq) FDG diluted in saline and then 100 mL of the same
diluted iodinated contrast material was manually injected, using a
catheter-tip syringe, into the dependent portion of the bottle over a period
of 1 minute. We chose to inject the iodinated contrast material into the
dependent portion of the bottle because at our institution CT contrast
material is given after FDG and because the ureters insert into the posterior
(dependent) portion of the bladder. PET/CT images of both phantoms were
obtained at 0, 5, and 10 minutes after agitation using the same imaging
parameters as described for our clinical in vivo studies.
Results
In Vivo Studies
Anterior layering of excreted FDG in the bladder was seen on 61 of 86
studies (71%) performed with IV iodinated contrast material but was not seen
on any of the 14 studies performed without IV contrast material (Fig.
2A,
2B). This difference was highly
significant (p < 0.001). Attenuation correction did not affect the
appearance of anterior layering of FDG in the urinary bladder in any patient.
In one patient, FDG uptake in a malignant posteriorly located bladder mass was
unmasked by the anterior layering phenomenon (Fig.
3A,
3B,
3C). This mass was later
biopsied at cystoscopy, with a final pathologic diagnosis of intestinal-type
adenocarcinoma.
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Fig. 3A —72-year-old man with colon cancer and new bladder metastasis. IV and
oral contrast-enhanced CT image obtained through pelvis shows enhancing mass
(arrow) arising from left posterior bladder wall.
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Fig. 3B —72-year-old man with colon cancer and new bladder metastasis. PET
image obtained at same level as A shows anterior layering of excreted
18F-FDG (arrow), thereby revealing FDG-avid mass in
posterior bladder.
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Fig. 3C —72-year-old man with colon cancer and new bladder metastasis. Fused
PET/CT image. Note that some misregistration (arrow) occurred due to
gradual filling of bladder with urine between time of CT and time of PET image
acquisitions.
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In Vitro Phantom Study
Imaging of the agitated phantom containing diluted FDG and iodinated
contrast material showed the gradual development of a separation gradient,
with the FDG activity becoming gradually localized to the nondependent portion
of the phantom (Fig. 4A,
4B). Images of the nonagitated
phantom revealed a sharp fluid-fluid level with no evidence of mixing on
delayed imaging (Fig. 5).
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Fig. 4A —Agitated PET/CT phantom. Volume-rendered CT reconstruction of bottle
containing 18F-FDG solution and dilute iodinated contrast material
that was shaken 5 minutes before imaging. Note gradual CT attenuation gradient
with increased CT attenuation fluid in dependent portion of bottle.
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Fig. 5 —Nonagitated PET/CT phantom. Volume-rendered reconstruction of
fused PET/CT images shows well-defined fluid-fluid level (arrow)
after slow, dependent injection of iodinated contrast material, which
anteriorly displaced 18F-FDG solution.
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Discussion
The results of the in vivo arm of our study indicate that anterior layering
of excreted FDG in the bladder is seen exclusively on scans obtained with IV
iodinated contrast material. This phenomenon was reproduced with in vitro
imaging of a phantom containing diluted FDG and iodinated contrast material.
These observations suggest the layering phenomenon is due to anterior
displacement of excreted FDG by excreted iodinated contrast material. The
disparity in physical density between normal urine (normal range of specific
gravity, 1.005-1.030) and iohexol (range, 1.16-1.41 for CT)
[7] presumably allows this
separation.
The clinical utility of this finding might seem limited, but in one case
the anterior displacement of FDG did help unmask an FDG-avid posteriorly
located bladder malignancy. Bladder activity merits close scrutiny on PET/CT
studies, particularly if performed with iodinated IV contrast material, and
should not be perfunctorily dismissed as always representing only excreted
FDG. A more speculative interpretation of this finding is that it suggests the
displacement phenomenon could potentially be exploited for improving bladder
imaging at PET by imaging the patient in the supine or prone position or in
both positions depending on the location of the lesion in question. This would
avoid other burdensome and cumbersome techniques that have been suggested to
assist evaluation of bladder abnormalities at PET, such as bladder lavage.
However, the value of this potential application remains to be tested and may
be limited by the fact that separation of the excreted FDG and iodinated
contrast material is not observed in all studies performed with IV iodinated
contrast material (71%).
It should be noted that our protocol was to acquire the contrast-enhanced
CT images after administration of FDG but before acquisition of the PET
images. Presumably at the time of PET image acquisition, iodinated contrast
material had accumulated in the bladder in sufficient quantity to cause
anterior layering of FDG, although we did not directly confirm this by
performing delayed CT of the bladder at the exact same time as PET. Further
study is needed to determine contributing factors to anterior layering,
possibly with delayed CT and correlation with additional clinical information
(including renal function). Another limitation of this study is that images
were interpreted by a single observer. However, the appearance of anterior
layering of FDG in the bladder is distinctive at PET and is unlikely to be
misinterpreted.
In conclusion, anterior layering of excreted FDG in the bladder is commonly
seen on PET/CT scans obtained with IV iodinated contrast material and is due
to displacement of FDG by excreted iodinated contrast material; this
phenomenon may unmask FDG-avid bladder disease.
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Abstract
Introduction
