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PET/CT Imaging of Pelvic Pathology: Spectrum of Disease and Pitfalls in Diagnosis

Naveen Subhas MD, Harpreet K. Pannu MD, Pavni Patel MD, Elliot K. Fishman MD, Richard L. Wahl MD

Introduction

18F-FDG PET is routinely used in the evaluation of pelvic malignancies. Numerous studies have shown FDG PET to be superior to CT in the evaluation of recurrent or metastatic colorectal carcinoma with an overall sensitivity of 97% and specificity of 76%. In cervical cancer, the sensitivity and specificity of FDG PET has been reported as 90% and 76%, respectively, in detecting early recurrence and 86% and 94%, respectively, in detecting metastases in advanced disease. FDG PET imaging has had similar results in detecting recurrence post-therapy in endometrial carcinoma with a sensitivity of 96% and a specificity of 78%. FDG PET has even shown promise in staging bladder cancer despite the inherent difficulty associated with the urinary excretion of the radiotracer. Despite the high sensitivity of FDG PET in these pelvic malignancies, one of the inherent pitfalls of metabolic imaging has been false positive readings due to increased FDG uptake in benign processes, inflammatory conditions, normal organs and artifacts which can complicate image assessment. Increased uptake of FDG-PET has been reported in many benign pelvic processes including retroperitoneal fibrosis, uterine fibroids, and endometriosis and anecdotally related to the menstrual cycle. Inflammatory changes from recent surgery and radiation therapy have also demonstrated increased FDG uptake. Focal physiologic retention of radiotracer in bowel loops, the urinary system, uterus, bone marrow and skeletal muscle can similarly lead to false positive interpretations. Hip prostheses and surgical clips can also cause artifactual foci of increased FDG uptake. With the advent of PET/CT scanners, the ability to anatomically localize areas of increased FDG uptake using CT promises not only to increase the diagnostic accuracy but avoid many of the pitfalls that have been challenges for PET imaging.

 

Technique

MINIMIZING PITFALLS
Proper patient preparation and optimal scanning technique can improve diagnostic accuracy.

BOWEL PREPARATION
Bowel definition can be enhanced by using low density barium for CT oral contrast which does not cause significant artifact on PET.11 Bowel cleansing regimens are not recommended as they may result in bowel irritation and increased FDG uptake.

BLADDER EMPTYING
Oral hydration is recommended as long as glucose containing fluids are not in the liquids. Full voiding immediately before starting the PET is suggested as well as imaging the pelvis early in the study.

METALLIC ARTIFACTS
Artifactual foci of apparent increased FDG uptake near metallic prostheses can be reduced by minimizing patient motion as well as by using an attenuation-weighted iterative reconstruction technique.10 Artifacts near metal can also be minimized by examining non–attenuation corrected images.

PATIENT PREPARATION
The patient is asked to remain NPO for 4 hours prior to examination. Upon arrival to the department, weight and height are obtained and a weight-based dose of 18F-FDG is prepared. A 22 or 24 gauge intravenous line is placed contralateral to any prior lymph node resection to ensure proper distribution of radiotracer. Blood glucose is tested to ensure that the patient is not hyperglycemic (level > 200mg/dL). Approximately one hour prior to imaging, the patient is then given 900 mL of low density barium oral contrast (Readi-cat, 1.3% weight/volume barium sulfate suspension, E-Z-EM, Inc., Westbury, NY) to drink. An additional 100 mL of oral contrast is given approximately 30 minutes prior to imaging. Approximately 45 minutes prior to imaging, the FDG dose is injected. The patient is then placed in a quiet, dimly lit room. To minimize skeletal muscle uptake during this period, the patient is instructed not to talk and to keep arms by the side and legs uncrossed. Just prior to imaging, the patient is asked to void completely to minimize bladder activity.


SCAN PROTOCOL
For the scan, patient is positioned supine and head first on a fused PET/CT scanner with a single gantry and table. The arms are raised above the head and the patient is allowed to breath quietly. The CT scan is performed first. CT scanning parameters are a slice thickness of 4.25 mm, a weight adjusted mA (average of 80), and a kVP of 140. The scan is performed from the skull base to the midthigh level. The images are reconstructed with a 512 X 512 matrix and a 50 cm field of view. For fusion with the PET data, images are also reconstructed with a 128 X 128 matrix.

After the CT scan is completed, the table is moved into the PET scanner. The images are acquired in a caudal to cranial direction from the midthigh to the skull base to minimize bladder filling. 5 minute emission acquisitions per field of view are obtained. The PET images are reconstructed on a 128 X 128 matrix, ordered subset expectation maximum iterative reconstruction algorithm (2 iterations, 28 subsets), 8 mm Gaussian filter and 50 cm field of view.

The CT transmission map is used for attenuation correction. Attenuation correction accounts for differences in activity due to location within the body i.e. normally photons from deep sites are attenuated to a greater degree than photons from superficial sites. Both attenuation corrected and uncorrected PET images are reviewed along with the CT and fused images on a workstation.

Normal 18F-FDG Activity

An analog of glucose, 18F-FDG, is distributed via the bloodstream after intravenous injection and taken up by metabolically active tissues. Imaging is typically performed 60 minutes after injection of radiotracer to allow for sufficient blood pool clearance to improve target-to-background ratio. Normal physiologic uptake is seen in the brain and myocardium and to a lesser extent in the liver, spleen, bone marrow, GI tract, renal cortex, testes and skeletal musculature. Mycocardial uptake is variable in fasting patients but often intense in nonfasting individuals. Skeletal muscle uptake is dependent of recent utilization of the muscle group. Activity within the blood pool particularly in the mediastinum can also be seen. Other less frequent sites of uptake include the thyroid gland, endometrium, breast and brown or USA fat in the supraclavicular and paraspinal regions. Since 18F-FDG is excreted by the kidneys, intense activity can be seen in the renal collecting system, ureters and bladder.

Increased 18F-FDG uptake can also be seen in many benign processes. Healing fractures, granulomatous diseases, inflammatory and degenerative joint diseases, infectious processes including pneumonia, sinusitis and abscesses, inflammatory processes such as pancreatitis and foci of wound healing and repair such as ostomy sites have all been reported to have increased 18F-FDG uptake.

 

Classic Appearance of Pelvic Malignancies on PET/CT


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FIGURE 1: PRIMARY RECTAL CARCINOMA WITH LYMPH NODE METASTASIS
Increased foci of FDG uptake in the rectum and right inguinal region on PET (a). Enlarged right inguinal lymph node (arrow) and normal appearing rectum on CT (b). Increased uptake localized to enlarged right inguinal lymph node and rectum on the fused image (c).

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FIGURE 2: RECURRENT ENDOMETRIAL CARCINOMA AFTER HYSTERECTOMY
Increased foci of FDG uptake in the left obturator region and paraortic regions bilaterally on PET (a) corresponding to enlarged lymph nodes (arrows) on CT (b) and confirmed on fused image (c). Normal physiologic activity is seen in the bladder (B).

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FIGURE 3: ADVANCED CERVICAL CANCER
Increased foci of FDG uptake in the pelvis above the bladder (B), along the paraaortic and iliac regions bilaterally, and in the left supraclavicular region on PET (a) which localize to the cervix and lymph nodes (arrows) on CT (b) and fused image (c).

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FIGURE 4: METASTATIC BLADDER CANCER PET
(a) demonstrates markedly increased FDG uptake in the left pelvis and sacral region which correspond to the soft tissue masses on CT (b) along left pelvic side wall destroying the left acetabulum, in the presacral region destroying the sacrum and in the left inguinal region on the fused image (c). Normal physiologic activity is seen in the bladder (B).

Pitfallson PET/CT


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FIGURE 5: PHYSIOLOGIC BOWEL UPTAKE OF FDG
Multiple foci of increased FDG uptake are seen in the right and mid anterior abdomen on PET (a) in a patient with history of endometrial cancer. At the same level, CT (b) demonstrates normal appearing bowel loops containing oral contrast. On the fused image (c), the increased foci of uptake localize to bowel loops confirming normal physiologic uptake of radiotracer.

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FIGURE 6: PATHOLOGIC FDG UPTAKE IN ABDOMINAL IMPLANTS
In contrast to the previous example of normal physiologic bowel uptake of FDG in this patient with reticulum cell sarcoma of the right iliopsoas muscle, PET demonstrates an increased focus of FDG activity in the right lower quadrant near bowel loops (a) and in the left mid abdomen also near bowel loops (b). On CT (c,d) and fused images (e,f), these areas localize to a soft tissue density nodule (arrow) adjacent to a loop of small bowel opacified with oral contrast in the right lower quadrant and to nodularity (arrow) adjacent to the descending colon. Normal physiologic activity is seen in the inferior pole of the left kidney (K) and the bladder (B). These findings confirm that the areas of increased activity are not due to normal physiologic uptake of FDG within bowel but indeed represent implants adjacent to bowel loops.

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FIGURE 7: FOCAL RETENTION OF FDG IN URETER
Increased focus of FDG activity is seen in the right retroperitoneum on coronal (a) and axial (b) PET images. Normal physiologic activity is seen in the bladder (B) on the coronal image. On CT (c), no abnormally enlarged lymph nodes or soft tissue masses are seen. A normal right ureter (arrow) is seen. Fused image (d) shows the increased radiotracer activity localizing to the right ureter confirming focal retention of FDG in the ureter.

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FIGURE 8: FOCAL RETENTION OF FDG IN BLADDER DIVERTICULUM
Increased FDG activity is seen on PET (a) just posterior and to the left of the bladder (B) where a bladder diverticulum (arrow) is seen on CT (b). The fused image (c) demonstrates the increased radiotracer activity localizing to the bladder diverticulum confirming focal retention of FDG in the bladder diverticulum.

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FIGURE 9: PHYSIOLOGIC FDG UPTAKE IN OVARY AND ENDOMETRIUM
In this 18 year-old female with Hodgkin’s lymphoma, two foci of FDG activity are seen in the pelvis on PET (a). On CT (b), a right ovarian cyst (O) and a portion of uterus (U) are seen. The two foci of FDG activity localize to the right ovary and uterus on the fused image (c) compatible with normal physiologic uptake of FDG in a functional ovarian cyst and the endometrium.

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FIGURE 10: INCREASED FDG ACTIVITY AROUND HIP SCREW DUE TO POSTOPERATIVE CHANGES AND ARTIFACT
Increased FDG activity is seen around a left hip screw (seen as a photopenic defect) on PET (a) in this patient with incidentally found lung cancer after a femur fracture one month prior to the scan. Artifact from the hardware is seen on the CT (b). Increased radiotracer activity localizes to the bone and soft tissues surrounding the screw on the fused image (c) compatible with recent surgery and metallic artifact.

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FIGURE 11: FALSE NEGATIVE DUE TO FDG ACTIVITY IN BLADDER AND MISREGISTRATION
In this patient with recurrent ovarian cancer, CT (b) demonstrates minimal nodularity near the bladder dome which was surgically confirmed to be metastatic disease. On the PET (a) and fused image (c), activity in the tumor nodule is not detected due to FDG activity in the bladder and misregistration artifact. There is also increased FDG uptake in an enlarged right inguinal node compatible with metastasis.

Summary

PET/CT has proven to be valuable in a range of disease states including rectal cancer, cervical cancer and endometrial cancer. Through representative cases, this exhibit illustrates the typical appearance of gynecologic, gastrointestinal and genitourinary malignancies on PET/CT. Common pitfalls including normal physiologic uptake in bowel, ovaries, and endometrium, focal retained activity in bladder diverticula and ureters and artifacts due to motion and metallic hardware are also presented. Specific scanning techniques to reduce diagnostic errors and optimized protocol design including the use of oral contrast material is addressed. The use of CT in a PET study provides additional information that may better help to localize and define disease and help avoid potential pitfalls in diagnosis.

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