Authors: Christiaan Schiepers Magnus Dahlbom
Publish Date: 2010/12/21
Volume: 21, Issue: 3, Pages: 548-554
Abstract
In the last decade PETonly systems have been phased out and replaced with PETCT systems This merger of a functional and anatomical imaging modality turned out to be extremely useful in clinical practice Currently PETCT is a major diagnostic tool in oncology At the dawn of the merger of MRI and PET another breakthrough in clinical imaging is expected The combination of these imaging modalities is challenging but has particular features such as imaging biological processes at the same time in specific body locationsIn oncology detection of tumor staging of disease and monitoring of therapy are important for patient management and prognosis 1 Cancer patients undergo a number of different imaging studies throughout the course of their disease Various imaging investigations are available and the resulting images are reviewed on different viewing stations The gathered information is synthesized and integrated by the interpreting physicians The multitude of images need to be correlated and can be coregistered or aligned retrospectively and fused in computer memory Post hoc image fusion based on tomograms acquired at different institutions on separate days using varying equipment and protocols is a tedious and timeconsuming task Moreover welldefined and reproducible landmarks are necessary to provide the coordinate system in which the images can be aligned scaled and registered Since patient positioning varies widely between PET and CT eg arms up or down different patient and organ axes the post hoc fusion technique is prone to misalignment inconsistencies and errors Also there may be problems due to patient movement as well as motion of internal organs Changes related to breathing and organ movement such as a beating heart are inevitable and cannot be controlled when a patient is imaged on different machines and at different times even when care is taken to ensure that the body position of the patient is the same eg with external lasers in 3D Other movements such as bowel peristalsis may be minimized with medication Brown adipose tissue uptake may be aborted with sympathetic nervous system receptor blocking agentsIn the last decade this journal has reported on the developments of imaging in oncology from the progress of PETonly 2 to combining PET and CT 3 and PET and MRI 4 Reviews have been published on some of the achievements of the dual imaging modality PET/CT a powerful routinely and frequently applied molecular imaging tool for diagnosis staging and therapy monitoring in oncology 5 6 7 8For an oncology study the most often used radiopharmaceutical is 18FFDG 9 FDG mimics the glucose utilization which is usually deranged in cancer Typically 200–600 MBq of 18FFDG is administered to the patient followed by a 1 h uptake interval During this uptake interval oral contrast medium may be administered eg at 5 30 and 55 min relative to the FDG injection for improved detection of abdominal and pelvic abnormalities by CT After voiding the patient is positioned in the PET/CT system The photon emissions from the 18F label do not interfere with the CT detectors which have lower sensitivity for the highenergy photons from the annihilation In addition the photon flux from 18FFDG is a power of 4 lower than from the Xray tubeA scout view is obtained and the imaging field defined Subsequently a helical CT dataset is acquired without and/or with intravenous contrast medium The complete CT data acquisition takes 05–15 min depending on the axial field of view and type of CT system incorporated into the PET/CT machine The patient bed is then translated axially into the PET fieldofview and PET data 10 acquired over the same axial range as the CT The duration of the PET data acquisition is 5–25 min dependent on the axial fieldofview the type of detectors and mode of acquisition 2D vs 3D The CT data are used for attenuation correction omitting the lengthy transmission with radioactive sources which was used previously 3 If an enhanced CT study is requested or deemed appropriate the additional CT acquisition is performed after the standard PETCT study is completed to ensure correct attenuation correction 3 Intravenous contrast medium usually about 300 mg/ml iodine at a rate of 1–2 ml/sec is administered up to 125 ml Current multidetector CT systems are sufficiently fast to permit multiphase imaging An enhanced CT following PET/CT is generally limited to a specific area of the body eg chest liver or pelvis in order to reduce the total radiation dose to the patient If a dedicated head neck CT examination is required the intravenous dose of contrast agent can be split in two 80 ml for the body followed by 45 ml for the HNThe contribution of breathing is less important for the abdomen than for the chest 11 The artifact caused by motion of the liver during CT acquisition does not pose a real clinical problem in staging primary hepatic cancer or extent of liver metastases 12 Nonattenuation corrected tomograms and projection images are always available to check for possible artifacts induced by misregistered PETCT slices Metal implants or contrast material may also induce artifacts Therefore nonattenuation corrected images are routinely interpreted to check for and eliminate possible imaging artifacts Modern reconstruction algorithms are less sensitive to metal artifacts dental implants oral and intravenous contrast agents 13 14
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