PET/CT > Physics > PET/CT Scanner

PET/CT Scanner

The PET/CT systems now in wide clinical use combine a multidetector PET system with a multidetector (currently 4-16 slice) computed tomography (CT) scanner in a single unit with a patient couch which traverses the bore of both imaging components. Approximately 30 to 60 minutes after intravenous FDG administration the patient is placed on the examination couch. The CT data is acquired first (lasting around 30 seconds) followed by a repeat slower transit of the patient through the bore for PET data acquisition (lasting around 30-45 minutes).

Simultaneous CT and PET imaging provides several distinct advantages over PET scanning alone, in which case PET images would typically be correlated with CT images acquired at a different time. Most importantly, acquiring the CT and PET data in very close temporal proximity with the patient in the same position on the imaging couch minimizes patient motion between the two acquisitions which allows more precise anatomic localization of FGD activity. This results in fewer equivocal findings, such as when activity can be determined as physiologic in nature rather than pathologic (e.g. excreted activity in a ureter rather than in an adjacent retroperitoneal lymph node). Another advantage of the combined modality is that a CT transmission scan provides more accurate and efficient attenuation correction compared to a transmission scan using a radioactive source as is typically used in dedicated PET systems not combined with CT. Also, total imaging times are shorter using CT transmission for attenuation correction rather than a radioactive transmission source.

The CT and PET data sets are fused or “coregistered” electronically by the scanner’s computer system and presented to the interpreter on a work station. The data can then be simultaneously and interactively viewed as CT data, PET data, and superimposed CT and PET data in any percentage combination of these data sets desired (e.g. 100% PET data, 100% CT data, 50% CT / 50% PET data).

Top row: 100% PET data     Middle row: 100% CT data     Bottom row: 50% PET/50% CT data


PET/CT is increasingly becoming preferred over PET scanning alone in most clinical settings because it generates images that combine both exquisite anatomic detail and physiologic information (i.e. level of metabolic activity).

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