PET/CT > Physics > Interpretation


Interpretation

Most PET/CT workstations allow visualization of the data in several different display protocols with the interpreter’s being able to move between the different display protocols easily and quickly at the workstation.

At the University of Virginia, one “page” of the display protocols includes the attenuation and non-attenuation data sets displayed simultaneously in three orthogonal planes: axial, sagittal, and coronal. That “page” also includes the two PET data sets (corrected and non-corrected) in three-dimensional rotating displays. The rotating images often provide a general overview of the FDG distribution within the body and draw attention to possible areas of focal and/or abnormal activity to be closely inspected on subsequent complete data review.

The top row displays attenuation-corrected PET data; the bottom row shows non-attenuation-corrected PET data. The image to the far left on each row would be rotating on the nuclear medicine work station.

 

Another display protocol “page” includes the fused PET and CT data in the same three orthogonal planes with the interpreter’s being able to scroll through the data in all three planes simultaneously or his/her being able to select any one plane for review in an enlarged manner, encompassing a greater percentage of the monitor space.

Simultaneous three-plane view of fused PET/CT data

 

Semiquantitative methods of assessing FDG activity can also be used to aid in diagnosis and are quite helpful in clinical practice. When a PET camera is appropriately calibrated, it is able to assess the amount of FDG activity per volume (millicuries/millileter or mCi/mL) of tissue. The standard uptake value (SUV) is the semiquantification of activity within an area of interest based on administered FDG activity and patient body weight. The SUV is defined by the following equation:

SUV=Mean region of interest activity (mCi/mL)

administered activity (mCi)/body weight (g)

                                                =grams/mL

The attenuation-corrected data is used for SUV determination. Special software allows for immediate calculation of SUV at the workstation by the interpreter’s placing a cursor or custom drawn region of interest (ROI) over a lesion or portion of tissue visually demonstrating the greatest FDG activity and thus containing the maximum value pixel. Body weight obviously alters SUVs with obese patients having higher SUVs compared to thinner patients in both normal tissue and in areas of pathology. SUVs can be calculated based on lean body mass or body surface area in selected cases.

An SUV greater than 2.5 should raise the possibility of malignancy in a tissue thought possibly involved by cancer on CT images (e.g. soft tissue mass, lymph node, etc.). Using 2.5 as a threshold above which an area of uptake is considered significantly suspicious for malignancy is based on studies of SUVs in pulmonary nodules which were pathology proven to be malignant. Currently, the 2.5 threshold is applied to all malignancies evaluated with PET although further studies are needed to possibly establish specific threshold values for different tumor types in different tissues. It should also be noted that SUVs may be falsely low in small lesions (less than about 1 cm) due to volume averaging. For example, if an 8 mm pulmonary nodule being evaluated with PET in a patient with significant risk factors for lung cancer demonstrates only low level FDG activity (SUV < 2.5) the clinical concern rather than the SUV alone should guide further evaluation of the lesion (e.g. CT-guided biopsy, short-term follow-up imaging, etc.).



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