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.).
© 2006 by the Rector & Visitors of the University of Virginia