2.3. Positron Emission Tomography/Computed Tomography Synthesis

As shown in Fig. 1, bone metastases manifest pathologically distinct characteristics on CT and PET images. Although most bone metastases are distinguishable on CT, some are difficult to detect merely using CT [Fig. 1(a)]. For certain lesions, PET provides complementary and enhancing information. To make use both of CT and PET in the detection, we create a synthetic image by enhancing the CT image with PET standardized uptake value (SUV) uptake.

The PET image is first resampled to the CT resolution. For sclerotic lesions, we need to increase the CT value to enhance its appearance, while for lytic lesions, we need to decrease the CT value. Therefore, we generate two synthetic CT images, one for each type of metastasis. The synthesis process can be formulated as

where I is the CT value, S is the PET SUV value, and I′ is the CT/PET synthesis value. I¯c is the mean CT value for cortical bones, and I¯m is the mean CT value for medullary bones, which can be computed from the spine segmentation in Sec. 2.1. Smax and Smin are the maximum and minimum PET SUV value in the image. w is the weight for enhancing, which is set empirically from the training cases. We set w=2 for sclerotic PET/CT synthesis and w=−1 for lytic synthesis. The synthesis process is, therefore, patient and study specific since it is normalized by the max SUV of the PET study and bone density of the CT study. The enhancement is bounded by the contrast between cortical and medullary bones to prevent distortion of the overall appearance of vertebra. Figure 3 shows the PET/CT synthesis for lytic and sclerotic lesions in Fig. 1.

PET/CT synthesis. (a) Sclerotic synthesis of Fig. 1(a), sclerotic lesion becomes visible (arrow); (b) lytic synthesis of Fig. 1(b), mixed lesion is enhanced (arrow); (c) sclerotic synthesis of Fig. 1(c), mixed lesion is enhanced; and (d) lytic synthesis of Fig. 1(d), lytic lesion is enhanced.