Applied Radiology

Dr. Tran is the Director of the Division of Nuclear Medicine in the Department of Radiology at Albert Einstein Medical Center; an Assistant Professor of Radiology at Jefferson Medical College; and the Director of Nuclear Radiology at Germantown MRI & PET Center, Philadelphia, PA.

Fusion of anatomic (CT, MRI) with metabolic (PET, SPECT) scanning has improved the diagnostic accuracy of tumor imaging for a decade ¹ ; however, it was practiced at only a few centers due to its difficulty. Recent advances in imaging hardware and computer software have made this exciting technique significantly easier to adopt. This article will review fusion methods and discuss the benefits of software fusion.

Mental fusion

With mental fusion, the anatomic and metabolic studies are placed side-by-side on the viewbox, and the physician attempts to correlate the two. There is a relatively high degree of interobserver variability in interpretation, depending on the skill and experience of the physician.

Software fusion

Several independent software products, including Medical Image Merge (Zalen, Cleveland, OH), Hermes Multi-Modality Image Fusion & Review (Nuclear Diagnostics, Stockholm, Sweden), and software from the PET camera vendors, allow importation of DICOM-compliant data from multiple modalities such as PET, SPECT, CT, or MRI in order to perform software fusion. Fusion can be rigid, where the organs remain fixed in size; or deformable, where the organs can be warped and stretched in order to attempt to account for changes in relative organ position and shape between scans. This is especially important when scans are taken more than a few minutes apart, as the patient's lungs, gastrointestinal tract, and bladder are likely to change in size. Without positioning aids, the patient may also bend the neck or torso, and the extremities are commonly positioned differently. A CT scan of the chest is usually done with the arms up over the head, while a PET scan is usually done with the arms down by the chest.

Software fusion can be used to fuse OctreoScan (Indium-111 Pentetreo-tide, Mallinkrodt, St. Louis, MO) SPECT with CT (Figure 1). In this example, the initial CT scan was not diagnostic. On a subsequent day, the SPECT scan was positive, but the location of the lesion was uncertain. The DICOM-compliant CT and SPECT scans were imported into a MiraView/Reveal MVS dual-monitor workstation (Mirada Solutions Ltd., Oxford, UK, and CTI Medical Imaging Inc., Knoxville, TN). Automatic rigid fusion was applied, the results were inspected, manual correction was performed by translating and rotating the PET data, then automatic deformable fusion was performed. At each step, the fusion was inspected visually along the transverse, sagittal, and coronal planes by changing the transparency of the pseudocolored overlay SPECT and adjusting the overlay position until the liver, spleen, and kidneys matched up with their locations on the CT scan. Displaying the data on two large monitors was very helpful. Then, simultaneous SPECT and CT crosshairs were turned on, pinpointing an insulinoma at the junction of the body and tail of the pancreas on the CT scan. Software fusion works better if there are organs concentrating physiologic radiopharmaceuticals near the region of interest; these can be used as landmarks to check the accuracy of the registration. In addition, a more similar body position between scans will improve the success rate of the registration. ^2-4

Figure 2 presents an example of ¹⁸ F-FDG-PET/CT software fusion in the chest and was done during preoperative staging of a head and neck squamous cell carcinoma. The initial CT scan was done in a breath-hold at maximum inspiration, while the PET scan was done during quiet breathing. Automatic rigid fusion was done. Then direct manual translation and rotation of the PET data was performed using internal landmarks to adjust the PET to the CT, superimposing the FDG in vertebral body red marrow and in mediastinal blood pool in the ascending and descending aorta, right and left pulmonary arteries, and superior vena cava to their locations on the CT. Then automatic deformable fusion was applied to compensate for differences in lung volumes between the PET and CT scans. The PET image is converted to a semitransparent orange pseudo-color, and this is overlaid onto the CT image. The PET overlay transparency is varied dynamically with the computer mouse, localizing the tumor to a normal-size, right hilar node on the CT scan. Note that the arms are positioned farther away from the chest walls on the CT scan, whereas they are touching the chest on the PET scan; after fusion, the semi-transparent orange FDG uptake in arm muscles is projected over empty space between the chest walls and the arms on the CT scan. If this were the region of clinical interest, additional processing with registration using manually placed landmarks would be necessary to overlay the arm on the PET scan to the arm on the CT scan. Fiducial markers can be placed externally ^1,5 or internally ⁶ to help guide the fusion.

Software fusion with positioning devices

By ensuring the patient is positioned the same way during each scanning session, fixation devices allow accurate fusion. ^7,8 This is especially important for radiation therapy planning. Radiation treatment sessions take place over weeks, so radiation oncologists use a variety of devices to keep the patient immobilized in order to treat tumors without over-radiating normal tissue.

In another example, following diagnosis of squamous cell cancer of the head and neck, a CT scan was done in a custom-molded mask for radiation treatment planning (Figure 3). The patient was to undergo radiation treatments for several weeks while wearing the same mask. On a different day at a different location, a FDG-PET scan was done using the same mask. Automatic rigid fusion was applied with a single command. Because the patient positioning was similar, the software was able to pinpoint an unsuspected metastasis, changing the radiation treatment plan. Before performing studies in positioning devices, both CT and PET technologists should be trained in the proper application of these devices.

Hardware fusion

Despite new artifacts that need to be accounted for, ^9-12 hardware fusion with combined PET/CT scanners has been shown to significantly improve diagnostic accuracy over mental fusion. ^13,14 The scans are taken on the same table only a few minutes apart, and the patient should stay in the same position. The study is much faster because the CT scan is also used for attenuation correction of the emission PET data. There is still a time gap of several minutes between scans, so there may still be patient motion and artifact from differences in stages of respiration between PET and CT, bladder filling, and peristalsis.

Conclusion

Good software fusion and hardware fusion are synergistic, and having both will lead to a more accurate, confident diagnosis and localization. Hardware fusion gives better data for software fusion to work with, and software fusion can complement hardware fusion by tweaking the registration more precisely.

Despite these proven benefits, there remains significant difficulty in obtaining adequate reimbursement for the additional cost of the integrated scanner and the increased complexity of acquisition and interpretation. Not every facility will be able to afford combined PET/CT. If a PET/CT scanner is not available, software fusion still conveys significant clinical benefits by adding anatomic information to metabolic imaging.