Attenuation corrections

Finally, newer scanners are being developed with both PET and another modality, such as computed tomography (X-ray CT), where a subject would be scanned first with X-ray CT for anatomy and for attenuation correction, and then imaged for their PET or SPECT measurements. 

In 45 patients with known colorectal cancer. Cohade etal. [28] compared retrospectively the accuracies for staging and restaging of [ F]-FDG-PET/CT versus [1 F]-FDG-PET alone. All p F]-FDG-PET and [ F]-FDG-PET/CT studies were separately evaluated in randomized order. [ Ge] attenuation-corrected images were assessed to reassure that only the added value of CT information was... 

Moreover, the coherent scatter interaction is also elastic, and hence photons have identical energy before and after the interaction. Hence, a first-order attenuation correction simply normalizes the coherent scatter spectrum against the spectrum of transmitted rays. Further refinement of the first-order correction is possible, but its discussion is beyond the scope of this chapter. 

This section is based on the description published in reference [40], The geometry of the data acquisition for XRDCT is shown in Fig. 18. A fan beam illuminates the object, and the transmitted fan (in the plane of the drawing) reaches the 2-D detector at its central row. These data can be used for attenuation correction and also for a conventional transmission-CT image. The detector columns are indexed by the variable t, whereas the relative angle between measurement system (source, detector, etc.) and suitcase is described by the variable 4>. In conventional CT, a 2-D data set, I(f, ), is measured for many angular positions, 4>, of the object with respect to the device. 

Before the data can be reconstructed, an attenuation correction has to be performed by normalizing the scatter signal for a certain column of detectors against the primary beam... 

Dedicated PET systems are usually based on lull ring detector systems with an axial field of view exceeding 15 cm, and can be operated in septa extended (2D mode) or septa retracted mode (3D) for patient examinations. Some systems only provide 3D-data acquisition modes. Most systems allow the simultaneous acquisition of 36 transversal slices and more with a theoretical slice thickness of 2-5 mm. Transmission scans for a total of 10 min are obtained prior to the radionuclide application, for the attenuation correction of the acquired emission tomographic images. All PET images are attenuation corrected and iteratively reconstructed. 

PET/CT systems are gaining major attention in the last few years. Due to the sequential acquisition of CT and PET data, morphological and functional information can be easily correlated and analysed. The CT data can also be used for attenuation correction of the PET images, thus decreasing the total examination time of the patient. 

Gallium-68 68 Ga 68.1 min Study of thrombosis and atherosclerosis, PET imaging, detection of pancreatic cancer, attenuation correction... 

Fig. 8 shows all located events. The signal attenuation in CFRP-materials is usually quite high, typically 50 - 60 dB within 200 - 300 mm. Attenuation correction for located events has not been attempted. The sensor distance of 210 mm therefore implies that events located at the centre between both sensors (minimum propagation distance) show a lower detection threshold (amplitude) than those located near either sensor. 

Eleming, J.S. A technique for using CT images in attenuation correction and quantification in SPECT. Nucl. Med. Commun. 1989,10, 83-97. 

Pitcairn G. Tissue attenuation corrections in gamma scintigraphy. J Aerosol Med 1997 3 187-198. 

The computer code Correct has been used to perform this normalization and also the background, multiple scattering and container attenuation corrections to the neutron diffraction data. 

Pratapa, S., O Connor, B.H. and Low, I.M. Use of Compton Scattering for Attenuation Corrections in Rietveld Phase Analysis . In preparation. 

An important advantage of a CT transmission scan in the PET/CT system is that the scan data can be used for attenuation correction of PET emission images, obviating the need for a separate lengthy transmission scan in the dedicated PET system. The use of CT scans for attenuation correction reduces the whole-body scan time significantly. CT attenuation correction and fusion of CT and PET images are discussed in detail in Chap. 3. 

Figure 3.8. (a) Profile of raw acquired data showing depression in activity at the center. (b) The same profile with attenuation correction. (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved)... 

Note that the probability P is the attenuation correction factor, which is independent of the location of positron annihilation and depends on the total thickness of the tissue. If an external radiation passes through the body,... 

Theoretical Method. A simple theoretical calculation based on Eq. (3.5), also called the Chang method, can be applied for attenuation correction based on the knowledge of p, and the contour of an organ, such as the head, where uniform attenuation can be assumed. However, in organs in the thorax (e.g., heart) and the abdomen areas, attenuation is not uniform due to the prevalence of various tissue structures, and the theoretical method is difficult to apply and, therefore, a separate method such as the transmission method is employed. 

Using the currently available rod sources, the above method requires the transmission scan to be taken before the emission scan and obviously requires a long time to acquire enough counts for good accuracy of the measured attenuation correction factors. An alternative to this approach is to collect the transmission scan postinjection immediately after the emission scan without moving the patient from the bed. This method obviates the need for patient repositioning and hence the need for correction for patient motion... 

Figure 3.9. Illustration of attenuation correction by using CT transmission data on transverse whole-body images at the liver level, (a) Attenuation corrected images (b) Uncorrected images. Notice that the attenuation correction improves the images significantly showing better contrast and details.

Although PET/CT modality is widely employed for diagnostic purpose, CT data acquired either for fusion of PET and CT images or for photon attenuation correction are affected by several factors that cause artifacts in PET/CT images. The factors include patient positioning, metal objects, CT contrast agents, truncation artifacts, and respiratory movement, which are discussed below. 

Figure 3.14. Illustration of the effect of contrast agents on PET/CT scans. Attenuation correction factors are overestimated in tissues where contrast agent localizes and thus results in false-positive scan. (Reprinted with permission from Parker et al at Web site, http //www.jpnm.org)...

A simple method of recovering the truncated data is to linearly extrapolate the truncated CT projections to the extent of PET FOV and then setting the values to zero at the edge of the PET FOV. The attenuation correction by this method is adequate to remove the truncation artifacts in PET/CT images (Fig. 3.15). Using the iterative method rather than the filtered back projection technique also has shown to help to eliminate these artifacts. 

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