Attenuation Correction Methods

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 

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The transverse images of cold and hot regions insert were obtained by the filtered back projection image reconstruction technique. It was corrected for uniformity and center of rotation (COR). Chang s attenuation correction method was applied by choosing 0.111, 0.121, 0.131, 0.141 and 0.151 cm linear attenuation coefficient values. Butterworth filter with cut off frequency 0.35 cycles/cm and order 5 was used. Scatter correction technique was not applied due to non-availability with this gamma camera system. 

Glick SJ, Penney BJ, King MA (1991) Filtering of SPECT reconstruction made using Bellini s attenuation correction method a comparison of three pre-construction filters and a post reconstruction Wiener filter. IEEE Trans Nucl Sci 38 663-669... 

Sayed IS, Ahmad Z, Norhafiza N (2008) Comparison of Chang s with Sorenson s Attenuation Correction Method by Varying Linear Attenuation Coefficient Values in Tc-99m SPECT Imaging. LNCS... 

The projection data acquired in the form of sinograms are affected by a number of factors, namely variations in detector efficiencies between detector pairs, random coincidences, scattered coincidences, photon attenuation, dead time, and radial elongation. Each of these factors contributes to the sinogram to a varying degree depending on the 2D or 3D acquisition and needs to be corrected for prior to image reconstruction. These factors and their correction methods are described below. 

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. 

Describe a method of attenuation correction for PET acquisition data. 

Figure 4.10. Comparison of filtered backprojection (FBP) and iterative (OSEM) methods with attenuation correction, (a) Lungs, (b) Normal Liver, (c) Liver with tumor, (d) Breast. FBP images with attenuation corrections are noisier than OSEM images with attenuation correction. (Reprinted by permission of Society of Nuclear Medicine from Riddell C et al (2001) Noise reduction in oncology FDG PET images by iterative reconstructions a quantitative assessment. J Nucl Med 42 1316)...

Attenuation correction factors are calculated from the blank transmission scan (Step 12) and patient transmission scan (Step 19). Reconstruct images using the corrected data with appropriate reconstruction algorithms (FBP or iterative method), provided by the manufacturer. 

Attenuation correction factors are applied to acquired PET data (Step 15). PET images are then reconstructed from corrected data using either the FBP or the iterative method depending on the vendor s software. 

Two-line background correction The two-line correction method, which was proposed in the late 1970s [22], is based on measuring the absorption at a second, non-resonant line. This line should be close to the resonance Hne of the element that is measured but should not be absorbed by the analyte. If these conditions are met sufficiently well, it can be assumed that the attenuation at this second line is only due to the background absorption in the sample. 

In our experience, other than absolute or relative SUVs, contrast-oriented algorithms are quite robust under clinical conditions. However, the cahbration to the PET and radiotherapy planning system used is mandatory (Nestle et al. 2005,2006,2007). As technical factors like the methods of reconstruction, attenuation correction of PET images, and data transfer do influence PET imaging, they have to be defined before and kept constant after calibration. 

We decided at the outset that we would not use a method that would require first dismembering the spore and this stricture forced us to invent the technique of /3-attenuation. The method depends on the expectation that the closer a tritium-labeled material is to the center of the spore, the less likely that the /5-particles will escape the dimensions of the spore and be scored by the surrounding scintillation fluid. It was possible to empirically prove the correctness of this assumption by preparing spores labeled with tritium-marked uracil, diaminopimelate or lysine, added late in the sporulation sequence, to selectively label the core, cortex and coat respectively. The observation of increasing attenuations of 3, 18 and 26 % as one placed the marker at positions nearer the spore center, validated the proceduie.< >... 

Chang LT (1978) A method for attenuation correction in radionucUde computed tomography. IEEE Trans Nucl Sci 35 638-643... 

Murase K, Tanada S, Higashino H et al. (1989) A unified computer program for assessment of attenuation correction and data acquisition methods in single photon emission computed tomography (SPECT). Eur J Nucl Med 15 284-253... 

It can easily be shown that an experimentally generated curve coincides with the synthetically generated DGS curve having applicable corrections (transfer loss and sound attenuation) with acceptable tolerances. However, our intention is not to replace the reference block method by the DGS method. In practical application you may encounter many cases, where only the reference block method can be applied, e.g. ... 

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