Partial volume effect

Corrections need to be applied for overestimation or underestimation of activities to these smaller structures in the reconstructed images. A correction factor, called the recovery coefficient (RC), is the ratio of the reconstructed count density to the true count density of the region of interest that is smaller than twice the spatial resolution of the system. The recovery coefficient is determined by measuring the count density of different objects containing the same activity but with sizes larger as well as smaller than the spatial resolution of the system. Normally, the recovery coefficients would be 1 for larger 

Discuss the methods and merits and disadvantages of filtered backprojec-tion and iterative methods. 

What is the common initial step that is taken in the reconstruction of 3D data  

When do you apply various corrections (e.g., detection efficiency variations, noise components, random and scatter coincidences, attenuation) in the FBP and iterative methods of image reconstruction  

In a six-ring PET scanner, how many direct and oblique sinograms can be obtained in 3D acquisition  

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Finite resolution and partial volume effects. Although this can occur in other areas of imaging such as MRS, it is particularly an issue for SPECT and PET because of the finite resolution of the imaging instruments. Resolution is typically imaged as the response of the detector crystal and associated electron to the point or line source. These peak in the center and fall off from a point source, for example, in shapes that simulate Gaussian curves. These are measures of the ability to resolve two points, e.g. two structures in a brain. Because brain structures, in particular, are often smaller than the FWHM for PET or SPECT, the radioactivity measured in these areas is underestimated both by its small size (known as the partial volume effect), but also spillover from adjacent radioactivity... 

Another issue for quantification is the effect of the finite resolution of PET/SPECT relative to small structures being imaged. This can result in the so called Partial Volume Effect which is characterized by an underestimation of the true radioactivity. This is an area of increasing research, but is not a major limitation when there is not a large difference between patients and controls in the volume of... 

Another method to optimize image contrast includes the addition of an inversion recovery pulse before the typical SE sequence to suppress signal from cerebrospinal fluid (CSF). This yields so-called fluid-attenuated inversion recovery (FLAIR)-or CSF-suppressed DWI images or ADC maps. With this approach, the partial-volume effect of cerebrospinal fluid can be substantially reduced, which is particularly relevant in cortical regions due to the frequent CSF contribution to individual voxels (Latour and Warach 2002). The technique may enhance lesion conspicuity in the vicinity of CSF and has been shown to be of value in research studies utilizing pixel-wise analyses to predict tissue fate. The technique is not yet implemented in many centers, although this may change in the near future. 

A mathematical model has been used to determine the metabolic rates in human grey and white matter from isotopic labelling data from H-observed/ C-edited NMR. Spectra were obtained from 22.5 cm and 56 cm voxels located in the grey and white matter, respectively. When partial volume affects were ignored the measured rate of the TCA cycle was 0.72 0.22 and 0.29 0.09 pmol min g in voxels of 70% grey and 70% white matter, respectively. After correction for partial volume effects the rate of the TCA cycle was 0.80 0.10 pmol min g in grey matter and 0.1710.01 in white matter. In a separate study the mean values of the TCA cycle rate were 0.8810.12 pmol min" g in the occipital grey matter and 0.2810.13 pmol min" g" in the occipital white matter. ... 

Figure 4.12. An illustration of partial volume effect, (a) Cross section of a sphere phantom with six different spheres (b) PET images of the spheres demonstrating partial volume effect. (Reproduced with permission from Rota Kops E, Krause BJ (2000) Partial volume effects/corrections. In Wieler HJ, Coleman RE (eds) PET in Oncology. Springer, Darmstadt)...

Jucius RA, Kambic GX (1980) Measurements of computed tomography x-ray fields utilizing the partial volume effect. Med Phys 7 379-382. 

The major challenge in using CTP for the assessment of the core infarct is the questionable reliability of dynamic perfusion imaging. CBV measurements are thought to be more robust than CBF and MTT measurements because the calculation of cerebral blood volume is relatively insensitive to bolus delay and dispersion [85, 86]. However, as a calculated parameter, it remains dependent on postprocessing (e.g., partial volume effects) and patient-specific issues (e.g., hematocrit levels) [87-89]. Additionally, as demonstrated in the DIAS-2 study [90], there is poor interrater agreement in assessing the CBV lesion. 

Limited spatial resolution and partial volume effects, as well as scatter and septal penetration, further degrade image quality. These effects are hard to compensate for, and even if this is attempted, object-independent spatial resolution is not feasible with SPECT imaging [20]. 

Strul, D., Bendriem, B. Robustness of anatomically guided pixel-by-pixel algorithms for partial volume effect correction in positron emission tomography. J. Cereb. Blood Flow Metab. 19(5), 547-559 (1999)... 

Region Oriented segmentation can be sensitive to noise, causing extracted regions to have holes or even become disconnected, also, partial volume effects can cause separate regions. 

On the other hand, thinner slices can be used. This has the advantage of reducing partial volume effects and, if a very small slice thickness, e.g. 1 mm, is used, offers the option of performing secondary image reconstruction that is comparable to the axial source images in terms of spatial resolution. 

Multiplanar reformation (MPR) Delineates structures that maybe overlooked with axial slices (often due to partial volume effects)... 

For routine evaluation, it is generally sufficient to reconstruct 5 mm-thick slices with 4 mm increment, resulting in 20% overlap. Overlapping reconstruction is mandatory for adequate depiction of smaller structures (e.g., pulmonary nodules) between slices. Without overlapping reconstruction, smaller structures at the edge of a slice might be obscured by partial volume effects (see Figs. 14.4,14.5 and 14.6). 

Fig. 14.6. Top three slices without over-lap-the small is located half in the red, half in the green slice and may vanish due to partial volume effect. Bottom the entire nodule is now in the green slice-so chances are higher that the nodule will he seen...

Even in contrast-enhanced studies, coronary calcifications can easily be detected and quantified because the density of calcium (> 350 HU) is beyond the density of contrast media in the coronary artery lumen (250-300 HU) (Hong et al. 2002). However, because of partial volume effects, it is much more difficult to quantify non-calcified rather than calcified plaques. The optimal quantification algorithm for the different presentation of coronary atherosclerosis as seen in MDCT is still under development (Fig. 15.4). 

Fig. 15.4. The same software also may provide information about the composition of atherosclerotic plaque. The final way of quantifying these lesions in the coronary artery wall has not yet been determined and remains problematic, since the display of these lesions is very much influenced hy partial volume effects...

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