Evaluation of a Triple Energy WIndow Scatter Correction Method for SPECT Imaging

Background

For each pixel in the photopeak projections, the number of scatter counts is determined using the following equation

C_scat ~ (C_lower/W_s + C_upper/W_s)*W_m/2

where

C_lower = counts in left window
C_upper = counts in right window
W_s = width of left and right scatter windows (keV)
W_m = width of photopeak window (keV)
C_scat = number of scatter counts

Phantom studies and computer simulations performed by Ogawa et al and Ichihara et al² showed the method could estimate the number of scattered photons fairly accurately. However, neither paper addresses the potential for increased noise when the scatter counts are subtracted.

A Monte Carlo investigation of the method by Ljungberg et al³ questioned the use of the upper (right) scatter window noting that using the right scatter window might make the TEW method more susceptible to noise. When only the lower energy window is used, C_lower = 0 and the estimated number of scatter counts becomes

C_max = (C_lower*W_m)/(2*W_s)

Monte Carlo simulations performed by Buvat et al⁴ demonstrated an 18% overestimate of the scatter counts when both scatter windows were used, and a 14% underestimate of the scatter counts when only the lower scatter window was used. Good relative activity quantification was also demonstrated by the simulations when only the lower window was used.

Equipment

• Jaszczak phantom with cold rods and spheres
• ACNP kidney phantom
• Picker PRISM 3000 triple head SPECT camera

Method

1. Photopeak - 15% window centered at 140 keV.
2. Scatter1 - 3 keV window centered at 126 keV.
3. Scatter2 - 3 keV window centered at 153 keV.

with these acquisition parameters: 120 projections, 30 s/projections, 3 degrees/projections and a 128x128 projection matrix. The images from each energy window was stored in a separate file.

The ACNP (American College of Nuclear Physicias) kidney phantom consists of two fillable kidney objects with a cold spot defect located in the middle of the right kidney and superior portion of the left kidney. The objects were filled with approximately 370 MBq (10 mCi) of Tc-99m and placed within a water bath. A SPECT acquisition was acquired using the same energy windows used for the Jaszczak phantom, 120 projections, 30 s/projection, 3 degrees/projection and a 128x128 projection matrix.

Processing

TEW2

1. Divide the counts in each pixel of the scatter projections by the width in keV of the scatter window.
2. Add the two scatter window projections together.
3. Multiply the resultant projections by half of the photopeak window width in keV.
4. Subtract the result from the original projections.

TEW1

1. Divide the counts in each pixel of the scatter projections from the lower window by the width in keV of the scatter window.
2. Multiply the resultant projections by half the photopeak window width in keV.
3. Subtract the result from the original projections.

With the window settings used in this experiment (3 keV scatter windows, 15% (21 keV) photopeak window), the scatter correction using both lower and upper scatter windows was performed by adding the scatter projections together and multiplying the result by a factor of 3.5. The scatter projections were then subtracted from the main photopeak projections. For the TEW1 method, the pixels of the lower scatter window projections were scaled by a factor of 3.5 and subtracted from the photopeak window projections

Reconstruction

The ACNP kidney phantom was reconstructed using a ramp filter and single pixel thick slices, and then filtered using a Wiener 3-D postfilter. The filtered images were reformatted to 3 pixel thick coronal and transverse slices. No attenuation correction was applied to the phantom images.

Results and Discussion

Circular ROIs was drawn through the each sphere of the Jaszczak phantom to obtain the mean counts/pixel within the sphere for the corrected and uncorrected images. The same ROIs were used to obtain the mean counts/pixel from the center of the phantom. The percent contrast for each sphere was calculated using the equation

% Contrast = (bkg counts - ROI counts)/bkg counts

Table 1: Contrast levels for solid spheres

% Contrast
	Uncorrected	2 Window	1 Window
36 mm	80	90	92
31 mm	66	75	83
25 mm	52	68	69
19 mm	49	60	65
15 mm	23	41	34

The most noticeable problem with the scatter corrected images is a significant decrease in counts and increase in noise. The inherent noisiness of the ramp filter also contributes to the noise in the reconstructed images. An interesting item to note is the improved contrast when only the single window is used compared to when both windows are used. This suggests that using only the lower scatter window may produce better results as suggested by Ljungberg et al³. Neglecting the higher scatter window for the higher energy photopeak is also recommended by Ogawa to avoid increasing statistical noise.

The additional noise introduced by the TEW scatter correction may be compensated for somewhat by applying a different filter to the projections or postfiltering the reconstructed images.

The slices from the uniform section of the phantom was used to determine the uniformity and noise of the corrected and uncorrected slices. The mean, standard deviation, maximum and minimum counts per pixel from a 15x15 pixel ROI were used to calculate the integral uniformity and the RMS noise level for the uniform section. The relatively high uniformity values are a result of the ramp filter which is inherently noisy.

Table 2: Integral uniformity using 15x15 pixel ROIs with no scatter correction applied. The last row gives the average over all ROIs used.

Mean cts/pix	Max cts/pix	Min cts/pix	Int Unif
19619	24975	14598	26.2%
19750	23922	16182	19.3%
19134	24246	14715	24.5%
19371	24336	14940	23.9%
18785	23391	14598	23.1%
19331.8	24174	15006.6	23.4%

When both the upper and lower energy windows are used the integral uniformity increases significantly. This a result of the subtraction of the scatter counts from the projections.

Table 3: Integral uniformity (15x15 pixel ROIs) using upper and lower energy windows for scatter correction. The last row gives the average over all ROIs used.

Mean cts/pix	Max cts/pix	Min cts/pix	Int Unif
11789	20709	5661	57.1%
12451	18261	7326	42.7%
12139	17955	5661	52.1%
12391	18513	6084	50.5%
11921	20781	6039	55.0%
12138.2	19243.8	6154.2	51.5%

Using a just the lower energy window to estimate the scatter improves the integral uniformity slightly, although the values remain relatively high compared to the uncorrected images.

Table 4: Integral uniformity (15x15 pixel ROIs) using only the lower energy window for scatter correction. The last row gives the average over all ROIs used.

Mean cts/pix	Max cts/pix	Min cts/pix	Int Unif
13551	21951	67.32	53.1%
13896	19827	8163	41.7%
13227	21870	6732	52.9%
13999	19611	7785	43.2%
13512	20601	7614	46.0%
13637	20772	7405.2	47.4%

The noise level in the reconstructed images can be largely alleviated by filtering the images as is commonly done with clinical studies. However, the uniformity of the scatter corrected filtered images will still be greater than the uncorrected images simply because of the loss of counts incurred when the estimated scatter counts are subtracted.

A Wiener 3-D postfilter was applied to the same images resulting in a significant improvement in image noise. However, the scatter corrected filtered images still demonstrated greater non-uniformity and appeared noisier than the uncorrected filtered images.

Evaluation of the ACNP kidney phantom was performed qualitatively on both the filtered and unfiltered images. In all images, both defects were clearly visualized, although the scatter corrected images showed a lower count density and noise was increased significantly. Applying a Wiener 3-D postfilter improved the appearance of the images considerably.

Conclusion

Bibliography

1. Ogawa K, Harata Y, Ichihara T, Kubo A, Hashimoto S, A practical method for position dependent Compton scatter correction in single photon emission CT, IEEE Trans Nucl Med, 10:408-412 1991
2. Ichihara T, Ogawa K, Motomura N, Kubo A, Hashimoto S, Compton scatter compensation using the triple-energy window method for single and dual isotope SPECT, J Nucl Med 34:2216-2221 1993
3. Ljungberg M, King MA, Hademenos GJ, Strand SE, Comparison of four scatter correction methods using Monte Carlo simulated source distributions, J Nucl Med 35:143-151 1994
4. Buvat I, Rodrigues-Villafuerte M, Todd-Pokropek A, Benali H, Di Paola R, Comparative assessment of nine scatter correction methods based on spectra analysis using Monte Carlo simulations, J Nucl Med 36:1476-1488 1995