Image quality, accuracy of attenuation and scatter corrections

General

The performance, quality of a tomograph is determined by different aspects. It’s necessary to have a method to check and compare the usually image parameters in clinical-like conditions.

Purpose

The purpose of this measure is to simulate a whole body scan. The body has hot and cold lesions, which are in the measure spheres with different diameter and different activity concentration. The phantom has non-uniform attenuation, and activity is alsp present outside the scanner (outside the FOV). The contrast at cold and hot spots, and the homogeneity of the background on the picture are good quality parameters of a picture. From this measurement can be calculated the accuracy of the attenuation and the scatter correction as well.

Method

1. Symbols
Contrast:
Qxxx: one sphere contrast in a warm background
QH: hot sphere contrast
QC: cold sphere contrast
Background variability: used as part of the image quality measurement
Nxxx: parameters of image quality
Nj: coefficient of variation for all ROIs of size j in the image volume
Relative count error:
ΔC: the difference between the expected counts and the measured counts, expressed as a percentage
ΔC_lung: relative error in lung insert
Standard deviation
SD_XXX: used as part of the background variability measurement
SD_j: standard deviation for all ROIs of size j in the image volume

2. Radionuclide
₁₈F shall be used for this measurement. The activity concentration of the background shall be 5,3 kBq/cm³, this activity concentration means 10 mCi/70 liter (the volume of a man with 70 kg weight), as in an ordinary PET whole body scan. The volume of the phantom is ~9300 cm³ (if the spheres and lung insert are in). So around 50 MBq activity shall be injected in the background volume. The activities shall be measured in dose calibrator. The cold lesions are simulated with water, the hot lesions are simulated with 4, 8 times more activity concentration like the background. In this measurement the scatter phantom (used for the measures written 3. and 5. chapter of this document) simulates the human body outside the FOV. It has to contain 116 MBq activity. If a lower background activity is used in the phantom, then a corresponding lower activity shall be used in the line source, as well.

3. Source Distribution
The image quality phantom consists of four parts:
• body phantom (at least180 mm long)
• 6 fillable sphere (inside diameters: 10, 13, 17, 22, 28 and 37 mm, wall thickness: less than 1 mm)
• An insert to simulate the attenuation of the lung with a specific gravity 0.3±0.1 g/cm³. Outside diameter 50±2 mm, in the center of the body phantom
• The phantom simulating the human body outside the FOV (same as the phantom for scattered fraction measurement).

The two biggest spheres shall be filled with water, these give the cold lesions. The fourth small ones shall be filled with water mixed F-18, these gives the hot lesions. The distance between the centers of the spheres and the end of the phantom shall be 68 mm, and they have to be in one transverse slice. The centers of the spheres shall be 5.72 cm far away from the center of the phantom. The phantom shall be positioned that the sphere with the diameter of 17 mm is along the horizontal axis. The volumes of the spheres and the activity are shown in the following table:

The body phantom filled with the appropriate activities shall be placed on the patient bed. It shall to be positioned parallel with the long axis of the equipment, the centers of the spheres have to be in the center of the FOV. The phantom simulating the human body outside the FOV shall to be placed direct next to the body phantom on the patient bed.

Data collection

 
The acquisition time is determined by the axial step of the tomograph during whole body scan in normal clinical use. The whole body length is 100 cm and this could be imagined in 60 minutes. The acquisition time of this measure is proportional as follows:

It’s offered to measure the whole measurements several times (corrected with the decay) to improve the reliability of the results.

Data processing

 
All slices shall be reconstructed with all available corrections. Images shall be reconstructed using the standard parameters (image matrix size, pixel size…) for whole body studies. The parameters of the reconstruction shall be reported.

Analysis

1. Image quality
A transverse slice crossing hot and cold lesions shall be used for the analysis. The same slice shall be used for all the spheres. ROI shall be drawn on the hot and the cold spheres, circular ROI, same diameter as the sphere.
ROIs of the same sizes as the ROIs drawn on the hot and cold spheres shall be frawn in the background of the phantom on the slice centered on the spheres. Twelve 37 mm diameter ROIs shall be drawn throughout the background at a distance of 15 mm from the edge of the phantom but no closer than 15 mm to any sphere. ROIs of the smaller sizes shall be drawn concentric to the 37 mm background ROIs. The ROIs shall also be drawn on the slices as close as possible (± 2 cm) on either side of the central slice (5 slices with the center slice). The average counts in each background ROI shall be recorded. The percent contrast (Q_H,j) for each hot sphere j is calculated as follows:

where:

• C_H,j: the average counts in the ROI of sphere j
• C_B,j: the average of the background ROI counts for sphere j
• a_H: the activity concentration in the hot spheres
• a_B: the activity concentration in the background

The percent contrast Q_C,j for each cold sphere j is calculated as follow:

where:

• C_C,j: the average counts in the ROI of sphere j
• C_B,j: the average of the 60 background ROI counts for sphere j.

The percent background variability (Nj) for sphere j:

where S_Dj is the standard deviation of the background ROI counts for sphere j:

2. Accuracy of attenuation and scatter corrections
A circular ROI, 30±2 mm in diameter shall be drawn centered on the lung insert. The average pixel value (C_lung,j) shall be calculated for each slice i. 12 circular background-ROIs, 30±2 mm in diameter shall be drawn on every slice.
To measure the residual error in scatter and attenuation corrections, the relative error (ΔC_lung,i) in percentage units for each slice i shall calculated as follows:

where:

• C_lung,i: the average counts in the lung insert ROI
• C_B,i: the average or the 60 37-mm background ROIs drawn for the image quality analysis

 

Report

The following items are to be reported:
• Background concentration used to fill the phantom
• Acquisition parameters
• The method of reconstruction
• The percent contrast and percent background variability for each sphere size and for both concentration ratios
• The value of ΔClung,i for each slice.
• For both activity ratio as an image (the transverse image through the center of all the spheres) and a coronal image through the center of the 17 mm sphere.

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