Accuracy of PET/CT image registration

5.3.1. Accuracy of PET/CT image registration

5.3.1.1. Aim The aim of this test is to assess qualitatively the accuracy of the regis-

tration of the images obtained with the PET and CT scanners [48]. Since the fusion of PET and CT images assumes perfect registration of the two modalities, it is crucial to ensure that the two studies are registered in different parts of the FOV (axial and transaxial) for a reasonable range of patient weights. The accuracy of PET/CT image registration becomes even more important when considering the scanner in conjunction with radiotherapy applications. However, due to the complex interplay of different factors affecting the acquisition (e.g. table deflection and patient weight distribution), tration of the images obtained with the PET and CT scanners [48]. Since the fusion of PET and CT images assumes perfect registration of the two modalities, it is crucial to ensure that the two studies are registered in different parts of the FOV (axial and transaxial) for a reasonable range of patient weights. The accuracy of PET/CT image registration becomes even more important when considering the scanner in conjunction with radiotherapy applications. However, due to the complex interplay of different factors affecting the acquisition (e.g. table deflection and patient weight distribution),

This test is especially important if the images are to be used for the planning of radiation therapy treatment. In general, PET/CT systems are supplied with a special PET/CT offset procedure for the initial establishment, and subsequent checks, of the registration of the PET and CT fields of view. The procedure described here may be useful as an alternative means of verifying the accuracy of registration under the influence of the factors mentioned above.

5.3.1.2. Frequency The PET/CT image registration accuracy test must be performed by a

qualified medical physicist at the time of acceptance testing, as part of the end- of-warranty testing, and whenever the CT system is serviced in a way that might have an impact on image registration accuracy, including, but not limited to, servicing of the table, after separating the PET and CT gantries for servicing, system calibration and software changes or upgrades.

5.3.1.3. Materials The phantom needed for this test is the ‘image quality phantom’

described in IEC Standard 61675-1 [40], which is also used to assess image quality and accuracy of attenuation and scatter corrections during acceptance testing (Fig. 27). The phantom consists of:

(a) A ‘body compartment’ that is at least 18 cm in interior length in order to cover the whole axial FOV of the PET scanner; (b) Six hollow spheres with internal diameters of 1.0, 1.3, 1.7, 2.2, 2.8 and

3.7 cm, and a wall thickness of no more that 1 mm; (c)

A cylindrical insert (5.0 ± 0.2 cm outside diameter) filled with a material of low atomic number that mimics lung attenuation (average density: 0.3 ± 0.1 g/mL), is centred inside the body compartment, and extends axially through the entire phantom.

In addition to the image quality phantom, this test requires the use of heavy weights (total weight of about 100 kg) to mimic the scanning of a heavy patient. Lead bricks or other heavy materials can be used for this purpose.

The body compartment shall be filled with an 18 F solution of 5.3 kBq/mL radioactivity concentration. The 2.8 and 3.7 cm spheres shall be filled with cold water to mimic cold lesion imaging. The 1.0, 1.3, 1.7 and 2.2 cm spheres shall be

FIG. 27. Image quality phantom (IEC standard 61675-1 [40]). The phantom material is polymethylmethacrylate. This diagram is not to scale.

filled with an 18 F solution that is 8 times hotter than the background (sphere:background = 8:1), i.e. with a concentration of 42.4 kBq/mL. If a lower dose is recommended by the manufacturer for whole body scans, then a lower radioactivity concentration than 5.3 kBq/mL could be used and reported for this test. A practical approach to accurately achieve the 8:1 sphere-to- background ratio without spills is to fill the body compartment with an eighth of its total volume, add the radioactivity intended for the background compartment, use this solution to fill the spheres and then fill the body compartment with water and cover the phantom with the lid to which the spheres are attached. The spheres shall be positioned in such a manner that the centres of all spheres shall be in the same transverse slice, at a 5.72 cm radius from the centre of the phantom, with the 1.7 cm sphere positioned along the horizontal axis of the phantom.

The lead bricks (or equivalent heavy weights) shall be uniformly distributed over 1.5 m length of the table adjacent to the quality phantom. The phantom shall be positioned at the end of the table in a supine position. The phantom shall

be positioned axially in the scanner so that the centre of the spheres is at the middle slice of the scanner, and positioned transaxially so that the centre of the phantom is centred in the scanner.

The phantom should also be aligned so that the plane through the centres of the spheres is coplanar to the middle slice of the scanner to within 3 mm throughout the length of the phantom. The lead bricks (or equivalent heavy weights) shall be uniformly distributed over a 1.5 m length on the table adjacent to the image quality phantom.

5.3.1.4. Data acquisition The phantom shall be scanned on the PET and CT scanners using a

modified version of the standard whole body protocol, in which the CT acquisition matrix is set to 512 × 512 and the PET acquisition matrix is set to 512 × 512, or, if those values are not available, to the largest values possible. Next, the lead bricks should be removed and a second whole body scan performed with the image quality phantom alone on the table.

5.3.1.5. Analysis Both whole body scans shall be reconstructed in the manner

recommended by the manufacturer for the standard protocol for whole body imaging, except for allowing for larger acquisition matrices (e.g. a PET recon- structed volume of 512 × 512 instead of the standard 128 × 128). The recon- structed CT and PET volumes shall be displayed simultaneously using the image fusion software provided by the manufacturer.

For both cases (in the presence and in the absence of heavy weights), the centres of all spheres shall be visually examined in all three directions on both PET and CT to ensure that they are adequately registered, spatially within

1 voxel. The edge of the phantom shall also be examined to ensure that the edge of the phantom, as seen on the PET scan, appropriately matches the phantom boundaries, as seen on the CT scan.

5.3.1.6. Suggested tolerances The user should set reference values, tolerances and action levels (i.e. to

trigger the decision to place a call for maintenance). An appropriate tolerance criteria for the accuracy of PET/CT registration is a registration within ±1 pixel (or ±1 mm, whichever is smaller) when using a 512 × 512 matrix.

5.3.1.7. Corrective action The accuracy of PET/CT registration is crucial for accurate attenuation

correction and lesion localization. Therefore, this accuracy should not deviate correction and lesion localization. Therefore, this accuracy should not deviate