Quality control of micro-computed tomography systems

The importance of routine quality control for reproducible pulmonary measurements by in vivo micro-CT

Micro-computed tomography (CT) imaging provides densitometric and functional assessment of lung diseases in animal models, playing a key role either in understanding disease progression or in drug discovery studies. The generation of reliable and reproducible experimental data is strictly dependent on a system's stability. Quality controls (QC) are essential to monitor micro-CT performance but, although QC procedures are standardized and routinely employed in clinical practice, detailed guidelines for preclinical imaging are lacking. In this work, we propose a routine QC protocol for in vivo micro-CT, based on three commercial phantoms. To investigate the impact of a detected scanner drift on image post-processing, a retrospective analysis using twenty-two healthy mice was performed and lung density histograms used to compare the area under curve (AUC), the skewness and the kurtosis before and after the drift. As expected, statistically significant differences were found for all the selected parameters [AUC 532 ± 31 vs. 420 ± 38 (p < 0.001); skewness 2.3 ± 0.1 vs. 2.5 ± 0.1 (p < 0.001) and kurtosis 4.2 ± 0.3 vs. 5.1 ± 0.5 (p < 0.001)], confirming the importance of the designed QC procedure to obtain a reliable longitudinal quantification of disease progression and drug efficacy evaluation.

Human fetal whole-body postmortem microfocus computed tomographic imaging

Perinatal autopsy is the standard method for investigating fetal death; however, it requires dissection of the fetus. Human fetal microfocus computed tomography (micro-CT) provides a generally more acceptable and less invasive imaging alternative for bereaved parents to determine the cause of early pregnancy loss compared with conventional autopsy techniques. In this protocol, we describe the four main stages required to image fetuses using micro-CT. Preparation of the fetus includes staining with the contrast agent potassium triiodide and takes 3-19 d, depending on the size of the fetus and the time taken to obtain consent for the procedure. Setup for imaging requires appropriate positioning of the fetus and takes 1 h. The actual imaging takes, on average, 2 h 40 min and involves initial test scans followed by high-definition diagnostic scans. Postimaging, 3 d are required to postprocess the fetus, including removal of the stain, and also to undertake artifact recognition and data transfer. This procedure produces high-resolution isotropic datasets, allowing for radio-pathological interpretations to be made and long-term digital archiving for re-review and data sharing, where required. The protocol can be undertaken following appropriate training, which includes both the use of micro-CT techniques and handling of postmortem tissue.

Investigation of optimal sample preparation conditions with potassium triiodide and optimal imaging settings for microfocus computed tomography of excised cat hearts

OBJECTIVE

To determine optimal sample preparation conditions with potassium triiodide (I₂KI) and optimal imaging settings for microfocus CT (micro-CT) of excised cat hearts.

SAMPLE

7 excised hearts (weight range, 10 to 17.6 g) obtained from healthy adult cats after euthanasia by IV injection of pentobarbital sodium.

PROCEDURES

Following excision, the hearts were preserved in 10% formaldehyde solution. Six hearts were immersed in 1.25% I₂KI solution (n = 3) or 2.5% I₂KI solution (3) for a 12-day period. Micro-CT images were acquired at time 0 (prior to iodination) then approximately every 24 and 48 hours thereafter to determine optimal sample preparation conditions (ie, immersion time and concentration of I₂KI solution). Identified optimal conditions were then used to prepare the seventh heart for imaging; changes in voltage, current, exposure time, and gain on image quality were evaluated to determine optimal settings (ie, maximal signal-to-noise and contrast-to-noise ratios). Images were obtained at a voxel resolution of 30 μm. A detailed morphological assessment of the main cardiac structures of the seventh heart was then performed.

RESULTS

Immersion in 2.5% I₂KI solution for 48 hours was optimal for sample preparation. The optimal imaging conditions included a tube voltage of 100 kV, current of 150 μA, and exposure time of 354 milliseconds; scan duration was 12 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results provided an optimal micro-CT imaging protocol for excised cat hearts prepared with I₂KI solution that could serve as a basis for future studies of micro-CT for high resolution 3-D imaging of cat hearts.

A multipurpose quality assurance phantom for the small animal radiation research platform (SARRP)

In this work, the suitability and performance of a mouse-size MOSFET (Mousefet) phantom is investigated for routine quality assurance (QA) of the small animal radiation research platform (SARRP). This Mousefet phantom is a simple construction consisting of five micro-MOSFETS custom integrated in a quincunx pattern within a tissue-equivalent phantom, allowing repeat/multiple QA tasks to be quickly performed in one experimental set-up. The Mousefet phantom is particularly evaluated for facilitating SARRP QA tasks which may warrant daily evaluation, including output constancy, isocenter congruency test and cone beam computed tomography (CBCT) image geometric accuracy. Results for the output constancy measurements showed a maximum daily variation of less than 2.6% for all MOSFETS, in consonance with observations from concurrent ion chamber measurements. It is also shown that the design of the Mousefet phantom allows the output check data to be used for prompt verification of beam energy and cone profile constancy. For the isocenter congruency test, it is demonstrated that the Mousefet phantom can detect 0.3 mm deviations of the CBCT isocenter from the radiation isocenter. Meanwhile, results for CBCT image geometric accuracy were consistently found to be within 2% of the expected value. Other CBCT image quality parameters could also be assessed in terms of image intensity constancy, noise and image uniformity. Overall, the results establish the Mousefet phantom as a simple and time-efficient multipurpose tool that could be employed effectively for routine QA of the SARRP.

Micro-CT for anatomic referencing in PET and SPECT: radiation dose, biologic damage, and image quality

CT is widely used for anatomic referencing of PET and SPECT images of small animals but requires sufficiently high radiation doses capable of causing significant DNA damage. Therefore, we described the relationship between radiation dose, biologic damage, and image quality to determine whether CT can be used without significantly compromising radiotherapy and tumor development studies.

METHODS

The CT dose index generated by the nanoSPECT/CT system was compared with measurements using EBT2 gafchromic film. The effects of micro-CT were evaluated in 2 mouse strains that differ in sensitivity to radiation. γH2AX foci analysis to determine leukocyte, liver, and jejunum DNA damage and hematoxylin and eosin staining to investigate macroscopic jejunum damage were performed. Signal-to-noise ratio, contrast-to-noise ratio, and scanner linearity were determined to assess image quality.

RESULTS

For the standard settings, that is, as set by the manufacturers, EBT2 gafchromic film dosimetry showed that the nanoSPECT/CT system underestimated the absorbed dose. Moreover, significant doses were obtained, resulting in a significant increase in γH2AX formation in leukocytes, liver, and jejunum 40 min after CT, using preset parameters when compared with nonimaged controls. The jejenum response was more pronounced for the more radiosensitive strain. In contrast to leukocytes, the liver and jejunum still showed evidence of DNA damage 3 d after CT. Contrast-to-noise ratio, signal-to-noise ratio, and scanner linearity were sufficient to allow for anatomic referencing for both imaging protocols tested.

CONCLUSION

Anatomic reference images can be produced with no observable DNA damage or compromising image quality using low radiographic voltage, flux, and duration.