Longitudinal studies offer a wealth of information, particularly for diseases progression or development studies. In addition, smaller sample sizes are possible to accumulate significant data, as subject assessment is possible at multiple time points.
Recent longitudinal studies
In the study conducted by Namati et al.1 only 8 animals (5 treated; 3 controls) were necessary to identify and track the development of lung cancer tumours in their longitudinal study. Intubated A/J mice, a strain with high susceptibility to carcinogen-induced tumours,2 were assessed at month 2, 3 and 4 following cancer induction via urethane3. Subsequently, images were taken with a Micro-CT scanner as it has the capacity to differentiate tissues of differing density such as bone, cardiac or lung4. This is a well-established technique described in both anesthetized free-breathing and intubated, ventilated mice5,6 publications.
Consistent lung inflation pressures are critical to provide reproducible images and properly assess tumour size, as the impact of distending forces surrounding parenchymal tissues affects images and their subsequent analysis. This in mind, the flexiVent FX is well suited for these studies as it controls ventilation and offers precision breath holds when completing the Micro-CT imaging of the lung. Refinement of the anaesthetic and intubation protocols from previous studies allow a reduction in subject mortality from 30% to under 10%. This is a chief accomplishment as maintaining the necessary subject group size over the lifespan of the experiments is among the challenges posed by longitudinal experiments and has a large impact on the feasibility of any given study very early in the initial design stages1.
In Figure 7 below from Namati et al.‘s work, the progression of a tumour (indicated by the white arrow) in a single subject over the 3 timepoints is clear and quantifiable.
In addition, image-based analysis was found to be highly reproducible for tumour doubling rates. Tumours of a diameter of 0.11 to 0.2 mm in diameter were identified, an improvement over the typical minimum diameters which at the time of the study ranged between 0.2 and 0.5 mm7,8. Moreover, the tumour incidence rates were found to be consistent between imaging and histological analysis.
Longitudinal Studies with SCIREQ equipment
The SCIREQ flexiVent, inExpose and VivoFlow can all be used to enable and complement data collection in data collection at both single and longitudinal time points. Indeed, the in vivo evaluation of murine lungs continues to be a topic of great interest in the scientific community9. Visualizing the development of cancer over time, especially early stage tumour growth as done in Namati et al.’s study, showcases the importance and power of conducting longitudinal assessments in deepening scientific understanding of disease progression.
- 1Namati et al. 2010. Longitudinal assessment of lung cancer progression in the mouse using in vivo micro-CT imaging. Med. Phys.; 37(9): 4793-4805.
- 2Witschi et al. 2006. Lung tumours in 2 year old strain A/J mice exposed for 6 months to tobacco smoke. Cancer Letters; 241:64-68. DOI: 10.1016/j.canlet.2005.10.002
- 3Xu et al. (2016). Inflammation has a role in urethane-induced lung cancer in C57B/6J mice. Mol. Med. Reports; 14(4): 3323-3328. DOI: 10.3892/mmr.2016.5661
- 4Badea et al. (2008). In vivo small-animal imaging using micro-CT and digital subtraction angiography. Phys. Med. Biol.; 53(19): R319-R350.
- 5Ford et al. (2007). In vivo characterization of lung morphology and function in anesthetized free-breathing mice using micro-computed tomography. J. Appl. Physiol.; 102(5): 2046–2055.
- 6Sera et al. (2008). Development of high-resolution 4D in vivo-CT for visualization of cardiac and respiratory deformations of small animals,” Phys. Med. Biol. 53(16): 4285–4301.
- 7De Clerck et al. (2004). High-resolution x-ray microtomography for the detection of lung tumors in living mice. Neoplasia; 6(4): 374–379.
- 8Hori et al. (2008). Periodic analysis of urethane-induced pulmonary tumors in living A/J mice by respiration-gated x-ray microcomputed tomography. Cancer Sci. 99(9): 1774–1777.
- 9Gradl et al. (2019). Visualizing treatment delivery and deposition in mouse lungs using in vivo x-ray imaging. J. Cont. Release; 307(10): 282-291. DOI: 10.1016/j.jconrel.2019.06.035