The Rat Thoracic Ultrasound protocol: scanning technique and normal findings

Abstract

Respiratory diseases (especially pneumonia) are very common disorders in pet rats. The suspected diagnosis is mostly based on the clinical signs, thoracic auscultation, and thoracic radiography. However, auscultation is insensitive in determining the severity of the disease, and radiographs are often unremarkable. Non-cardiac thoracic ultrasonography is increasingly used in veterinary medicine; however, it has not been described in detail in rats. Thoracic ultrasonic examination was conducted on 400 client-owned conscious pet rats. The rats were examined in the period from June 2023 to August 2023 in two veterinary clinics. Due to the small size of the animal, different anatomical considerations, and different evaluation protocols, as well as to meet the optimal outcome of detailed thoracic ultrasound, a standard methodological protocol was developed, and the name RATTUS (Rat Thoracic Ultrasound) was proposed. Typical signs of normal RATTUS were described (bat sign, lung sliding, A-lines, abdominal curtain sign, ski jump sign, lung pulse, seashore sign in M-mode, and bamboo sign). The new evaluation of lung inflation symmetry by substernal access was also described. The methodical approach presented and the normal findings description are proposed to be used for a standard/routine thoracic ultrasound examination in pet rats.

Keywords: RATTUS; lung; pleura; rat; respiratory disease; thorax; ultrasonography.

Figure 1

RATTUS performance. The rat was placed on a towel with careful restraint, and the fur was not shaved. We used a lot of gel to obtain ideal contact. The examination commenced with the probe being placed perpendicular to the ribs and the marker pointing cranially (A—examination of the axillary line, first zone, B—examination of the scapular line, second zone—as is visible in the picture, both zones were examined without changing the placement of the probe). Then, the probe was turned 90° with the marker pointing towards the sternum (C). The whole thorax was clearly scanned. For a better demonstration, a Dumbo fuzz (hairless) rat was used.

Figure 2

Substernal access. The probe was placed perpendicularly to the sternum, with the sternum in the middle of the probe surface. The probe was moved from the cranial border (brachial muscles) to the caudal border (abdominal curtain sign) with special attention to the area of the heart. The marker was placed on the left side to obtain a standardised view.

Figure 3

The bat sign is an abstract sign established to help the examiner with lung surface orientation. The image obtained is called a ‘bat sign’, as the rib heads and pleural line resemble the wings and body of a bat.

Figure 4

A-lines are hyperechoic horizontal lines (yellow arrows) and are reverberation artefacts of the pleural line.

Figure 5

The abdominal curtain sign is visualised when the lung contracts over mediastinal or abdominal organs and hides the sonographic view into them as a curtain hides a window; hidden organs are again visible when the lung contracts during expiration.

Figure 6

The ski jump sign is visualised when the probe is placed parallel to the ribs, with the lung curving away from the sonogram, and this creates a sonographic picture like a ski jump.

Figure 7

The seashore sign is visualised in M-mode when pleura creates a horizontal hyperechoic line (the border between waves and sand). The aerated lung creates a sandy appearance, and the chest wall lying above creates stratifications. T-lines are vertical lines going from the pleural line to the bottom.

Figure 8

The bamboo sign is the sign when, between two A-lines, a smaller hypoechoic horizontal artefact is visible and is called sub-A lines or Pi-lines (white arrow, A-lines yellow arrow).

Figure 9

Reverberation artefact, where the subpleural line does not show anatomical lung tissue but a reverberation artefact of the chest wall is displayed above the pleura as A-lines. A-lines are reverberation artefacts of the pleura, and the space between the pleura and A-lines and the space between each A-line is a reverberation artefact of the chest wall.

Figure 10

Single B-line (arrow) on linear probe proven by changing depth (A−E—A: depth 2 cm, E: depth 9 cm). The micro-convex probe is usually recommended for vertical artefact assessment; however, by increasing the depth of the image, there is the ability to confirm that B-lines are ‘true’ B-lines and did not terminate before reaching the far field of the ultrasound image.