Musculoskeletal Scintigraphy of the Equine Athlete

Equine Nuclear Medicine in 2024: Use and Value of Scintigraphy and PET in Equine Lameness Diagnosis

Scintigraphy and Positron Emission Tomography (PET) are both nuclear medicine imaging techniques, providing functional information of the imaged areas. Scintigraphy is a two-dimensional projected imaging technique that was introduced in equine imaging in the late 1970s. Scintigraphy allows imaging of large body parts and can cover multiple areas, remaining the only technique commonly used in horses for whole body imaging. PET is a cross-sectional imaging technique, first used in horses in 2015, allowing higher resolution three-dimensional functional imaging of the equine distal limb. This manuscript will cover current use and values of these two modalities in equine lameness diagnosis.

Equine skeletal scintigraphy: Comparing lesion detection ability of methylene diphosphonate and hydroxymethylene diphosphonate in the caudal cervical and proximal metacarpal/metatarsal regions

BACKGROUND

Data regarding the lesion detection ability of different radiotracers are lacking in equine bone scintigraphy.

METHODS

In this prospective study, hydroxymethylene diphosphonate (HMDP) and methylene diphosphonate (MDP) were compared in horses with increased radiopharmaceutical uptake either in the caudal cervical region (CS group) or in the proximal metacarpal/metatarsal region (PMR group). Region of interest analysis was used to determine normal bone-to-soft tissue ratios, lesion-to-normal bone ratios and lesion-to-soft tissue ratios. Qualitative scoring and total count rates were recorded for each image.

RESULTS

A total of 213 scintigrams were included. Within the PMR group, there were significantly higher lesion-to-normal bone ratios for MDP compared with HMDP (p = 0.02). In the CS group, normal bone-to-soft tissue ratios were significantly higher for HMDP (p = 0.01). The interobserver agreement with regard to the qualitative assessment of the scintigrams was poor.

LIMITATION

Paired studies, comparing the different radiotracers in the same patient, were not feasible.

CONCLUSION

This study revealed minor differences between the two radiotracers, although these have no practical implications. Both radiopharmaceuticals are well suited for detecting lesions at the investigated sites using equine bone scintigraphy.

Poor Association between Facial Expression and Mild Lameness in Thoroughbred Trot-Up Examinations

Musculoskeletal injuries in racehorses are difficult to detect prior to catastrophic breakdown. Lameness is commonly attributed to orthopaedic pain in horses, therefore, subtle lameness may be a pre-clinical sign of injury and, if identified early, could allow for preventative intervention. Our objective was to determine if facial expressions could be used to detect mild lameness as an indicator of orthopaedic pain in 'fit to race' horses. The Horse Grimace Scale (HGS) and the facial expressions in ridden horses (FEReq), were used to score images (n = 380) of mildly lame (n = 21) and non-lame (n = 17) Thoroughbred horses by two independent observers. Using an Equinosis Lameness Locator®, the lameness status of each horse was determined according to published thresholds [forelimb lameness (>|14.5 mm|) and hindlimb lameness (>|7.5 mm|)]. Inter and intraobserver reliability were assessed using two-way random-effects models. Univariable associations between lameness and facial expression parameters were identified using logistic and linear regression. Interobserver reliability was moderate (κ 0.45; 95% CI 0.36, 0.55). Horses with moderate mouth strain (HGS) and tense and extended upper lip (FEReq) were less likely to be lame (p = 0.042 and p = 0.027, respectively). Exposed sclera was associated with lameness (p = 0.045). Higher orbital tightening (HGS) scores were associated with a lower degree of maximum head amplitude (HDmax) lameness (p = 0.044). Tension and moderate tension above the eye, for the HGS and FEReq scores, were associated with increasing amplitude of HDmax (p = 0.048 and p = 0.034, respectively). Inconsistent associations between lameness status and HGS and FEReq scores may limit the potential use of the facial expression for the prediction of mild orthopaedic pain during pre-race lameness examinations. More objective parameters associated with mild orthopaedic pain should be explored.

Equine skeletal scintigraphy: Comparing normal bone-to-soft tissue ratio 2 and 4 hours after injection with either hydroxymethylene diphosphonate or methylene diphosphonate

BACKGROUND

In equine skeletal scintigraphy, there is no information about the possible influence of different phosphonate compounds on image quality.

METHODS

This prospective randomised study determined bone uptake changes and image quality for hydroxymethylene diphosphonate (HDP) and methylene diphosphonate (MDP) in equine patients at different time points. Scintigraphic images of the radius and the tibia of 20 horses were acquired at 2 and 4 hours after injection of either technetium-labelled HDP or MDP. Three regions of interest were identified-in the bone diaphysis, adjacent soft tissue and background area-to determine the normal bone-to-soft tissue ratio (BSR). Qualitative analysis was performed using a modified scoring system.

RESULTS

In terms of BSR and count rates, HDP showed a slightly better incorporation into bone compared to MDP, but significant differences were only detected for count rates at 4 hours after injection (p = 0.048). The radiopharmaceutical used did not influence qualitative image scoring, which was correlated with the BSR (ρ = 0.49; p ≤ 0.001).

CONCLUSION

The choice of HDP or MDP for equine skeletal scintigraphy does not seem to substantially affect BSR and qualitative image scoring. Further studies with a larger sample size, including the evaluation of lesion detection ability for both bone-seeking agents, are needed.

The effect of stress fracture occurring within the first 12 months of training on subsequent race performance in Thoroughbreds in Hong Kong

BACKGROUND

Racehorses are at an increased risk of stress fracture within the first 12 months of racing and when resuming training after a break. Research in these high-risk periods and on the effect of performance post-recovery is limited.

OBJECTIVES

To describe the occurrence of stress fractures, diagnosed by nuclear scintigraphy (NS), in racehorses' first 12 months training in Hong Kong, and their impact on racing performance and career length.

STUDY DESIGN

Retrospective 1:2 matched case-control study.

METHODS

Clinical records of horses with NS-diagnosed stress fractures within 365 days of import between 2006 and 2018 were collated. Cases and controls were matched on import date. Univariable conditional logistic regression compared signallment, pre-fracture training and post-recovery racing performance between cases and matched controls. Shared Frailty Cox regression analysed time from import to fracture and total career length.

RESULTS

Eighty-seven horses sustained a NS-diagnosed fracture within their first year in Hong Kong (incidence risk 1.7% [95% confidence interval (CI) 1.4%-2.1%; N = 5180]). The humerus (42.0%; 95% CI 31.8%-52.6%; n = 39) and tibia (28.0%; 95% CI 19.1%-38.2%; n = 26) were the most common stress fracture sites. Cases missed a median of 63 days (Interquartile range (IQR) 49-82) of training because of fracture. Within the 12 months following diagnosis, case horses had a median of four (IQR 2-4, P < .0001) fewer race starts and were down HK$ 206 188 (IQR HK$ 0-436 800, P = .007) in race earnings compared to controls. Career length did not significantly differ between cases and controls (median 2 years and 3 months; IQR 15.3-39.1 months; P = .2).

MAIN LIMITATIONS

Only stress fractures diagnosed by NS were included, hence, the study is not representative of all stress fractures occurring in racehorses in Hong Kong.

CONCLUSIONS

Racehorses sustaining a stress fracture within 1 year of entering Hong Kong lost significant time in training, earnings and race starts. However, overall career length was unaffected.

State-of-the-Art Diagnostic Methods to Diagnose Equine Spinal Disorders, With Special Reference to Transcranial Magnetic Stimulation and Transcranial Electrical Stimulation

Spinal cord disorders are a common problem in equine medicine. However, finding the site of the lesion is challenging for veterinarians because of a lack of sensitive diagnostic methods that can assess neuronal functional integrity in horses. Although medical imaging is frequently applied to help diagnose corticospinal disorders, this approach does not reveal functional information. For the latter, transcranial magnetic stimulation (TMS) and more recently transcranial electrical stimulation (TES) can be useful. These are brain stimulation techniques that create either magnetic or electrical fields passing through the motor cortex, inducing muscular responses, which can be recorded either intramuscularly or extramuscularly by needle or surface electrodes. This permits the evaluation of the functional integrity of the spinal motor tracts and the nerve conduction pathways. The interest in TES in human medicine emerged these last years because unlike TMS, TES tends to bypass the motor cortex of the brain and predominantly relies on direct activation of corticospinal and extrapyramidal axons. Results from human medicine have indicated that TMS and TES recordings are mildly if not at all affected by sedation. Therefore, this technique can be reliably used in human patients under either sedation or full anesthesia to assess functional integrity of the corticospinal and adjunct motor tracts. This opens important new avenues in equine medicine.

Thoracic processi spinosi findings agree among subjective, semiquantitative, and modified semiquantitative scintigraphic image evaluation methods and partially agree with clinical findings in horses with and without thoracolumbar pain

Impinging processi spinosi in the equine thoracic spine are a common cause of poor performance in the horse. A modified semiquantitative scintigraphic image analysis has been proposed for the evaluation of equine processi spinosi. This technique showed a high inter- and intraobserver agreement when compared to subjective and semiquantitative image analysis. The aim of this retrospective, method comparison study was to evaluate the agreement of the modified semiquantitative scintigraphic image assessment with previous methods of interpretation and to compare these scintigraphic evaluation techniques with radiographic and clinical findings. Two hundred twenty-three Warmblood horses that underwent scintigraphic, radiographic, and clinical examination of the thoracic spine were included in the study. Scintigraphic images were assessed using subjective, semiquantitative, and modified semiquantitative techniques. Radiographs were subjectively graded and horses were assigned to a group with or without thoracolumbar pain. Total radiographic and total scintigraphic grades were higher in horses with thoracolumbar pain (P < 0.05). Both the semiquantitative and the modified semiquantitative uptake ratios did not differ significantly in horses with or without thoracolumbar pain. The kappa agreement showed a substantial agreement between the modified semiquantitative scintigraphic and the semiquantitative scintigraphic evaluation techniques. The agreement between subjective scintigraphic and modified semiquantitative scintigraphic image evaluations was fair. There was a slight agreement between all scintigraphic techniques and radiographic findings. Limitations were the definition of thoracolumbar pain and the image analysis being restricted to the caudal thoracic processi spinosi. In conclusion, the modified semiquantitative scintigraphic image assessment obtained consistent results but did not perform better than previous evaluation methods. Further comparison to a defined diagnosis is warranted.

Semi-quantitative methods yield greater inter- and intraobserver agreement than subjective methods for interpreting 99m technetium-hydroxymethylene-diphosphonate uptake in equine thoracic processi spinosi

Scintigraphy is a standard diagnostic method for evaluating horses with back pain due to suspected thoracic processus spinosus pathology. Lesion detection is based on subjective or semi-quantitative assessments of increased uptake. This retrospective, analytical study is aimed to compare semi-quantitative and subjective methods in the evaluation of scintigraphic images of the processi spinosi in the equine thoracic spine. Scintigraphic images of 20 Warmblood horses, presented for assessment of orthopedic conditions between 2014 and 2016, were included in the study. Randomized, blinded image evaluation was performed by 11 veterinarians using subjective and semi-quantitative methods. Subjective grading was performed for the analysis of red-green-blue and grayscale scintigraphic images, which were presented in full-size or as masked images. For the semi-quantitative assessment, observers placed regions of interest over each processus spinosus. The uptake ratio of each processus spinosus in comparison to a reference region of interest was determined. Subsequently, a modified semi-quantitative calculation was developed whereby only the highest counts-per-pixel for a specified number of pixels was processed. Inter- and intraobserver agreement was calculated using intraclass correlation coefficients. Inter- and intraobserver intraclass correlation coefficients were 41.65% and 71.39%, respectively, for the subjective image assessment. Additionally, a correlation between intraobserver agreement, experience, and grayscale images was identified. The inter- and intraobserver agreement was significantly increased when using semi-quantitative analysis (97.35% and 98.36%, respectively) or the modified semi-quantitative calculation (98.61% and 98.82%, respectively). The proposed modified semi-quantitative technique showed a higher inter- and intraobserver agreement when compared to other methods, which makes it a useful tool for the analysis of scintigraphic images. The association of the findings from this study with clinical and radiological examinations requires further investigation.