Selection of density standard and X-ray tube settings for computed digital absorptiometry in horses using the k-means clustering algorithm

BACKGROUND

In veterinary medicine, conventional radiography is the first-choice method for most diagnostic imaging applications in both small animal and equine practice. One direction in its development is the integration of bone density evaluation and artificial intelligence-assisted clinical decision-making, which is expected to enhance and streamline veterinarians' daily practices. One such decision-support method is k-means clustering, a machine learning and data mining technique that can be used clinically to classify radiographic signs into healthy or affected clusters. The study aims to investigate whether the k-means clustering algorithm can differentiate cortical and trabecular bone in both healthy and affected horse limbs. Therefore, identifying the optimal computed digital absorptiometry parameters was necessary.

METHODS AND RESULTS

Five metal-made density standards, made of pure aluminum, aluminum alloy (duralumin), cuprum alloy, iron-nickel alloy, and iron-silicon alloy, and ten X-ray tube settings were evaluated for the radiographic imaging of equine distal limbs, including six healthy limbs and six with radiographic signs of osteoarthritis. Density standards were imaged using ten combinations of X-ray tube settings, ranging from 50 to 90 kV and 1.2 to 4.0 mAs. The relative density in Hounsfield units was firstly returned for both bone types and density standards, then compared, and finally used for clustering. In both healthy and osteoarthritis-affected limbs, the relative density of the long pastern bone (the proximal phalanx) differed between bone types, allowing the k-means clustering algorithm to successful differentiate cortical and trabecular bone.

CONCLUSION

Density standard made of duralumin, along with the 60 kV, 4.0 mAs X-ray tube settings, yielded the highest clustering metric values and was therefore considered optimal for further research. We believe that the identified optimal computed digital absorptiometry parameters may be recommended for further researches on the relative quantification of conventional radiographs and for distal limb examination in equine veterinary practice.

Comparison of 18F-sodium fluoride positron emission tomography and computed tomography for imaging of the fetlock in 25 nonracehorses

BACKGROUND

18F-sodium fluoride (18F-NaF) positron emission tomography (PET) has been validated as a useful imaging technique in the racehorse fetlock. The use of 18F-NaF PET in the nonracehorse fetlock has not been reported.

OBJECTIVES

To describe 18F-NaF PET findings in nonracehorse fetlocks, to compare with computed tomography (CT) findings and to compare PET findings between horses with and without fetlock pain.

STUDY DESIGN

Retrospective observational study.

METHODS

All horses undergoing 18F-NaF PET and CT imaging of the fetlock between October 2016 and March 2021 were included in the study. Medical records were reviewed for associated lameness information.

RESULTS

Thirty-six fetlocks (33 front and 3 hind) from 25 horses were included. The interobserver agreement for PET [0.62 (95% confidence interval [CI] 0.59-0.65)] was higher than for CT [0.47 (95% CI 0.43-0.51)]. Increased 18F-NaF uptake was most common in the medial subchondral bone of the proximal phalanx (23/36), the dorsomedial (20/36) and dorsosagittal (16/36) metacarpal/metatarsal distal subchondral bone, whereas sclerosis was identified on CT in these areas in 22 of 36 (p > 0.9), 33 of 36 (p = 0.001) and 23 of 36 (p = 0.15), respectively. Significant correlations were found between PET grades and both subchondral sclerosis and resorption CT grades (p < 0.001, Spearman r = 0.19 and 0.21, respectively). Abnormal 18F-NaF uptake in the proximal sesamoid bones (PSBs) was identified in 12 of 36 of the fetlocks, which was more common than CT abnormalities (3/36, p = 0.02). Periarticular increased uptake was seen in 7 of 36 fetlocks, whereas osteophytosis was recognised on CT in 20 of 36 (p = 0.1). Maximal standardised uptake values (SUVmax) were significantly higher in painful than in nonpainful fetlocks (22.0 and 11.9, respectively, p = 0.038).

MAIN LIMITATIONS

Retrospective study and limited sample size.

CONCLUSIONS

Overall PET and CT findings follow a similar distribution in their location. PET identified more abnormalities in the PSBs than CT. SUVmax is a pertinent factor to take into consideration for the assessment of the clinical significance of findings.

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.

Fractures in Thoroughbred racing and the potential for pre-race identification of horses at risk

Risk rates for and predisposing factors to fractures occurring in Thoroughbred racing that have been published in peer reviewed journals are documented. The potential for currently available techniques to identify horses at increased risk for fracture is discussed on the bases of principles, practicalities, advantages, disadvantages and current data. All are reviewed in light of justifiable decision making and importance of fractures to horseracing's social license.

Comparison of skeletal scintigraphy and standing 18 F-NaF positron emission tomography for imaging of the fetlock in 33 Thoroughbred racehorses

This retrospective, methods comparison study aimed to compare skeletal scintigraphy and ¹⁸ F-NaF positron emission tomography (PET) for the detection of abnormalities in the fetlocks of Thoroughbred racehorses. Thirty-three horses (72 limbs) imaged with both scintigraphy and ¹⁸ F-NaF PET, for investigation of lameness or poor performance related to the fetlock, were included. Seven observers, including experienced racetrack practitioners, surgery and imaging residents, and a board-certified veterinary radiologist, independently reviewed all data for evidence of increased radiopharmaceutical uptake in 10 different regions of interest. The interobserver agreement was higher for PET (Kappa-weighted (K-w) 0.73 (0.51-0.84)) (median (range)) than for scintigraphy (0.61 (0.40-0.77)) (P < 0.0001). When scintigraphy and PET were compared, the agreement was fair (K-w 0.29). More sites of increased uptake were identified using PET compared with scintigraphy. Agreement between the two modalities was higher for the palmar/plantar metacarpal/metatarsal condylar regions (K-w 0.59) than for the proximal sesamoid bones (K-w 0.25). Increased radiopharmaceutical uptake was detected in the medial proximal sesamoid bone in 6.9% and 22.2% of limbs with scintigraphy and PET, respectively. The high interobserver agreement for PET, despite the recent introduction of this technique, demonstrates the ease of clinical interpretation of PET scans. The higher number of lesions detected with PET compared with scintigraphy can be explained by the higher spatial resolution and cross-sectional nature of this modality. Study findings supported using PET in a clinical population of racehorses, in particular for the assessment of the proximal sesamoid bones.

Imaging techniques in veterinary medicine. Part II: Computed tomography, magnetic resonance imaging, nuclear medicine

Radiography and ultrasonography are the most used techniques in veterinary clinical practice, due to organizational, managerial and, mostly, economic reasons. However, in the last decades, Computed tomography (CT), Magnetic Resonance Imaging (MRI) and, to a lesser extent, Nuclear Medicine (MN) are increasingly used. As we said in the previous article, all the Diagnostic Imaging techniques are actually "indispensable" in Veterinary Medicine, where many patients do not show any symptoms.This second part describes Computed Tomography (CT), Magnetic Resonance (MRI) and Nuclear Medicine techniques in Veterinary Medicine are described.

Use of a novel helical fan beam imaging system for computed tomography of the distal limb in sedated standing horses: 167 cases (2019–2020)

OBJECTIVE

To evaluate the diagnostic capabilities of a novel helical fan beam CT system used for imaging of horses with a range of clinical distal limb problems.

ANIMALS

167 horses.

PROCEDURES

Medical records were reviewed of horses presented for CT of the distal limb at 2 university-based veterinary hospitals. The following data were recorded: age, sex, breed, presenting complaint, sedation used for imaging, scanning time, procedure time, other diagnostic imaging methods performed, imaging diagnosis, clinical diagnosis, and complications during imaging.

RESULTS

Most horses were Thoroughbreds and Quarter Horses. Procedure times ranged from 15 to 40 minutes, with scanning completed in 15 to 45 seconds for each region of interest. The foot or pastern region was commonly scanned (88/167 [53%] horses), with navicular bone disease diagnosed in 42 of 88 (48%) horses. The fetlock region was also commonly scanned (42/167 [40%] horses), with palmar or plantar osteochondral disease diagnosed in 17 of 42 (40%) horses. Horses were compliant during scanning, and no complications with sedation or damage to the scanner occurred. A specific imaging diagnosis for the lameness was achieved more frequently with CT imaging (166/167 [99%]) than with planar digital radiography (26/58 [45%]).

CLINICAL RELEVANCE

The helical fan beam CT system could be used safely to scan sedated standing horses from the carpal or tarsal region distally. Subjectively, the machine was easy to operate, allowing CT to be incorporated into lameness investigations. CT imaging was very likely to result in a clinical diagnosis in horses with distal limb lameness.

Application of Advanced Imaging Modalities in Veterinary Medicine: A Review

Veterinary anatomy has traditionally relied on detailed dissections to produce anatomical illustrations, but modern imaging modalities, now represent an enormous resource that allows for fast non-invasive visualizations in living animals for clinical and research purposes. In this review, advanced anatomical imaging modalities and their applications, safety issues, challenges, and future prospects of the techniques commonly employed for animal imaging would be highlighted. The quality of diagnostic imaging equipment in veterinary practice has greatly improved. Recent advances made in veterinary advanced imaging specifically about cross-sectional modalities (CT and MRI), nuclear medicine (PET, SPECT), and dual imaging modalities (PET/CT, PET/MR, and SPECT/CT) have become widely available, leading to greater demands and expectations from veterinary clients. These modalities allow for the creation of three-dimensional representations that can be of considerable value in the dissemination of clinical diagnosis and anatomical studies. Despite, the modern imaging modalities well established in developed countries across the globe, it is yet to remain in its infancy stage in veterinary practice in developing countries due to heavy initial investment and maintenance costs, lack of expert interpretation, a requirement of specialized technical staff and need of adjustable machines to accommodate the different range of animal sizes. Therefore, veterinarians should take advantage of these imaging techniques in designing future experiments by considering the availability of these varied imaging modalities and the creation of three-dimensional graphical representations of internal structures.

Validation of a dedicated positron emission tomography scanner for imaging of the distal limb of standing horses

A positron emission tomography (PET) scanner, with an openable ring of detectors, was specifically designed to image the distal limb of standing horses. The goals of this prospective, preclinical, experimental, methods comparison study were to validate the safety of the scanner, assess image quality, and optimize scanning protocols. Six research horses were imaged three times (twice standing, once anesthetized) and six horses in active race training were imaged once under standing sedation. Multiple scans of both front fetlocks were obtained with different scan durations and axial fields of view. A total of 94 fetlock scans were attempted and 90 provided images of diagnostic value. Radiotracer uptake was the main factor affecting image quality, while motion did not represent a major issue. Scan duration and field of view also affected image quality. Eight specific lesions were identified on PET images from anesthetized horses and were all also independently recognized on the PET images obtained on standing horses. Maximal standardized uptake values (SUVmax) had good repeatability for the assessment of specific lesions among different scans. Three feet and six carpi were also successfully imaged. This study validated the safety and practicality of a PET scanner specifically designed to image the distal limb in standing horses. Proper preparation of horses, similar to bone scintigraphy, is important for image quality. A 4-min scan with 12 cm field of view was considered optimal for clinical fetlock imaging. Evaluation of a larger clinical population is the next step for further assessment of the clinical utility of PET imaging in horses.

Imaging the Equine Foot

Over the past 5 years, advancements in diagnostic imaging technology have led to improvement of radiographic technique and development of standing computed tomography (CT) and PET-CT scanners. Although these modalities are in their initial stages of development and clinical applications, they are meant to revolutionize the diagnosis and management of diseases of the foot in the standing patient, in particular detecting subclinical lesions, and the establishment of computer-assisted surgical suits. This article also reviews the improved radiographic projections of the equine foot and benefits of high-field and contrast magnetic resonance imaging (MRI) in diagnosis of cartilage and ligamentous pathologies.

Equine flexor tendon imaging part 2: Current status and future directions in advanced diagnostic imaging, with focus on the deep digital flexor tendon

Flexor tendon injuries are a common cause of lameness and early retirement in equine athletes. While ultrasonography is most frequently utilised, advanced diagnostic imaging modalities are becoming more widely available for detection and monitoring of flexor tendon lesions. Part two of this literature review details current experience with low- and high-field magnetic resonance imaging (MRI) and computed tomography (CT) for the diagnosis of equine flexor tendinopathy with a focus on the deep digital flexor tendon. Implications of the 'magic angle' artefact as well as injection techniques and the use of contrast media are discussed. Future developments in tendon imaging aim to gain enhanced structural information about the tendon architecture with the prospect to prevent injury. Techniques as described for the assessment of the human Achilles tendon including ultra-high field MRI and positron emission tomography are highlighted.

Comparison of 18F-sodium fluoride positron emission tomography and CT: An exploratory study in 12 dogs with elbow pain

18F-Sodium Fluoride (18F-NaF) positron emission tomography (PET) provides high resolution functional information about bone activity and can be fused with CT images to improve three-dimensional localization and characterization of lesions. This prospective, observational study assessed 18F-NaF PET-CT for imaging of canine elbows, compared PET with CT findings, and assessed correlation with lameness. Twelve patients with elbow pain were included. Cases included primarily young, large breed dogs. A three-level clinical lameness score was assigned to each forelimb. All dogs had bilateral elbow joints imaged with CT and PET under general anesthesia, approximately 1.5 h after intravenous injection of 3 MBq/kg of 18F-NaF. Imaging findings were independently reviewed by two radiologists using a three-level scoring scheme over nine anatomical regions in the elbow. PET imaging identified areas of bone activity where minimal change was identified on CT. PET imaging also demonstrated absence of uptake in areas where modeling was present on CT. A stronger correlation was observed between clinical grades and PET scores (r2  = 0.38, P = .001) than between clinical grades and CT scores (r2  = 0.17, P = .048). The total PET scores were significantly different for each clinical grade (P = .013) but total CT scores did not differ (P = .139). This exploratory study suggests that PET improves the ability to detect lesions and to determine the clinical significance of CT findings in dogs with elbow pain.

18 Fluorine-fluorodeoxyglucose positron emission tomography for assessment of deep digital flexor tendinopathy: An exploratory study in eight horses with comparison to CT and MRI

Lesions of the deep digital flexor tendon (DDFT) are a cause for foot lameness in horses. Positron emission tomography (PET) could provide valuable information regarding the metabolic activity of these lesions. The aims of this exploratory, prospective, methods comparison study were to assess the ability of ¹⁸ fluorine-fluorodeoxyglucose (¹⁸ F-FDG) PET to detect DDFT lesions and to compare the PET findings with CT and MRI findings. Eight horses with lameness due to pain localized to the front feet were included. Both front limbs of all horses were imaged with ¹⁸ F-FDG PET, noncontrast CT, and arterial contrast-enhanced CT; 11 limbs were also assessed using MRI. Two observers graded independently ¹⁸ F-FDG PET, noncontrast CT, arterial contrast CT, T1-weighted (T1-w) MRI, and T2-weighted (T2-w)/STIR MRI. Maximal standardized uptake values were measured. Lesions were found in seven of 16 DDFT on PET, 12 of 16 DDFT on noncontrast CT, six of 15 DDFT on arterial contrast CT, eight of 11 DDFT on T1-w MRI, and six of 11 DDFT on T2-w/STIR MRI. Positron emission tomography was in better agreement with arterial contrast CT (Kappa-weighted 0.40) and T2-w/STIR MRI (0.35) than with noncontrast CT (0.28) and T1-w MRI (0.20). Maximal standardized uptake values of lesions ranged from 1.9 to 4.6 with a median of 3.1. Chronic lesions with scar tissues identified on noncontrast CT or T1-w MRI did not have increased ¹⁸ F-FDG uptake. These results demonstrated that ¹⁸ F-FDG PET agreed more closely with modalities previously used to detect active tendon lesions, i.e. arterial contrast CT and T2-w/STIR MRI. ¹⁸ Fluorine-fluorodeoxyglucose PET can be used to identify metabolically active DDFT lesions in horses.

Subchondral focal osteopenia associated with proximal sesamoid bone fracture in Thoroughbred racehorses

BACKGROUND

Proximal sesamoid bone (PSB) fracture is the most common fatal injury in Thoroughbred (TB) racehorses in the United States. Epidemiological and pathological evidence indicates PSB fracture is likely the acute culmination of a chronic stress-related process. However, the aetiopathogenesis of PSB fracture is poorly understood.

OBJECTIVE

To characterise bone abnormalities that precede PSB fracture.

STUDY DESIGN

Two retrospective case-control groups of PSBs from TB racehorses with, and without, unilateral biaxial PSB fracture.

METHODS

Proximal sesamoid bones were harvested post-mortem from TB racehorses subjected to euthanasia for unilateral biaxial PSB fracture (cases) or causes unrelated to PSB fracture (controls) while racing or training. The fractured medial PSB (FX-PSB) and contralateral intact medial PSB (CLI-PSB) from racehorses that sustained PSB fracture, and an intact medial PSB (CTRL-PSB) from racehorses that did not have a PSB fracture were collected as case and control specimens. Study 1 distributions of morphological features were compared among case and control groups using visual examination, photographs, radiographs and histology of whole PSBs and serial sagittal sections (10 FX-PSB, 10 CLI-PSB and 10 CTRL-PSB). Study 2 local bone volume fraction and mineral densities were compared among case and control PSBs using microcomputed tomography (9 FX-PSB, 9 CLI-PSB and 9 CTRL-PSB).

RESULTS

A focal subchondral lesion characterised by colocalised focal discoloration, radiolucency, osteopenia, low tissue mineral density and a surrounding region of dense cancellous bone was identified in most case horses but not in controls. This subchondral lesion was found in a slightly abaxial mid-body location and was bilaterally present in most case horses.

MAIN LIMITATIONS

The post-mortem samples may not represent the spectrum of abnormalities that occur throughout the development of the subchondral lesion. Lateral PSBs were not examined, so their contribution to biaxial PSB fracture pathogenesis is unknown.

CONCLUSION

Abaxial subchondral lesions are consistent with pre-existing injury and likely associated with PSB fracture.

Chondrosesamoidean ligament enthesopathy: Prevalence and findings in a population of lame horses imaged with positron emission tomography

BACKGROUND

Increased ¹⁸ F-Sodium Fluoride (¹⁸ F-NaF) uptake at the chondrosesamoidean ligament (ChSL) attachment on the distal phalanx was identified in an exploratory positron emission tomography (PET) study. The prevalence and significance of this lesion has not been previously investigated.

OBJECTIVES

The goal of this study was to assess the prevalence of this lesion, its association with other imaging findings and with clinical signs.

STUDY DESIGN

Retrospective cross-sectional analytical study.

METHODS

All horses with ¹⁸ F-NaF PET and computed tomography (CT) imaging of the feet performed between October 2016 and December 2017 were included in the study. All PET scans were independently assessed by two radiologists for increased uptake at the ChSL attachment site and concurrent imaging was reviewed. Clinical findings, treatment and outcome were retrieved from the medical records.

RESULTS

Fourteen of 30 horses (20/56 feet) had increased ¹⁸ F-NaF uptake in the region of interest. ChSL enthesopathy was the primary lesion in three horses. Other PET abnormalities included navicular bone uptake (13 feet) and ipsilateral palmar process uptake (9 feet). There was no significant association between ChSL enthesopathy and other lesions. ChSL enthesopathy was significantly associated with foot lameness. CT abnormalities at the ChSL attachment were initially identified in one foot, and retrospectively noted in another five following the results of PET imaging.

MAIN LIMITATIONS

The study is retrospective and there was a limited sample size.

CONCLUSIONS

PET led to identification of ChSL enthesopathy in a large proportion of horses with foot pain. This finding is most commonly associated with other lesions but may also represent the main abnormality. The axial border of the palmar processes of the distal phalanx should be carefully assessed on cross sectional imaging to identify this lesion. ChSL enthesopathy may be an important but previously not recognised component of foot pathology in horses.

Role of Positron Emission Tomography in Imaging of Non-neurologic Disorders of the Head, Neck, and Teeth in Veterinary Medicine

Positron Emission Tomography (PET) is an imaging technique that provides functional information, in addition to structural information obtained with computed tomography (CT). The most common application is cancer staging, using ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG), a radioactive analog of glucose. Although limited data are available in the veterinary literature, human studies have demonstrated benefit with the addition of PET both for assessment of the primary tumor and for detection of metastatic disease. ¹⁸F-FDG PET appears to be more accurate at detecting the margin of oral neoplasia, in particular for tumors arising from highly vascularized tissue, such as the lingual and laryngeal areas. ¹⁸F-FDG PET has a high sensitivity for the detection of lymph node metastasis, however the specificity is variable between studies. Tracers beyond ¹⁸F-FDG can also be used for oncology imaging. ¹⁸F-Fluoride (¹⁸F-NaF) is an excellent osseous tracer, useful in assessing bone involvement of primary tumors or osseous metastasis. Other specific tracers can be used to assess cell proliferation or hypoxia for tumor characterization. ¹⁸F-FDG is also an excellent tracer for detection of inflammation. Human studies have demonstrated its value for the assessment of periodontitis and dental implant infection. ¹⁸F-NaF has been used to assess disorders of the temporomandibular joint in the human literature, demonstrating good correlation with arthralgia and therapeutic outcome. Both ¹⁸F-NaF and ¹⁸F-FDG had good concordance with localization of cervical pain in people. PET will likely have a growing role in veterinary medicine not only for oncologic imaging but also for assessment of inflammation and pain.