CONTRAST ENHANCEMENT IN MAGNETIC RESONANCE IMAGING USING INTRAVENOUS PARAMAGNETIC CONTRAST MEDIA: A REVIEW

Interest in magnetic resonance (MR) imaging as a major diagnostic tool in veterinary clinical medicine is increasing. Most MR studies are performed with the use of contrast enhancement via intravenous injection of paramagnetic gadolinium-containing contrast media. A vast number of publications are available regarding the use of contrast media in humans. The purpose of this paper is to assist practicing veterinary radiologists in understanding mechanisms of MR contrast enhancement. This paper reviews certain aspects of MR contrast enhancement, including physical, chemical and biologic characteristics of most common MR contrast media targeted primarily at the central nervous system (CNS). Authors also describe processes that explain changes in signal intensity on the MR images.     

STANDING MYELOGRAPHY IN THE HORSE USING A NONIONIC CONTRAST AGENT

Standing myelography in the horse has been previously described. In that study, metrizamide was used and significant complications were reported. In recent years, the introduction of less-toxic nonionic contrast media has reduced the incidence of complications. This study was undertaken to determine whether standing myelography using a nonionic contrast medium could provide a diagnostic study and be performed safely in the equine patient. Standing myelography was performed in eight horses. The contrast medium used was iohexol. In five horses a myelogram of diagnostic quality was achieved; in one horse contrast flowed only to the level of C6 and in two horses contrast medium did not reach the cervical subarachnoid space. Owing to the difficulty in achieving good flow of the contrast medium in some horses, this procedure may be of limited utility. However, if puncture of the lumbosacral subarachnoid space can be achieved easily and quickly, standing myelography may be a clinically useful procedure. It may be attempted in cases in which the economic value of the patient makes myelography under general anesthesia impractical. In patients presenting for evaluation of ataxia it may be possible to perform a standing myelogram at the time of CSF sample collection from the lumbosacral space.     

EVALUATION OF IOHEXOL AS A GASTROINTESTINAL CONTRAST MEDIUM IN NORMAL CATS

The non-ionic, iodinated contrast medium, iohexol (240 mg I/ml) was evaluated as a gastrointestinal (GI) contrast medium in cats. Iohexol, both undiluted and diluted with tap water, was administered via a percutaneous endoscopically-placed gastrotomy (PEG) tube to 4 mature clinically normal cats. The dilution of contrast medium administered was 1:1, 1:2, and 1:3, and doses were 10 ml/kg and 5 ml/kg body weight. All combinations of dilution and dose of iohexol provided adequate visualization of the contrast medium column within the GI tract, and results were not significantly different than those observed using 30% w/v barium sulfate. Dehydration and diarrhea were not observed after contrast medium administration, but vomiting occurred within 15–30 minutes after administration of undiluted iohexol in all experimental cats. Renal opacification did not occur on exposures made through a 2 hour period, and dilution in transit was not apparent.     

INADVERTENT INTRATHECAL ADMINISTRATION OF IONIC CONTRAST MEDIUM TO A DOG

A hyperosmolar ionic contrast medium, ioxithalamate (Telebrix), was inadvertently injected intrathecally to a dog during myelography. The resultant severe adverse effects were myoclonus, uncontrollable seizures, and hyperthermia. These symptoms have been described by some authors as "ascending tonic-clonic seizure syndrome". The dog completely recovered within 24 h. The literature on 47 humans receiving intrathecal ionic contrast medium after 1966, one dog and one horse was reviewed, including the drugs involved, the circumstances of their use, and the symptoms, treatment and outcome of patients who received the drugs intrathecally. Recommendations to prevent such a misuse are given. The present report and review are a reminder that ionic contrast media are absolutely contraindicated for myelography. Only nonionic contrast media can be used intrathecally. All of the hyperosmolar contrast media are ionic and therefore contraindicated for myelography.     

TRANSOSSEOUS VERTEBRAL VENOGRAPHY: A DIAGNOSTIC AID IN LUMBOSACRAL DISEASE

Transosseous vertebral venography as a technic for investigating disease of the lumbosacral region is described. The technic preferred by the author is an intraosseous injection of contrast medium into the body of L7. Using a caudal needle, 10 ml of contrast medium was injected manually and a radiograph was made as the last milliliter was introduced. Contrast medium filled the lumbar venous sinuses and, by reflux, filled the sacral sinuses caudally to the S2–3 intervertebral foramen. Lumbosacral disease caused an obstruction to contrast medium flow and narrowed or deviated the vertebral venous sinus. Complications of transosseous vertebral venography were extravasation of contrast medium into the epidural space and surrounding soft tissue, and trauma to the sixth lumbar nerve in one dog. The procedure was contraindicated in the presence of local infection and when anesthesia was an undue risk.     

A radiographic study of urine flow in the domestic fowl

The movement of radiopaque media in the ureters, cloaca, colon and caeca was studied radiographically in 12 in‐lay light hybrid hens. During direct screening of the conscious birds after intravenous administration of sodium diacetamidotriiodobenzoate, the contrast medium was observed to pass along the ureters to their distal extremity under the action of ureteral contractions. It accumulated in the coprodeum, which remained quiescent in some birds, but in others, periods of vigorous contraction of the coprodeum forced the material cranially into the colon. Retroperistaltic contractions of the colon which appeared to originate in the coprodeum were then seen to promote further retrograde movement and on several occasions the contrast medium entered the caeca. Retrograde movement of a suspension of barium sulphate from the coprodeum into the colon and caeca was also observed, but neither of the contrast media entered the ileum. Post‐mortem radiographs of the viscera confirmed the presence of the contrast media in the cloaca, colon and caeca. The findings are illustrated by radiographs and the contribution of retrograde flow to caecal filling is discussed.     

A RADIOGRAPHIC STUDY OF THE DISTAL INTERPHALANGEAL JOINT AND NAVICULAR BURSA OF THE HORSE

Radiographic contrast studies were used in 50 forelimbs from 13 live horses and 12 fresh adult cadavers to determine the frequency of communication between the navicular bursa and the distal interphalangeal joint. Injections of contrast medium were made into the dorsal aspect of the distal interphalangeal joint of one limb and into the navicular bursa of the other forelimb of each horse. In 25 limbs in which contrast medium was injected into the distal interphalangeal joint, no communication was demonstrated between the joint and the navicular bursa. In 20 of the 25 limbs in which injection was made into the navicular bursa, no communication between joint and bursa was seen. In five horses, contrast medium was visible in both the distal interphalangeal joint and the navicular bursa. However, in four of five horses the communication was clearly iatrogenic. In both limbs of one horse, contrast medium was seen to enter the digital flexor tendon sheath after injection into the navicular bursa.
There is probably no naturally occurring communication between the navicular bursa and distal interphalangeal joint in the horse.     

USE OF IOHEXOL AS A GASTROINTESTINAL CONTRAST MEDIUM IN THE DOG

Iohexol was administered orally in five dogs. The dose, gastrointestinal (GI) transit time, appearance of mucosal patterns and side effects were studied. Three different doses (525, 700, 875 mgI/kg) were used in each dog at 1-week intervals. GI transit time was rapid. In each dose, gastric emptying commenced immediately after administration of the contrast medium, and was completed within 30–60 min with doses of 525–700 mgI/kg and 90–120 min with 875 mgI/kg. Large intestinal filling was observed within 60-90 min. In the majority of studies, the mucosal border appeared as a thin homogeneous halo of lucency surrounding the more opaque bowel lumen contents. The contrast intensity was not adequate with the lowest dose. The image quality did not deteriorate along the GI tract. No adverse reactions were found. Iohexol is an alternative GI contrast medium in the dog when contrast media are contraindicated.     

A REVIEW OF NONCONVENTIONAL ULTRASOUND TECHNIQUES AND CONTRAST-ENHANCED ULTRASONOGRAPHY OF NONCARDIAC CANINE DISORDERS

Modern ultrasound contrast media are gas-containing stabilized microbubbles that remain intact in the circulating blood for several minutes after intravenous injection and increase the intensity of the backscattered ultrasound. When the microbubbles disappear from the blood, they can be detected in the parenchyma of the liver and the spleen for about 30 more minutes (late liver- and spleen-specific phase). The insonated microbubbles produce second harmonic ultrasound frequencies, whose detection requires nonconventional ultrasound modalities such as pulsed inversion imaging. Nonconventional ultrasound techniques can also be used without microbubbles because second harmonics can be generated by ultrasound in tissues as well. The physical principles and advantages of nonconventional ultrasound techniques are described. The circulating microbubbles can be used not only to enhance weak Doppler signals, but also to perform dynamic contrast studies. Contrast-enhanced dynamic ultrasound studies--similar to contrast-enhanced CT and MRI examinations--have been used in humans to characterize lesions noninvasively (i.e., without biopsies) found during conventional ultrasound examinations. To map the distribution of contrast medium in a nodule or in an organ, specific scanning techniques such as stimulated acoustic emission have been developed. Stimulated acoustic emission occurs when high acoustic pressure ultrasonic waves disrupt the stationary or slowly moving microbubbles. This results in the release of a large amount of harmonic ultrasound frequencies. When the stimulated acoustic emission technique is used for dynamic studies, scanning must be interrupted several times to allow the microvasculature of the lesion to refill with microbubbles (interval delay imaging). The contrast patterns of malignant and benign hepatic nodules in humans have been the most intensively studied. Another type of dynamic study in humans measures the transit time of the contrast medium; that is, how fast the peripherally injected microbubbles reach the hepatic veins. Hepatic cirrhosis can be differentiated from other diffuse parenchymal liver diseases by a shorter transit time. Introducing nonconventional ultrasound techniques and ultrasound contrast media in veterinary diagnostic imaging may have potential value; however, intensive research should be carried out before ultrasound contrast agents can routinely be used in clinical practice.     

IOHEXOL AS A GASTROINTESTINAL CONTRAST MEDIUM IN THE CAT

Iohexol was evaluated as a radiologic contrast medium in the gastrointestinal (GI) tract in cats. Three different doses (525, 700, 875 mg iodine/kg with an iodine concentration of 300 mg iodine/mL) diluted with tap water until a total volume of 10 ml/kg, were administered via an orogastric tube, to 5 cats at weekly intervals. The GI transit time was rapid and variable. Gastric emptying commenced immediately after administration of the contrast medium and was complete within 10–30 min. In each dose, iohexol reached the large intestine within 10–20 min. In 73% (11/15) of studies, the mucosal border appeared as a thin homogeneous "halo" of lucency surrounding the more opaque contents of the small intestine. Radiographic image quality of the GI tract was inadequate with the lowest dose (525 mg iodine/kg). Image quality did not deteriorate along the GI tract. Absorption of iohexol from the GI tract was observed in 40% (6/15) of examinations, where opacification of the urinary bladder was seen. No side effects were observed. lohexol should be considered as an alternative GI contrast medium in the cat when the use of other radiologic contrast media is contraindicated.     

A TECHNIQUE FOR ULTRASOUND-GUIDED PARAVERTEBRAL BRACHIAL PLEXUS INJECTIONS IN DOGS

Using cadaveric dogs, we established the ultrasonographic landmarks for performing paravertebral injections around the brachial plexus nerve roots in the dog, and assessed the accuracy and regional spread of the aliquots. A mixture of methylene blue dye and an iodinated contrast medium was used as the injectate. A 0.3 ml volume was used to assess accuracy and a 3.0 ml volume was used to assess regional spread. Accuracy and regional spread were assessed from computed tomography (CT) images acquired after injection by measuring the distance from each foramen to the nearest edge of contrast medium, and the dimensions of spread of contrast medium in each anatomic plane, respectively. The mean distance of small volume injections from each foramen was 0.9 cm (standard deviation [SD] 0.56 cm). The mean spread of contrast medium for the small volume injections measured 1.7 cm (SD 0.6 cm) cranial-to-caudal, 1.2 cm (SD 0.4 cm) dorsal-to-ventral and 7.4 cm (SD 0.4 cm) medial-to-lateral. The mean spread of contrast medium for the combined three large volume injections measured 7.4 cm (SD 1.7 cm) cranial-to-caudal, 3.1 cm (SD 0.8 cm) medial-to-lateral, and 2.8 cm (SD 0.5 cm) dorsal-to-ventral. After the CT studies, staining of each nerve root and any other regional structure was assessed grossly. Based on our results, ultrasound can be used to guide injections around the nerve roots of the brachial plexus in dogs.     

HARMONIC ULTRASOUND: A REVIEW

Harmonic ultrasound is a technique based on the principle of transmitting at frequency f and receiving at frequency 2f (or 1/2f). This technology has become available through the development of wide-bandwidth transducers. Microbubble contrast media produce a large amount of harmonic signal. Contrast harmonic ultrasound provides the opportunity to image patterns of high flow vasculature and overall perfusion. Regions of poor perfusion, including necrosis or infarction, can be identified with contrast harmonic ultrasound. While proportionately lower, tissues also produce harmonic signals. Tissue harmonic ultrasound sequences often improve subjective image quality compared to fundamental ultrasound in echocardiographic and abdominal examinations. This review will discuss the physical principles of harmonic ultrasound signal generation, medical and animal research applications, and an overview of current veterinary experiences.     

THE FUNCTIONAL ASPECTS OF THE NEPHROGRAM IN EXCRETORY UROGRAPHY: A REVIEW

The nephrographic phase of the excretory urogram may be used to qualitatively assess some functional aspects of renal disease. Functional abnormalities of the kidney which alter the normal pattern of nephrographic opacification may be associated with renal perfusion abnormalities, glomerular dysfunction, intra or extrarenal obstruction, renal tubular necrosis and adverse reactions (renal or systemic) to iodinated contrast media. The time of maximum nephrographic opacification following intravenous injection of contrast medium and variations in nephrographic opacity before and after maximum opacification may help differentiate these disease processes. Standard radiographic and urographic techniques must be used to prevent changes caused by variations in contrast medium dosage and filming sequence from being misinterpreted as functional renal abnormalities.     

A study of ultrasound-guided perineural injection of the caudal cervical spinal nerve roots in equine cadavers

ObjectivesTo develop an ultrasound-guided cervical perineural injection technique for horses and to evaluate and compare the distribution of contrast agent among perineural, intra-articular and periarticular injections.Study designProspective, experimental cadaveric study.AnimalsA total of 14 equine cadaveric necks.MethodsBilateral ultrasound-guided perineural injection technique for the caudal cervical spinal nerve roots (CSNRs 5–7) was developed. Paramagnetic or iodinated contrast was injected and the distribution of contrast was evaluated using magnetic resonance (MR) or computed tomography (CT) imaging, respectively. The presence of contrast in the CSNR region was determined by an observer unaware of the technique used for each injection performed. The ability of the perineural injection technique to distribute contrast agent to the CSNR region was compared with intra-articular and periarticular injection techniques.ResultsPerineural injection delivered contrast agent to the CSNR region 100% of the time and was significantly different when compared with intra-articular injection (p = 0.008). There was no difference in ability to deliver contrast agent to the CSNR region between the perineural and periarticular injection techniques or between the intra-articular and periarticular injection techniques.Conclusion and clinical relevanceThe ultrasound-guided perineural injection technique developed in this study accurately delivered contrast agent to the CSNR region in equine cadavers. This technique could potentially be used for the diagnosis and treatment of cervical pain in horses, particularly in cases where intra-articular cervical articular process joint injections have not been beneficial. Further studies are necessary to assess the effectiveness of the ultrasound-guided perineural injection technique in live horses.     

TRAUMATIC SUBARACHNOID-PLEURAL FISTULA IN A DOG

Subarachnoid-pleural fistula is a rare occurrence in humans as a result of trauma or spinal surgery. Such fistulas commonly remain undiagnosed until sufficient cerebrospinal fluid accumulates in the pleural space to cause respiratory distress. We describe a subarachnoid-pleural fistula in a dog that occurred subsequent to blunt trauma sustained during a fall, with concurrent acute, traumatic intervertebral disc rupture. The extruded disc material penetrated the dura mater, allowing communication between the subarachnoid space and the extrapleural thoracic cavity. Radiographic, myelographic, and computed tomographic (CT) findings are reviewed. Abnormalities noted during myelography included an intradural-extramedullary lesion at T11-T12, with epidural leakage of contrast medium from the region of T12 extending cranially. In images from myelography and CT there was extravasation of contrast medium extending from the subarachnoid and epidural space into the extrapleural thoracic cavity.     

A REVIEW OF PORTAL SCREEN-FILM TECHNOLOGY AND FIVE RADIOLOGISTS' EVALUATIONS OF SOME EXISTING PRODUCTS

Portal radiographs, radiographs made to document the accuracy of radiotherapy treatment fields, are typically of poor image contrast. Recently, a new portal film and screened-cassette system was marketed, the Kodak EC-L system, with the claim of greatly improved image contrast. This new EC-L system was tested on a canine cadaver exposed to Cobalt-60 teletherapy gamma radiation, and image quality was compared to earlier marketed Kodak portal film products. The EC-L system was found to provide portal images of improved contrast/quality.     

IMAGING DIAGNOSIS—MAGNETIC RESONANCE IMAGING FEATURES OF A MULTIFOCAL OLIGODENDROGLIOMA IN THE SPINAL CORD AND BRAIN OF A DOG

An 8‐year‐old neutered male Toy Poodle was presented with chronic, progressive tetraparesis, and possible seizures. Magnetic resonance images demonstrated an extensive, T1 and T2 hyperintense contrast enhancing mass in the cervical spinal cord. Three nodules were present on the surface of the thalamus, with enhancement most evident on delayed images. A diagnosis of high‐grade oligodendroglioma was confirmed with postmortem histopathology and immunohistochemical labeling. Oligodendroglioma should be considered as a differential for T1 hyperintense intraaxial or intramedullary lesions with contrast enhancement. If enhancement is not visualized on postcontrast images, delayed images may be beneficial.     

POSITIVE CONTRAST RETROGRADE URETHROGRAPHY: A USEFUL PROCEDURE FOR EVALUATING URETHRAL DISORDERS IN THE DOG

Positive contrast retrograde urethrography proved to be a useful and relatively simple technic for the evaluation of lower urinary tract disorders in the dog. The case histories of 12 dogs with such disorders were used to illustrate the role of positive contrast retrograde urethrography in the evaluation of urethral and urinary bladder integrity after caudal abdominal and pelvic trauma, and in the investigation of incontinence, stranguria, dysuria, and hematuria.     

SPINAL SUBARACHNOID CYST IN A CAT

A 7-year-old neutered female domestic short hair cat was presented with hind limb ataxia. A subarachnoid cyst in the T10-T11 spinal cord region was identified by myelography as a collection of contrast medium in the subarachnoid space.     

IMAGING DIAGNOSIS: MAGNETIC RESONANCE IMAGING OF A CERVICAL WOODEN FOREIGN BODY IN A DOG

This article describes the discovery of a chronic cervical wooden foreign body ventral to the left transverse processes of the cranial cervical spine using magnetic resonance imaging (MRI) in a dog that presented with chronic neck pain and lameness. The dog did not exhibit dysphagia or chronic draining tracts, the most common signs of the presumed cause, that of a penetrating oropharyngeal foreign body. The foreign body itself was represented on MR images as an oval straight-edged core within an inflammatory tissue reaction. The wood was slightly hyperintense on T2- and isointense on T1-weighted images relative to muscle. Surrounding this was a more conspicuous contrast-enhancing reactive tissue rim that was hyperintense on all pulse sequences. Adjacent musculature also exhibited diffuse edema and contrast enhancement that extended around the left cervical vertebral transverse processes and local intervertebral nerve roots. The foreign body was found to be a wooden stick upon surgical removal. MRI is an excellent method for visualizing the inflammatory tissue reactions associated with soft-tissue foreign bodies because of its contrast resolution and depiction of anatomy in multiple imaging planes.