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What is MRI?
A computer generated cross sectional image or “slice” of anatomy, based on the response of the various tissues within the slice to an intense magnetic field.
It is also a cross sectional image of anatomy. However, the characteristics of the image are based on the response of the various tissues within the slice to an intense magnetic field. Hence, MRI defines tissue characteristics, which yields more useful information about that tissue. Other modalities information based solely off attenuation of radiation.

Advantages and Disadvantages of MRI

  • MRI possesses superior soft tissue imaging.
  • Multi-planar image acquisition is possible. This means images can be acquired in any possible plan including sagittal, dorsal, axial, and oblique. Because no 2D reconstruction is required, there is no loss of information.
  • The spatial resolution within an MRI image is superior to all other imaging modalities.
  • There is no harmful ionizing radiation, which is paramount in humans and of no significance in animals. Because of this, the possibility of damaging effects (cancer induction, genetic defects, mutations) is null.


  • Cost. MRI is the most expensive commonly available imaging modality in veterinary medicine.
  • Time. Performing an MRI study is time consuming. However, with stronger magnets becoming more readily available, the study times are declining and are no longer considered a significant limiting factor in most patients.
  • Bone imaging is considered inferior to CT. This is true for cortical bone. MRI provides are great deal of information about the remainder of the tissues within the bone and is considered the modality of chose for certain disease processes. An example would be staging the extent of involvement of osteosarcoma within a patient considering a limb sparing procedure

When do I use MRI?
Appendicular Skeleton. Especially when only soft tissue swelling is present, radiographically.

Joints. Because of MRI’s superior soft tissue resolution, lesions involving soft tissues are clearly defined. MRI is an excellent modality for defining ligamentous and tendinous damage, partial vs. complete cranial crucial rupture, bone bruising (because of the accompanying subchondral edema and hemorrhage), and articular surface. MRI is sensitive for superficial soft tissue, musculature, capsular and synovial involvement, OCD lesion, and degenerative joint disease as well.

Axial Skeleton Spinal Disease. MRI is the gold standard in human medicine. Excellent for identifying subtle changes, especially when radiographs are normal. Severe soft tissue changes can occur before bony changes, making this a more sensitive modality than CT. Allows for differentiation between acute and chronic disease processes.

Neurological disorders. MRI is excellent for differentiating between congenital and acquired hydrocephalus and potentially identifying the cause. Anomalies such as Chiari malformation are easily identified.

 Acquired lesions. Including trauma related injuries (contusions, fractures, subacute and chronic hemorrhage, cerebral vascular accidents, spinal cord infarction, malacia and degenerative lesions.

 Abdomen: Portosystemic shunting. Standard protocols limited to the abdomen require less scan time than CT. Concurrent assessment of the brain for commonly accompanying alterations is possible. Intracranial changes may correlate with the shunt severity and influence surgical outcome.

Respiratory gated abdominal studies. Same studies as mentioned for CT with superior soft tissue detail. Longer acquisition times are required.

Thorax: Respiratory and cardiac gated thoracic studies. Relatively limited in veterinary medicine due to limited protocol design and scan time. Studies are useful
in assessing mediastinal, pulmonary, pleural and heart base pathology. Cardiac studies are also possible.

 CT and MRI Availability AIC cross-sectional imaging schedule is flexible and serve commonly available Monday through Friday evening. Our studies are performed at human diagnostic imaging facility utilizing state of the art equipment, which yields better image quality and shorter scan times. Shorter acquisition times mean less time spent under general anesthesia and therefore, less risk to patient.

What is CT?
A computer generated cross sectional image or “slice” of anatomy, which can be manipulated to accentuate a specific tissue density. The image is based on the x-ray beam attenuation characteristics of the different tissues within the field of view.

When and why would you use CT


  • CT provides superior bone imaging with excellent contrast resolution. Changes as small as 0.5% in contrast are easily detected with CT. On the contrary, radiographs require at least a 5% difference in x-ray attenuation before two adjacent tissues can be distinguished. Hence, radiographs are ten times worse than CT in regards to contrast.
  • CT study acquisition time is shorter than other cross sectional imaging modalities. In some instances, CT studies are faster than conventional radiography. Examples include CT bulla or nasal studies vs. skull radiographs, and three views of the thorax vs. a CT thoracic study, evaluating for pulmonary metastases.
  • The cost of CT is less than other cross sectional imaging modalities and in some instances, may be equal or less in cost than certain radiographic studies.
  • CT is capable of 2D and 3D computer reconstruction in sagittal, dorsal and oblique planes allowing for evaluation of a lesion through multiple plans. This yields a more complete study and aids in defining the type, exact positioning and extent of the lesion. This information is paramount for surgical planning, and protocol development (chemotherapeutic, radiation therapy, etc.)

To a limited extent, CT is capable of differentiating between certain soft tissue attenuating disease processes such as soft tissue tumors from cysts.

  • CT images possess adequate spatial resolution (the ability to tell two adjacent structure apart). However, the resolution is much less than radiographs.
  • Soft tissue resolution is inferior to MRI and superior to conventional radiography.

Image acquisition can only be performed in a single plane, usually axial. With smaller animals, multiple planes may be possible; however, this is time consuming and increases potential exposure rates to staff. The quality of the 2D and 3D reconstructions are limited to the quality of the scan (number of slices acquired, slice thickness, etc), and tend to be pixilated.

When should I use CT?
Axial Skeleton Bulla studies. CT is the modality of choice for differentiating infectious, inflammatory and neoplastic processes involving the ear. It can clearly delineate between otitis externa, otitis media, and neoplasia. It defines the extent of bone involvement, i.e. reactive ostitis, osteolysis.

Nasal and Sinus Studies. CT is superior to radiographs in defining destructive from non-destructive rhinitis, sinusitis and neoplasia, as well as the extent of involvement, and location. Also useful in the localization of foreign bodies and tooth root abscesses.

Brain (Benign and non-neoplastic) Pituitary dependent hyperadenocorticism. CT is helpful in differentiating between macro and microadenomas. This information is important in determining treatment. Macroadenomas respond well to radiation therapy. Most microadenomas are addressed medically.

Skull Hemorrhage. CT is the modality of choice for recent intracranial hemorrhage. There is a linear relationship between the amount of hemoglobin (Fe content) and x-ray beam attenuation. The hemoglobin content is highest within 24 hours of a bleed. Potential causes include trauma, DIC, neoplasia, anticoagulants (rodenticide toxicity), infectious (fungal granuloma formation with vascular erosion).

Trauma/Fractures. CT is excellent at defining the type and extent of fractures, as well as the extent of adjacent soft tissue involvement. This is important in determining prognosis, management (medical vs. surgical), and surgical planning.


Appendicular Skeleton Elbows Dysplasia. CT is superior to radiographs in defining lesions of the elbow. A CT scan is warranted in all young dogs with a history of persistent lameness localized to the elbow, especially if radiographs are normal. At most institutions, an elbow CT study is part of the routine protocol in all animals meeting this criteria. Lesions commonly diagnosed include fragmented medial coronoid process, ununitied anconeal process, OCD, DJD, joint incongruency and joint surface defects

Osteochondritis/Osteochondrosis Dissecans (OCD). MRI is excellent in defining the location of the lesion, extent of bone and soft tissue involvement and, if present, the severity of degenerative joint disease, all of which are important in determining prognosis. Radiographs will help determine whether a CT or MRI study is warranted. General guidelines: CT is used with bone changes, MRI when they are absent

Trauma. CT is superior to radiographs in defining small, non-displaced fractures (i.e. fissure fractures, cortical stress fractures), especially in the distal limbs (carpus, tarsus). In certain instances of acute trauma, initial radiographs may be unrewarding and would require 7-10 days before the fracture was visualized. In this situation, because of it’s superior contrast resolution and cross sectional capabilities, a CT of the effected bone would be warranted. CT also provided useful information regarding soft tissue/muscle trauma and is considered a viable option in debilitated, geriatric and potential anesthetic risk patients because of the rapid scan time or when cost maybe an issue.

AbdomenPortosystemic Shunts. MRI and contrast CT is a non-invasive, sensitive and specific means of identifying, localizing and determining the number of both intra and extrahepatic portosystemic shunts. MRI studies are actually faster than CT if limited to the abdomen. However, concurrent assessment of the brain is helpful, especially if the shunt is large. The extent of intracranial changes can influence the prognosis. Overall, both modalities can reduce surgical time while adding useful information.

Pancreatitis. The use of contrast CT in evaluating for pancreatitis is very sensitive in defining the effected region, the viability of the effected region, and whether surgery is indicated. CT is sensitive in differentiating between necrotizing and non-necrotizing pancreatitis. CT and ultrasound combined are extremely sensitive in determining between surgical and non-surgical pancreatitis.

Ectopic Ureters. Contrast CT is excellent for differentiating between unilateral vs. bilateral, intramural vs. extramural, and location of termination. This information is helpful in determining surgical approach; which in return reduces surgical time.

Mid abdominal masses. Based on tissue kinetics and the vasculature supply of different lesions, contrast CT is becoming increasingly accurate at differentiating between neoplastic, infectious and inflammatory processes. CT is also helpful in defining location, extent of involvement and surgical vs. non-surgical masses. Contrast wash-in and wash-out rate, intensity, and persistence are helpful in differentiating between neoplasia, abscessation, cyst and granuloma

Thorax Pulmonary Metastases. Comparing CT to conventional radiography, a recent human study revealed the following:

  • 4x more powerful than routine chest radiographs
  • Doubles the rate of early lung cancer detection
  • In certain cancer types, CT improves the 5 year survival rate from 48 to 78%
  • By the time pulmonary involvement is discovered on radiographs, so advanced, have 12-15% chance of survival
  • CT is considered the gold standard when evaluating for pulmonary metastases.
  • Other applications: Pulmonary thromboembolic disease (PTE), chylothorax, pulmonary contusions, fractures, bronchial or upper airway involvement, and chronic lower airway disease.


What is a “digital” radiograph?

Quite simply, it is a radiographic image that is made without the use of a conventional film-screen (CFS) system. These images are then store on a hard drive (or comparable digital storage media) and view on a monitor. Digital radiography embodies all system types including computed radiography (CR), digital radiography (direct radiography, DR) and hybrids.

Is DR imaging better than CFS?
No. The images produced by CFS are of equal or higher quality than those produced by any digital system. In fact, CFS is considered the gold standard by which all digital systems are measured.

 Then why go with digital?
DR offers many advantages over CFS. However, not all of these advantages apply to the veterinary field. Remember, an advantage is only truly advantageous or “real” if manifests in your particular practice. To better understand this, we will compare the top 10 advertised benefits of DR in human medicine and compare those to an average general veterinary practice:

  •  DR produces a ready to read image very quickly (3-8 seconds), which increases productivity in busy radiology departments (>20 cases per day). No Advantage. This would only be advantageous in large private practices, referral clinics and university settings.
  • DR allows for fewer repeat radiograph do to technique error (not positioning error). Advantage. A recent study found that 34% of repeat films were made because of technique error. This study is from the human field where extensive radiology technician training is commonplace. The percentage of repeat films due to technique error is undoubtfully higher in the veterinary field.
  • DR allows images to be sent through networked computers in hospital (intranet) as well as to other doctors/specialists in remote locations (internet) for consultation. Advantage.
  • DR offers increased dynamic range. This allows visualization of both soft tissue and bone details in the same exposure with post-processing manipulation of the image. Advantage.
  • With DR there are no films to lose. Images are preserved on a digital storage media (i.e. hard drive) forever. Advantage.
  • Stored DR images never change hence no information is lost. Unlike processed films which have the potential to degrade over time (i.e. brown films). Advantage.
  • DR does not require chemical processing, alleviating OSHA concerns. Advantage.
  • DR does not require film processing, eliminating the cost of film, chemicals, processor and processor maintenance, and time required to perform the maintenance. Advantage.
  • DR’s digital format allows for easy duplication for the client’s use and can be store on most mobile storage medias (i.e. floppy disk). Advantage.
  • DR eliminates the costs associated with film storage. Advantage. Hard drive space is cheap compared to physical storage space.

9 out of 10 advertised advantages of DR are directly applicable to the veterinary field.

How does DR work?

CR has many similarities to CFS radiography and is essentially used in the same manner. Both use cassette based phosphor screens, which absorb x-rays and promptly emit light. In CFS, the emitted light is use to create a latent image that is later chemically developed into a viewable image or radiograph. With CR, this light is useless. So, how does it work? The CR phosphor retains about 50% of the x-ray energy via electron trapping. This stored energy is in image form and is emitted or released when scanned by a laser. The intensity of the energy is proportional to the amount of attenuation. The released energy is recorded and displayed as a digital image. The CR image is available for interpretation in about the same time as CFS (approximately 90 seconds). The CR cassettes have the potential for thousands of exposures allowing for years of service. DR’s technology differs greatly from CR and CFS applications. Essentially, the only similarity is the x-ray tube. There are two types of DR, direct and indirect. Direct DR is a seamless process, using semiconductor material, which converts x-ray energy directly into electrical energy. The electrical energy is used to create a digital image. No light is emitted which is a major advantage of direct DR. Direct DR is the most expensive technology. Indirect DR has an added internal step and light emission, potentially leading to minute image degradation. Indirect DR is slightly less expensive than direct DR. Currently, most systems offered to the veterinary field are indirect DR. These systems or plates are retrofitted to work with your existing x-ray table. The DR plates are electronically erased and synchronized with the x-ray machine prior to each exposure. DR images are displayed within 3 to 8 seconds.

What is the difference between CR and DR?
One of the major advantages of both DR and CR is the inherently expanded latitude. So, what does this mean? In radiography, broader latitude means broader exposure flexibility. No more cumbersome CFS technique charts. All exposures are made using a select number of settings. Under and overexposed films are digital corrected, hence, as long as the animal is properly positioned, a repeat exposure is not needed. Increased latitude allows for high detail visualization of soft tissue structures and high contrast structures such as bone on the same image. The result is a decrease in personal exposure and an increased in profitability. The advantages to CR are the reusable detector plate, which can be used with any and multiple x-ray machines. With proper handling and care, the phosphor plates will last for years. There are no required x-ray table modifications. The cassettes can be used from tabletop, buck tray and positional radiography. The disadvantages are the increased time needed to produce an image (similar to film-screen systems) and the potential for cassette damage and cost of replacement. DR’s main advantage is time. Images are displayed rapidly allowing for essentially instantaneous assessment and positional correcting, if needed. DR’s biggest disadvantage is cost with the average system selling for approximately 2.5 to 3 times the price of CR. Another major disadvantage is the mounted or fixed detection plate, eliminating the ability to perform positional radiography and multiple machines.

So, is DR for me? If so, which system best suits my practice?
As always, cost is a major contributing factor when considering a technological investment such as DR. However, it is not considered paramount. The most important factor to look at is production or expected production. If the limiting step in your radiology department is time, then direct or indirect DR should be considered. If not, CR is a reliable, quality, alternative. Both CR and DR produce high quality images with only a negligible difference. If your practice is radiographing less than ten cases per day, CR is a smart choice.