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A Comparison of Imaging Modalities



Radiography, MRI, CT, and Myelography are different types of medical imaging specialties equally important to the diagnostic process. Though their similarities and differences are vast and varied I've tried my very best to simplify it as much as possible for you. Here are some of the major PROS and CONS of these modalities.



Radiography (X-Ray):



PROS:

Inexpensive and readily available, radiography is useful for the diagnosis of fractures, bony malformations, advanced neoplasia, as well as significant changes associated with diseases of the lungs and abdominal organs.


CONS:

Optimal radiographic technique is critical for imaging subtle lesions. Radiography can be time consuming if you are working with an uncooperative patient and cannot give the patient a sedative. The resulting image is only available in two dimensions meaning that the tissue structures will be superimposed over each other. If separation of the structures is required then multiple radiographs at different angles with need to be acquired. It is inadequate for imaging delicate tissue structures such as the brain and spinal cord. This modality uses ionizing radiation to create the images, often requiring radiation exposure to personnel when they are holding the animal.



Computed Tomography (CT):



PROS:

The rapid acquisition time of CT usually only requires minimal sedation for optimal cooperation as opposed to general anesthesia. CT is the modality of choice for advanced imaging of bones and the lungs. It is an exception tool for evaluating trauma cases, pre-surgical planning, and the major organs of the body. Cross-sectional imaging allows for 3D reconstructions negating the issues of superimposition which plague conventional radiography.


Cons:

This imaging study is rarely able to be performed with the patient awake and requires sedation or general anesthesia as well as a preanesthetic work up. CT utilizes much larger radiation doses than conventional radiography though the staff should not be exposed during a CT. Only limited information of the brain and spinal cord can be provided since the tissue densities of grey and white matter are so similar. A CT scan often requires the administration of an iodinated contrast agent as part of the study. Though this adds a significant amount of diagnostic information there have been reported cases of allergic reactions to the iodine.



Magnetic Resonance Imaging (MRI):




Pros:

Magnetic resonance imaging provides exception soft-tissue detail and the ability to image in any plane. MRI does not utilize ionizing radiation for image production but instead uses radio waves and strong magnetic fields. This is the modality of choice for imaging the central nervous system, joints, specific organs, musculature, and other soft tissue structures. The resulting images can reveal pathologic characteristics of a disease process as opposed to just its presence or absence. This information can provide enough information to begin treating a disease without always requiring a biopsy or surgical diagnosis.


Cons:

Firstly, MRI is more expensive and time consuming than other modalities. An MRI always requires the patient to have a pre-anesthetic work up and go under general anesthesia for the study. The image acquisition process is susceptible to image artifacts from metallic implants. Any Metallic foreign body may pose a health risk to the patient an must always be discussed prior to imaging. Many studies require a contrast injection though it is not related to allergic reactions like the CT contrast it doesn't come without its own additional risks and cost.



Conventional Myelography:


Pros:

This study can be performed with any diagnostic radiography system making it readily available to many clinics. Though a bit "old school" it is good for a relatively quick overview of the entire spinal column. A myelogram can accurately identify areas of significant compression of the spinal cord.


Cons:

A conventional myelogram is an invasive procedure requiring an injection of a contrast material into the epidural space and ionizing radiation for the image acquisition. Due to the delicate nature of the injection it requires a sterile field and may be difficult to perform successfully on obese patients. A conventional myelogram does not provide information on the brain or the peripheral nerves. It is important to note that a non-invasive myelographic sequence is a standard part of every spinal MRI protocol.


MRI Myelogram of the thoraco-lumbar spine


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