Angiography is utilized to image blood vessels. Traditional angiographic techniques utilize direct injection of iodinated contrast into an artery and obtaining radiographs of the vessels of interest. Newer angiographic techniques have been developed which are non-invasive. MR Angiography utilizes special sequences to image blood vessels, and is especially helpful in analyzing brain and neck arteries. Contrast may or may not be used, depending on the circumstances. CT Angiography utilizes special reconstruction techniques following the intravenous administration of iodinated contrast. Doppler duplex ultrasound may be utilized to evaluate arteries and veins. This technique combines the standard ultrasongraphic imaging of a vessel in addition to determining the blood velocity to assess both the degree of narrowing and morphologic characteristics of the vessel. No contrast is used in this technique.
Arthrograms are used to evaluate the internal characteristics of the joints. A needle in introduced into a joint, and iodinated contrast is injected. Radiographs are obtained. Arthrograms may detect ligament tears or cartilage abnormalities.
A radiograph (also known commonly as plain film or x-ray) is an image obtained by shining high energy light or x-rays through the body to a detector which may either be photographic film or a photo-sensitive plate in newer digital image systems. The body absorbs different amounts of radiation depending on the density of the tissue. Dense tissues such as bone absorbs more radiation and appears light, and less dense tissues such as lung absorb less radiation and appears dark. The image obtained of a body part therefore represents a shadowgram. Disease states often alter the absorption of a given tissue. For instance, pneumonia increases the density of lung and results in a lighter shadow on the radiograph corresponding to the area of infection. Conversely a fracture decreases the absorption of bone and is perceived as a dark line at the fracture site.
Computed Tomography (also known as CAT Scan, Computed Axial Tomography) is a technical advance over Conventional Tomography. A x-ray source and detector are moved about a focal plane in the body, as in Conventional Tomography, but a computer is used to generate an accurate cross-sectional image of the body. This powerful technique is used to evaluate pathology throughout the body. Administration of intravenous iodinated contrast aids in the evaluation of solid organ disease, for example metastatic lesions within the liver. Iodinated contrast may also be injected into the joints in the detection of ligament tears or other joint abnormalities.
Iodinated intravenous contrast is often used in body CT. It is especially helpful in the evaluation of neoplastic disease of the abdomen. Intravenous contrast enhanced chest CT is useful for the evaluation of mediastinal or hilar adenopathy. Iodinated contrast is generally safe but in rare incidences may cause an allergic type reaction, which may range from hives to life threatening anaphylaxis. Life threatening reactions are extremely rare. Reactions to intravenous iodinated contrast usually are self-limited and require no treatment; more severe reactions are treated at the site of the scan. Pre-medication with oral steroids reduces the incidence of contrast reactions in patients with known contrast allergies.
The use of intravenous gadolinium chelate contrast has been widely employed for specific applications. Contrast enhanced MRI is useful for determining scar versus recurring disk protrusion or bulge in the post-operative spine. Contrast enhanced imaging of the central nervous system is especially helpful in the evaluation of infection, white matter disease, and neoplastic disease. Contrast may be used in body imaging for tumor evaluation. Certain MR angiography techniques employ intravenous contrast administration. Dilute gadolinium contrast is used for MR arthrography.
Conventional tomograms (also known as plain film tomograms) are radiographs, which are used to evaluate a specific plane within the body by blurring out tissues above and below the plane of interest. The x-ray source and detector are moved about a focal plane centered in the body. The tissues above and below the plane of interest are blurred, and the focal plane remains sharp. This technique may be useful in determining fracture healing, evaluation of pulmonary nodules (though this has largely been replaced by Computed Tomography, see below), evaluating the kidneys in excretory urography, and evaluating the integrity of spinal fusion.
A CT arthrogram is similar to the plain film arthrogram discussed earlier. A needle is introduced into a joint and iodinated contrast is injected, outlining the joint capsule, ligaments, and articular surfaces. A CT is then obtained. Ligament tears and cartilage abnormalities are well demonstrated with this technique.
A diskogram is utilized to evaluate the internal architecture of a disk and to determine whether or not an individual disk is responsible for a patient's symptoms. A needle is introduced into an intervertebral disk under fluoroscopic guidance and iodinated contrast is injected into the disk. The patient is questioned as to whether the increased pressure within the disk caused by the contrast generates pain similar to the patient's usual symptoms. Radiographs are then obtained. The patient may then go onto CT imaging for a more precise evaluation of the disk and spinal canal.
There are several reasons for poor quality images with MR imaging. Perhaps the most common problem encountered is motion artifact, which may due to patient pain, claustrophobia, anxiety, or non-compliance. Obesity may result in poor scans secondary to decreased imaging signal and relatively increased image noise. The quality of a scan may also be affected by the intrinsic imaging abilities of a scanner. Low magnetic field strength may result in decreased resolution and image contrast. Metal distorts magnetic fields and may result in loss of imaging information adjacent to metallic objects. For instance, anatomic detail around spinal fusion hardware may be obscured, and similar areas of signal loss may be noted next to joint replacements, bullet fragments, or surgical clips.
In this technique a portion of the body is exposed to a continuous beam of x-ray radiation to generate a movie like image which is viewed on a TV monitor. This technique is helpful for the evaluation of motion of bones within joint, swallowing and gastro-intestinal studies, and evaluation of lung and diaphragm abnormalities. Fluoroscopy is used for accurate placement of needles for interventional procedures.
MRI is a powerful imaging technique which exploits the magnetic properties of hydrogen atoms within the body. This imaging technique uses magnetic fields and radio waves and does not employ ionizing radiation. MR is extremely helpful in evaluation of disorders of the central nervous system and muskuloskeletal system as well as for abdominal and chest imaging.
MR arthrography is useful for detection of ligament tears and cartilage abnormalities. A needle is introduced into the joints of interest under fluoroscopic guidance. Contrast material visible on MR imaging is injected, and the patient is then sent to the MR scanner to obtain images. Cartilage abnormalities, tendon tears, and ligament tears are well demonstrated with this technique.
The quality of an MR image improves with increasing strength of the imaging signal. Signal increases the closer an MR receiving antenna or coil is located to the body part imaged. Therefore, surface coils, which lie on the patient's skin, are extremely useful for looking at joints and other small anatomic regions requiring high-resolution imaging.
Myelography is utilized to evaluate for disk pathology and degenerative changes of the spine. A needle is introduced into the spinal canal under fluoroscopic guidance and iodinated contrast is injected into the thecal sac, which contains the nerve roots. Radiographs are then obtained. The patient may go on to CT imaging for a more precise evaluation of the disk and spinal canal. Please see CT myleogram.
Nuclear medicine imaging visualizes the distribution and uptake of radioactively labeled molecules called radiopharmecuticals. Different radiopharmecuticals are used depending on the body part or disease state being evaluated. A radiopharmecutical is typically injected intravenously, and after allowing an appropriate amount of time for target organ absorption the patient is imaged using a gamma camera, which detects the gamma radiation similar to x-rays emitted by the radiopharmecutical. Although the spatial resolution is limited relative to radiographic imaging techniques, this modality is unique in its ability to characterize the function of tissues. For instance, radioactive iodine is used to determine the ability of the thyroid gland to absorb iodine and also to evaluate increased or decreased function within a thyroid nodule, a characteristic helpful in assessing whether or not a thyroid nodule is potentially malignant. One of the most frequent uses of nuclear medicine is to evaluate skeletal abnormalities utilizing a bone scan. Bone scan radiopharmecuticals are taken up by osteoid, a compound present in newly forming bone. Many abnormalities of bone are associated with increased osteoid production, including fractures, infection, and tumors, and will show up as "hot spots" on a bone scan.
This technique utilizes sound waves to produce images. A transducer is placed on the skin, which generates high frequency sound. The sound penetrates the body and is returned as echoes at the interface of anatomic structures or areas of pathology. The echoes are received by the transducer and are combined to form an image. Ultrasound is excellent for evaluating the liver, gallbladder and kidneys, fetal development, and female pelvic disease.