Imaging Modalities
Radiography
General Notes
- Produced by ionizing radiation
- Patient compliance is high: only metal objects (i.e. jewelry) need to be removed, image acquisition is fast and examination is painless (other than positioning potential injured areas)
- Radiograph can be available for interpretation within minutes of completing the examination. It is also highly inexpensive compared to other imaging modalities
Imaging Gradient
- The images produced are seen as a grey-scale image with various densities
- Radiographic film prior to exposure is entirely white when viewed on a board
- As X-rays strike the film it darkens. If tissue is struck prior to the film less X-rays reach the film
- The color on the film depends on the density of the aforementioned obstructing tissue
- The less dense the object the more X-rays penetrate through it producing a darker contrast on the film
- Densities range from (black to white):
- Gas (black)
- Fat
- Fluid/Soft tissue
- Calcification(bone)
- Heavy metal/radiographic constrast (white)
- Gravy boat imagery helps as a recall tool
- Place a gravy boat in the fridge. After time, air appears on top, then rendered fat, then the fluid of the gravy, followed by bone and finally the metal of the gravy boat
General Clinical Approach
- Labels: always insure you are reviewing the film of the correct patient with the correct date and time
- Previous exams: if possible, have a previous image for comparison. Can be very important to identify chronicity or acuity
- Film quality: try to insure the film is of adequate quality.
- A good quality film is neither over exposed (to dark) or underexposed (to white). Also, note whether the position of the patient is acceptable
Terminology
- Radiolucent: describes dark apperance (i.e. air)
- Radio-opaque: describes white apperance (i.e. bone)
- Under exposed: the film is to white
- Over exposed: the film is to dark
Risks and Concerns
- Scattering of the beam which could expose other body tissues to radiation
- The effect of ionizing radiation on a patient is cumulative over the patient's life time
- Some radiosensitive areas include:
- Eye lenses
- Thyroid
- Gonads
- Bone Marrow
- In general, ionizing radiation is more harmful in a younger patient. Their cells are replicating more rapidly (more turnover), this increases the probability of a replication error
Ultrasound
General Notes
- Produced by sound waves directed by a transducer
- Transducer receives returning sound waves (echos) from the patient
- U/S is radiation free making it the image modality of choice during pregancy and often pediatrics
- U/S does not penetrate air and thus jelly is applied to eliminate the air interface
General Clinical Approach
- Labels: always insure you are reviewing the image of the correct patient with the correct date and time
- Previous exams: if possible, have a previous image for comparison. Can be very important to identify chronicity or acuity
Terminology
- Echogenic: echo producing material; the more echogenic a substance is the whiter it will appear (i.e. bone)
- Echolucent: echo non-producer; the more echolucent a substance is the darker it will appear (i.e. blood)
- Hyperechoic: material appears whiter than the surroundings
- Hypoechoic: material appears darker than the surroundings
- Anechoic: produces no echoes; completely black
- Near-field: part of the body closest to the probe; top half of the display
- Far-field: part of the body furthest from the probe; bottom half of the display
- Acoustic window: allows U/S waves to penetrate deep into the body without any interference/energy loss to surrounding tissues (i.e. full bladder or the liver)
Risks and Concerns
- Due to attenuation, not enough energy is reflected to produce an echo in obese individuals (image capture becomes difficult)
- May cause some discomfort for the patient depending on the type of U/S needed (i.e. transvaginal)
- Within the operating range used for diagnostic imaging there have been no known side-effects
Computer Tomography (CT)
General Notes
- Produced by ionizing radiation (higher amounts when compared to radiography)
- Requires a radiation source, a detector (spins around the patient) and a computer
- Relatively available, can be enhanced with contrast/manipulation
- The examination is quick and very good at evaluating bony structures
- Also effective for evaluation of lungs, chest and for tumor identification
- Equally effective compared to MRI for detection of acute intracranial hemorrhage
- Image orientation:
- The left side of the image is patient RIGHT (unless manipulated). Imagine your viewing the image from the patients feet
- Common planes
- Axial (transverse, the "slices" of the body)
- Note: data can be used to construct images in other (sagittal and coronal) planes
General Clinical Approach
- Labels: always insure you are reviewing the image of the correct patient with the correct date and time
- Previous exams: if possible, have a previous image for comparison. Can be very important to identify chronicity or acuity
Terminology
- Hyperdense: describes white appearance (i.e. acute hemorrhage)
- Hypodense: describes dark appearance (i.e. cerebrospinal fluid)
- Hounsfield Units (HU): measure of radiodensity on CT; ranges from -1000 to +1000; useful for distinguishing acute blood from bone for example
- -1000 (least dense structure)→Air
- -100→Fat
- 0→water/ CSF
- 20 to +40→Brain parenchyma
- +55 to +75→Acute blood
- +100s→Bone/Calcification
- 1000 (most dense structure)→Bone/Contrast
Risks and Concerns
- Higher radiation dose per study (cumulative effects of radiation)
- High cost to perform
- Patient concerns and allegic reaction to IV contrast (should investigate kidney function and previous reactions before administering contrast)
- Claustrophobia
- Size and weight restrictions (often pt.'s >350 lbs unable to use standard tables)
- MRI is more effective for soft tissue investigation and identification of ischemic stroke
Magnetic Resonance Imaging (MRI)
General Notes
- Produced by magnetic fields, thus no radiation exposure
- Best used to image soft tissue/ligamentous injuries. Most detailed soft tissue imaging available
- Common Planes:
- Axial (transverse, the standard "slices" of the body)
- Coronal (perpendicular with sagittal plane, splitting front from back)
- Sagittal (along the midline, splitting left from right)
- How to distinguish between T1 versus T2 images:
- CSF, Gray Matter, and Globes (eyes) appear Dark on T1 and White on T2 images
General Clinical Approach
- Labels: always insure you are reviewing the image of the correct patient with the correct date and time
- Previous exams: if possible, have a previous image for comparison. Can be very important to identify chronicity or acuity
Terminology
- Hyperintense: describes white appearance
- Hypointense: describes dark appearance
Risks and Concerns
- High cost to perform
- Not widely available
- Size and weight restrictions (often pt.'s >350 lbs unable to use standard tables)
- Claustrophobia (can require laying on hard surface in enclosed space for up to two hours)
- Patients with metallic foreign bodies or surgically inserted metallic devices (i.e. pacemakers, aneurysm clip) can not receive this study
- IV contrast reactions (investigate previous reactions and kidney function prior to adminstration)
- Side-effects can include accelerated bone healing, ventricular fibrillation, peripheral nerve stimulation and muscle twitching
- The magnet is ALWAYS ON; no metal objects are to come near the MRI
Nuclear Imaging
General Notes
- Radionuclides are given to patient via IV injection
- The bio-distribution of the nuclides are detected and utilized to formulate an image
- Radiation is detected in a crystal. The radiation is being emitted from the patient and in this case the rays are in fact originating from within the patient as compared to modalities previously described
- Images are produced as more rays are detected from the areas with the most radionuclide absorbed
- Essentially no allergic reaction as very small doses are utilized
- Can be used to investigate function, allows for applied physiology as isotope is taken where the body is meant to take it
General Clinical Approach
- Labels: always insure you are reviewing the image of the correct patient with the correct date and time
- Previous exams: if possible, have a previous image for comparison. Can be very important to identify chronicity or acuity
Types of Imagery
- Bone Scan: Not actually an endocrine scan, but part of nuclear med department. Used for orientation of later scans
- Thyroid Scan: Thyroid absorbs iodine, and thus radioactive iodine (I-131) is used for imaging
- Parathyroid Scan: Imaging of the parathyroid can be done, however, issues are often investigated using clinical exam and U/S
- Pituitary Imaging: Most often carried out with CT and MRI to investigate for masses producing an excess of pituitary hormones
- Adrenal Imaging: Can perform scans using I-131 Iodocholesterol if standard imaging modalities demonstrate no masses with hyper or hypo adrenal symptoms
Risks and Concerns
- Images take time to collect
- Much of the radiation is lost, therefore, enough of the isotope must be given. However, excess must be avoided to limit radiation exposure
- Radiation, although often a minor exposure
- Lack of anatomical detail
- Cost is expensive as cameras are expensive, compounds are expensive and storage of radioactive compounds is expensive