OnExam

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

Common Image Interpretation

General Approach

  • When reviewing any image, make sure that:
    • Correct patient, date and views
    • Note the clinical description provided on the requisition
    • Compare to previous images, if available
    • Insure adequate exposure and patient positioning

Chest Radiograph

Imaging Directions

  • Views: PA, AP, Lateral

Image Interpretation

  • PA/AP view:
    • Image quality by assessing rotation, inspiration and penetration
      • Rotation: spinous process midpoint between clavicles
      • Inspiration: diaphragm at level of 7th anterior rib
      • Penetration: vertebrae must be visible behind the heart
    • When assessing AP images: remember to recognize that structures such as the heart, will appear larger than they are due to x-ray beam scattering and the direction of the beam
    • Begin by assessing the soft tissue structures outside the chest cavity (beginning outside the chest or inside is examiner dependent; both are acceptable)
      • Subcutaneous air
      • Swelling
    • Bones
      • Fractures
    • Alignment of the trachea and bronchi
      • Midline
      • Deviation (Left or Right)
    • Mediastinum and Aortic Knuckle
      • Widened
      • Obscured
    • Cardiac silhouette
      • Width not more than half of the hemithorax
    • Lung fields
      • Consolidation
      • Foreign body
    • Costophrenic angles
      • Sharp
      • Obscured
      • Blunted
    • Cardiophrenic angles
      • Sharp
      • Obscured
      • Blunted
    • Diaphragm
      • Flatten
      • Elevated
      • Free air below
    • Gastric bubble
    • Comment on any devices/tubing
      • Proper placement
  • Lateral view:
    • Vertebral bodies
    • Retrotracheal space
    • Hemi-diaphgragms
    • Costophrenic angles
    • Middle lobe of right lung

What you CAN'T miss on Chest Radiography

  • Pleural Effusion/Hemothorax/Empyema
    • A collection of fluid between the visceral and parietal pleura (intrapleural space)
    • Can be serous fluid (pleural), blood (hemothorax) and pus (empyema)
    • Fluid will appear as a diffusely increased density in comparison to the normal air (more white). It will often displace the pleural line and may cause mediastinal shift if severe
    • Upright PA fliud tends to gravitate to the lateral costophrenic angle due to gravity (look their first with this view)
    • When in doubt order a decubitus view with the patient side-lying. This will bring the fluid between the lung and chest wall where it is easy to see
      • Note: if adhesions of pleura are present the fluid may not deviate. Termed a loculate effusion
  • Pneumothorax/Tension Pneumothorax
    • Air tends to rise to the highest point in the chest unlike fluid. In a upright PA view, look for air (black crescent) in the apex of the affected lung
    • Note visualization of the visceral pleural line
    • Absence of lung markings distal to the visceral pleural line
    • Shift of the heart, mediastinum and trachea to the contralateral side as the process continues to progress (i.e. increasing with each inspiration)
  • Congestive Heart Failure
    • May see blurring of the cardiophrenic angles and enlargement of the cardiac shadow >1/2 the total thoracic distance (cardiomegaly)
    • Pulmonary alveolar edema
    • Pulmonary interstitial edema
    • Pleural effusion
  • Cardiac tamponade
    • Progressive increase in size of cardiac silhouette
  • Pneumonia
    • Extensive infiltrate within lobes
  • Aortic Dissection
    • Widen mediastinum
    • Left pleural effusion
    • Left lower lobe atelectasis

Abdomen Radiograph

Imaging Directions

  • Views: erect (standing), supine (lying flat) and left lateral decubitus (supine while lying on left side)

Image Interpretation

  • Can follow Free ABDO pattern
    • Free fluid
    • Air
    • Bowel wall thickening
    • Densities: abnormal calcifications (appendicolith, gallbladder and renal stones)
    • Organs: outline of liver and spleen
  • Gas pattern:
    • Central gas pattern consistent with small bowel
      • Plica circularis
      • Circular fold around the entire length of small bowel
    • Peripheral gas pattern consisten with large bowel
      • Haustra
      • Markings do not cross the entire length of large bowel
    • Gas in the rectum
    • Recall that bowel can only be visualized on the radiograph when it contains air. The air outlines the interior of the bowel wall. Bowel without air is of fluid density and blends with other soft tissue dense structures around it
  • Air/Fluid Levels
    • Look for air within the intra and retroperitoneal cavity, branching air in the liver and for any abscesses
    • Note air in the stomach
    • Small bowel (2-3 levels)
  • Assess biliary tree for air
  • Assess gallbladder for stones (15% appear radio-opaque, ultrasound better at visualization)
  • Assess ureters for kidney stones (85% appear radio-opaque)
  • Diameter of abdominal aorta (less than 3cm, normal)
  • Soft tissues

What you CAN'T miss on Abdomen Radiography

  • Free air under the diaphragm
    • Seen in the erect position
  • Bowel obstruction
    • Dilated loops:
      • Small bowel diameter >3cm
      • Cecal diameter >12cm
      • Transverse colon diameter >10cm
      • Descending colon diameter >8cm
    • Multiple air fluid levels (more than 3 levels)
    • Opacity proximal to the obstruction with lucent bowel distal to obstuction as only air gets through
    • Look at the rectum for presence of air:
      • Air present: partial obstruction
      • None: complete obstruction
  • Lower lobe pneumonia
  • Pleural effusion
  • Abdominal Aortic Aneursym
    • Widened,>3cm in diameter, monitor for rapid changes, once >5cm requires surgical intervention
    • Calcifications

Cervical Spine Radiograph

Imaging Directions

  • Views: Lateral (most important), Odontoid (mouth open, through the teeth, AP), Anterior-Posterior (AP) and Swimmers (arms up, helps to visualize cervicothoracic junction, C1 and T3)

Image Interpretation

  • Lateral view:
    • Visualization
      • Count vertebral bones to ensure complete visualization
      • Ensure that the cervical-thoracic junction is visualized
        • If not visualized repeat with Swimmer’s view
    • Alignment
      • Ensure the anterior, posterior and spinolaminar lines are intact
      • Anterior line corresponds to the anterior longitudinal ligament
      • Posterior line corresponds to the posterior longitudinal ligament
      • Spinolaminar line corresponds to the curved line formed by the spinous processes
    • Bones
      • Trace bony vertebra to ensure no obvious fractures (irregular margins→lucent on radiograph
      • Ensure vertebral body height remains consistent
      • Examine spinous processes for fracture
    • Soft Tissue/Spacing
      • Inspect intervertebral discs for solidarity and structure
      • Assess for soft tissue swelling along the vertebra, paying particular attention to C3 and C7
      • Distance between C2 and C3, the soft tissue should not be greater than 7cm; if so follow up with CT
      • Distance between C6 and C7, the soft tissue should not be greater than 21cm; if so follow up with CT
      • Ensure intervertebral disc spacing is equal in height along the cervical spine
  • Odontoid view
    • Ensure the edges of the lateral masses of C1 and C2 are in line
      • Slippage of these masses is suggestive of fracture
    • The distance between the dens and the lateral masses of C1 should be equal on each side (not greater than 3mm on either side)
  • Anterior-Posterior view
    • Assess for spinous process fracture
    • Vertebral alignment
  • If indicated, CT is ordered after cervical spine radiograph. MRI if practitioner suspects spinal cord involvement
  • The Canadian C-Spine Rules (see imaging guideline below) are criteria used by physicians to determine when imaging is necessary in suspected C-Spine injuries

What you CAN'T miss on Cervical Spine Radiography

  • Misaligment/fractures of verterbra
  • Suspected spinal cord involvement
  • Fracture of first or second rib(AP vier)
  • Pneumothorax(AP view)
  • Soft-tissue swelling
  • Fracture of first or second rib (AP view)
  • Antlooccipital dislocation (dislocation of the atlas and the inferior aspect of the skull, often fatal)

CT Head

    Image Directions

    • Axial view is the primary view
    • Can digitally construct views from data including coronal and sagittal views

    Image Interpretation

    • Quickly recall that the image is viewed from the feet up. Thus, the left of the image is patient RIGHT
    • Scan through the entire image to assess for any obvious abnormalities
    • Assess: cisterns, cerebral parenchyma, ventricles and everything else
    • Cisterns:
      • Subarachnoid hemorrhage
      • Asymmetry
      • Effacement (suggestive of increased intracranial pressure)
    • Cerebral parenchyma:
      • Intraparenchymal bleeds
      • Masses
      • Changes in grey-white matter differentiation
    • Ventricles:
      • Subarachnoid hemorrhage
      • Hydrocephalus
      • Midline shift
    • Remaining tissue
      • Skull bones
        • Fractures (not to be confused with suture lines)
      • Extra-cerebral bleeds:
        • Epidural hematoma (biconvex lenticular shape)
        • Subdural hematoma (crescent-shaped)
      • Orbits
      • Sinuses/Mastoid cells
        • Symmetry
        • Hemorrhage
        • Air-fluid levels
    • The Canadian CT Head Rules (see imaging guideline below) are criteria used by physicians to determine when imaging is necessary in suspected head injuries

    What you CAN'T miss on Head CTs

    • Epidural hematoma
      • Biconvex hyperdense (acute) appearance, bound by suture lines but may cross dural reflections
      • Commonly due to disruption of the middle meningeal artery, often caused by significant trauma
      • Can lead to mass effect if not corrected, often associated with skull fractures
    • Subdural hematoma
      • Crescentic hyperdense (acute) appearance, NOT bound by suture lines but never crosses dural reflections (falx cerebri and tentorium cerebelli)
      • Commonly due to disruption of the bridging veins
      • Frequently occurs as a result of minor trauma with most patients not recalling the injury
    • Subarachnoid hemorrhage
      • Can be traumatic or non-traumatic (ruptured aneurysm) etiology
      • Hyperdense appearance (white appearance) compared to CSF within the suprasellar cistern and within the Sylvian fissures
      • Hyperdense appearance (white appearance) within the Circle of Willis suggestive of SAH
    • Cranial Fractures
      • Ensure that you are not mistaking fractures for suture lines (compare contralateral side of skull for symmetry)
    • Masses
      • Hypodense appearance, with the potential of hemorrhage or calcification appearing within to give a hyperdense appearance
      • Due to the fixed volume of the cranium, mass effect can cause a rapid increase in intracranial pressure
      • Posterior masses generally result in poorer outcomes due to 4th ventricle obstruction

Fractures

    Fracture Types

    • Fractures are described based on four criteria
      1. Direction of the fracture line
        • Transverse
        • Diagonal or oblique
        • Longitudinal
        • Spiral
        • Avulsion
        • Impacted
      2. Orientation of the fragments
        • Described by the relationship of the distal fragment relative to the proximal fragment:
          • Displacement
          • Angulation
          • Shortening
          • Rotation
      3. Number of fragments
        • Simple: involves two fragments
        • Comminuted: involves more than two fragments
      4. Communication with the surrounding tissue:
        • Closed
        • Open or compound

    Special Fractures

    • Salter-Harris Fractures
      • Fractures involving the epiphyseal (growth) plate are called Salter-Harris Fractures
        • Type I
          • Involving only the epiphyseal plate
          • Through the epiphyseal plate
          • Good prognostic outcome
        • Type II
          • Involving only the epiphyseal plate and metaphysis
          • Good prognostic outcome
        • Type III
          • Involving only the epiphyseal plate and epiphysis
        • Type IV
          • Involving only the epiphyseal plate, metaphysis, and epiphysis
          • Can result in early fusion of epiphysis plate
        • Type V
          • Crush fracture of the epiphyseal plate
          • Can result in early fusion of epiphysis plate
    • Odontoid Fractures
      • Fractures involving the odontoid process of the 2nd cervical vertebrae
      • There are 3 classifications:
        • Type I
          • Fracture through the tip of the dens process
          • < 5% of fractures
        • Type II
          • Fracture through the base of the dens at the attachment site to the body of the 2nd cervical vertebrae
          • > 60% of fractures
        • Type III
          • Fracture through the body of the 2nd cervical vertebrae (dens process and part of the body of the vertebrae as a unit)
          • 30% of fractures

Imaging Guidelines

Canadian Head CT rules

  • CT head is only required for minor head injury patients with any one of the following:
  • High Risk:
    • GCS score < 15 at 2hrs after injury
    • Suspected open or depressed skull fracture
    • Any sign of basal skull fracture
      • Hemotympanum, raccoon eyes, CSF otorrhea/rhinorrhea, Battles Sign (bruising of mastoids processes)
    • Vomiting > or equal 2 episodes post injury
    • Age > or equal to 65 years
    • Sensitivity 100%
  • Medium Risk:
    • Amnesia before impact > or equal to 30min
    • Dangerous mechanism of injury
      • Pedestrian struck by vehicle, occupant ejected from motor vehicle, fall from elevation > or equal to 3 feet or 5 stairs
  • Rule not applicable if:
    • Non-trauma cases
    • GCS< 13
    • Age < 16 years old
    • Taking anticoagulants or diagnosed with a bleeding condition
    • Obvious open skull fracture

Ottawa Knee Rules

  • Radiograph is necessary if any of the following are positive:
    • Patient is aged 55 or older
    • Isolated tenderness of patella (no other bony tenderness)
    • Tenderness at head of fibula
    • Inability to flex knee to 90 degrees
    • Inability to weight bear/walk 4 steps both immediately and in the emergency department

Ottawa Ankle Rules

  • Radiograph is necessary if any of the following are positive:
    • Mallor zone tenderness
    • Tenderness along the posterior edge or tip of the lateral malleolus within a 6cm zone
    • Tenderness along the posterior edge or tip of the medial malleolus within a 6cm zone
    • Inability to weight bear/walk 4 steps both immediately and in the emergency department
    • Sensitivity 100% (Combined with Ottawa foot and ankle rules)

Ottawa Foot Rules

  • Radiograph is necessary if any of the following are positive:
    • Midfoot tenderness
    • Tenderness at the base of the 5th metatarsal
    • Tenderness over the navicular bone
    • Inability to weight bear/walk 4 steps both immediately and in the emergency department
    • Sensitivity 100% (Combined with Ottawa foot and ankle rules)

References

  1. Herring W. Learning Radiology: Recognizing the Basics. 2nd Edition. USA. Elsevier-Saunders. 2012.
  2. Ouellette, H. & Tetreault, P. (2000). Clinical Radiology Made Ridiculously Simple. 2nd Edition. Miami Florida. MedMaster, Inc. 2002.
  3. Abdominal X-Ray Anatomy. Radiology Masterclass Web site. http://www.radiologymasterclass.co.uk/. Published 2007-2013. Accessed December 5, 2013.
  4. Chest X-Ray Anatomy. Radiology Masterclass Web site. http://www.radiologymasterclass.co.uk/. Published 2007-2014. Accessed January 3, 2014.
  5. Imaging Techniques. Neuroradiology Learning Module: Department of Radiology University of Wisconsin School of Medicine and Public Health Web site. https://sites.google.com/a/wisc.edu/neuroradiology/image-acquisition. Published 2011. Accessed December 2, 2013.
  6. Stiell IG et al. The Canadian C-Spine Rule Versus the NEXUS Low-Risk Criteria in Patients with Trauma. New Engl J Med. 2003; 349:2510-2518.
  7. Stiell IG et al. The Canadian CT Head Rule for patients with minor head injury. Lancet. 2001; 357:1391-1396.
  8. Stiell IG et al. Prospective Validation of a Decision Rule for the Use of Radiography in Acute Knee Injuries. JAMA. 1996; 275:611-615.
  9. Stiell IG et al. Decision Rules for the Use of Radiography in Acute Ankle Injuries: Refinement and Prospective Validation. JAMA. 1993; 269:1127-1132.
  10. Chalela, J., Kidwell, C. S., Nentwich, L. M., Luby, M., Butman, J. A., Demchuk, A. M., Hill, M. D., Patronas, N., Latour, L. & Warach, S. (2007). Magnetic resonance imaging and computer tomography in emergency assessment of patients with suspected acute stroke: a prospective comparison. The Lancet, Vol. 369, pp.293-298