Medical Imaging Modalities

Learning Objectives

• Describe the scientific basis (basic physics) for common imaging modalities like CT, MRI, US, and Radiographs
• Describe the indications, precautions, and contraindications of common imaging modalities
• Apply radiographic appearance three-point convention to describing normal anatomy on radiographs, CT, US and MRI
• Discuss the correlation between tissue properties such as density and their attenuation/absorption potential on radiation-based imaging

Common Imaging Referrals

• Computed Tomography (CT) is used for:
  • Head/Brain CT (Trauma Activation)
  • Suspected meningitis
  • Thunderclap headache
  • Stroke
  • GIT obstructions
• Angiography can be used in conjunction with CT scans
• Conventional Radiographs (Bone) are used for:
  • Appendicular Skeleton imaging
  • MSK Trauma/Fractures
  • MSK dislocations
  • Chest X-rays (CXR)
  • Mammogram
  • GIT obstructions (initial)
• Ultrasound (Soft Tissue) is used for:
  • Abdomen
  • Pelvis / Gynaecological
  • ENT/Thyroid gland
  • Soft Tissue Lumps
  • MSK tears, e.g., Achilles tendon
• Magnetic Resonance Imaging (MRI) is used for
  • MRI Head (unexplained seizures, suspected intracranial pathology)
  • Cervical spine – trauma and pathology
  • MRI Knee – ACL and acute meniscal tears)

X-Ray Production & Interactions

• When traversing matter, photons will penetrate without interaction, scatter or be absorbed
• With effective atomic number, there is an increased X-ray absorption
• Fat has an effective atomic number of 6.3 resulting in less absorption and darker images on film
• Bone has an effective atomic number of 13.8 resulting in greater absorption and whiter images on film

Radiographic Attenuation

• Bones, calcified structures & metal implants have the highest density, appearing radiopaque
• Soft tissues have a lower density, appearing semi-radiopaque
• Fat and air have the lowest densities, appearing radiolucent
• Comparative Terms:
  • HYPERdense = more dense/more white
  • HYPOdense = less dense/darker

Patient Position

• PA (Posterior-Anterior)
• AP (Anterior-Posterior)
• AP views of the chest show magnification of the heart and widening of the mediastinum
• AP views are less useful and should be reserved for patients who cannot stand erect

Fracture Evaluation

• Radiographs are the Gold Standard for simple fractures and dislocations
• CT is the Gold Standard for Head and Neck and complex fractures
• #RADIOLUCENT
• #SPIRAL

Scaphoid Series

• For a 30 year old male patient with FOOSH (Fall on outstretched hand), snuffbox tenderness
• Clinical Presentation: Pain in the anatomical snuffbox; sensitivity 90%, specificity 40%
• Initial radiograph can miss 5-20% of fractures in the acute setting
• If fracture is suspected and not seen on radiograph, follow up requested for 7-10 days time
• Angled Anteroposterior can also be used

Radiographic Appearance Terms

• Attenuation: the density or the intensity (colour)
• Radiographs are correlated to material density (HARDNESS)
  • Radiolucent
  • Semi-radioopaque
  • Radioopaque
• Magnetic Resonance Imaging is correlated to intensity of the signal emitted from the tissue
  • High signal intensity
  • Intermediate SI
  • Void

Computed Tomography Advantages

• Cross-sectional image format
• Superior low contrast resolution
  • RAD: 10%
  • CT: 0.3%
• Height and depth information
• Less superimposition of tissues

MSCT Imaging Guidelines

• Acute Head Injury for:
  • Infants
  • Adults and older children with minor injury
  • Adults with moderate-severe intracranial injury, penetrating injury, high speed collision depressed fracture etc
• **C1-C3 should be included in FOV - spiral fracture:
• Headache (Non-traumatic isolated headache)
  • Thunderclap headache; increased risk of SAH from aneurysm rupture
  • Symptoms/signs of raised ICP (Urgent)
• New Onset Seizures of

  • 40 years

  • Head injury
  • Drug/alcohol abuse
  • Focal signs
  • Biochemical evidence of systemic illness
• Stroke

Orientation and Sides

• Patient lies in SUPINE position (face up) in MRI or CT machine
• Coronal, Sagittal and Axial images acquired
• Coronal scans are viewed as though the clinician is facing the patient
• Axial scans are viewed as clinician is standing at the patient’s feet looking up toward the patient’s head

Trauma

• Rib fractures have the appearance of an abrupt discontinuity in the smooth outline of the rib, a lucent fracture line may be seen
• Fracture of the upper three ribs is associated with an increased risk of aortic injury because of the excessive force needed to fracture these ribs
• Fracture of the lower three ribs can be associated with liver or spleen injury and that multiple bilateral rib fractures in various stages of healing are associated with child abuse in children

Referral Protocol for MR Imaging

• MSK Soft Tissue Injuries: Ligaments, Menisci, Intervertebral Discs
• MSK Infections, bone bruising/contusions, bone cysts
• Brain Imaging: Abnormalities of CSF flow i.e. Hydrocephalus; intra-cranial masses/tumours

Magnetic Resonance Imaging (MRI)

• Measures the water/hydrogen atom concentration in structures
• Cortical bone = DEVOID of H+ = black

MRI Functioning

• The patient lies on the scanner couch (1) which slides into the bore of the scanner (2)
• Within the bore of the scanner there is a powerful magnetic field
• The scanner produces radiofrequency pulses to excite protons in the body
• As the excited protons in the body relax after each pulse, they give off radiofrequency signal which is detected by the receiver (3)
• The receiver is placed around or near the body part being imaged

Ultrasound Attenuation Terms

• Ultrasound uses short sound pulses that are transmitted into the body
• The more closely packed the molecules, the faster the speed of sound
• In biological tissues, the speed of sound is lowest in gases, faster in fluid, faster yet in soft tissue, and fastest in bones
• Bone has a lot of acoustic impedance = reflection of echoes
• Anechoic (Black)
• Hypoechoic (dark)
• Hyperechoic (bright)