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Flashcards in Alternate imaging Deck (37):

What is tomography? What are the different modalities?

  • Alternate imaging that depicts a slice of the body free of superimposition by overlying structures
  • US, CT, MRI


What is a problem with tomography? 

We are used to topographic anatomy, not tomographic anatomy



Picture element; tiny square, 2-d

Represents the average number of a voxel of tissue



Volume element--block of tissue, 3-d

Represented on an image matrix by the pixel--determined by the product of the pixel size and the thickness of the scan slice


Voxel on MRI vs. voxel on CT

  • MRI--represents the signal intensity 
    • Depends on the respective transverse magnetization
  • CT--represents the Hounsfield unit
    • Depends on the linear attenuation coefficient of tissue


When do you go for MRI? CT? Which is the standard for thoracic metastasis?

  • MRI--soft tissue evaluation
  • CT--bone and lung evaluation 
  • CT is the standard for thoracic metastasis


What are the 5 steps involved in preparation/planning for CT or MRI?

  • Proper clinical work needed
  • MRI and CT do not replace standard DI modalities, such as radiology and US
  • Anesthesia pre-rogative for MRI
  • Sedation or anesthesia for CT
  • Use standard protocols


CT-image based on X-ray attenuation: 5 steps

  1. X-rays are used to create an attenuation map of the patient
  2. X-ray tube rotates around the patient
  3. The x-rays are attenuated by a number of factors
  4. Transmitted beam intensity measured by detectors
  5. Reconstruction 


What is reconstruction (CT-image based on x-ray attenuation)?

  • The computer reconstructs the x-ray image
  • The number of the voxel represents the linear attenuation coefficient of the tissue
  • It is called the Hansfield unit (HU)
  • The HU is converted to a grayscale for the final image


What has the highest HU values on a CT? What has the lowest?

Bone is the highest (700-3000 HU); air is the lowest (black: -1000 HU)


A image thumb

What is the window level of CT? What is the window width?

  • Window level (WL) = center CT number of the window
  • Window width (WW) = range of CT numbers represented by the gray scale (above and below the window level)


What is a long scale of contrast in a CT window? What is a short scale of contrast?

  • Long scale
    • Wide window (1000)
    • Shows all subtle shades of gray
    • Low contrast
    • Aka bone window
  • Short scale
    • Narrow window (250)
    • Helps accentuate subtle differences
    • High contrast
    • Aka soft tissue window


Hypodense (hypoattenuating)



Hyperdense (hyperattenuating)



What are 5 reasons for CT-ing a thoracic mass?

  1. Additional imaging findings help with diagnoses
    1. More information--picks up subtle changes faster
  2. CT biopsy possible
  3. Prognostication
  4. Surgery planning
  5. Metastasis 


What is the CT mets check?

  • Gold standard
  • Routine in some institutions
  • Tumor type?
  • Treatment plan?


How is contrast achieved in CT scanning?

  • Iodinated
  • Given IV
  • Hyperattenuating signal

A image thumb

T/F: 3D reconstructions can be achieved with CT scanning



T/F: CT can only image 1 plane and it is usually transverse. Other planes cannot be reconstructed.

FALSE--other planes CAN be reconstructed


What are 6 advantages of CT (compared with conventional x-ray techniques)?

  1. No imposition of overlying structures
  2. Multiple imaging planes (via reconstruction)
  3. Superior soft tissue differentiation
    1. Fluid vs. soft tissue for example
  4. Manipulation of grayscales allows optimal visualization of all tissues within the slice
  5. Improved lesion detection
    1. Ex: lung metastasis
  6. CT-guided biopsy possible


What are 3 advantages of CT (compared to MRI)?

  1. Since also based on x-ray technique, usually easier to interpret and more familiarity from the start
  2. Faster and cheaper
  3. CT-guided biopsy possible 


What are 4 disadvantages of CT?

  1. General anesthesia or sedation
  2. Radiation (animal and humans)
  3. Limited to referral centers
    1. Image aquisition
    2. Image interpretation
  4. Compared to MRI: less soft tissue detail


What are the CT clinical applications?

  • Surgery planning (determine whether resectable)
  • Radiation therapy planning
  • Excellent for diagnosis and treatment:
    • Thoracic mets check
    • Thoracic disease
    • Nasal or oral cavity disease
    • Elbow dysplasia
    • Brain disease (MRI)
    • Spinal disease (MRI)


What are the 4 (basic) components of an MRI?

  1. Strong magnet (0.5-3.0 Tesla)
  2. Radio waves
  3. Patient (Hydrogen protons)
  4. Computer


What are the physics of an MRI (5)?

  1. Patient placed in a strong magnetic field
  2. Protons of Hydrogen atoms align with magnetic field
  3. RF pulse is applied and protons tip over
  4. Signal released as protons "relax"
  5. Signal is measured and converted to gray scale


T/F: Most legions result in increased tissue water (MRI)



What is the level of signal in MRIs referred to as?

Referred to in terms of its intensity:

Hypointense = black

Hyperintense = white


What are T1 characteristics?

Bone (hypointense) --> fluids --> gray matter --> white matter--> fat (hyperintense)

A image thumb

What are T2 characteristics of MRIs?

Bone --> white matter --> gray matter --> fat --> CSF

A image thumb

What are 3 advantages of MRI?

  1. Best low contrast resolution
  2. No ionizing radiation used
  3. Direct multiplanar imaging can be obtained


What are 6 disadvantages of MRI?

  1. Based on very complex physics
    1. Does not resemble any other previously used principle in veterinary science
  2. Cost
  3. Time consuming
  4. Focal areas should only be imaged
    1. Emphasizing importance of proper clinical work-up
  5. No ferromagnetic metals can be used
  6. Anesthesia and monitoring


What are the clinical applications of MRI?

  • Neuroimaging
    • Brain disease
    • Spinal disease
    • Nerve sheath tumors
  • Musculoskeletal
  • Tumor staging


When should T1 be used in MRI?

  • Best for anatomical view
  • Used for contrast studies (Gadolinium IV)
  • Fat is bright
  • Free fluid is dark
  • Edema is dark


How are MRI contrast studies done?

  • Gadolinium given IV
  • Hyperintense signal on T1

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When shoud T2 MRIs be performed?

  • For diagnosis of pathology
  • Fat is intermediate gray
  • Free fluid is bright
  • Edema is bright


What is sTIR? What can be useful?

  • Inversion recovery images with short TI;
  • Fat is selectively repressed (signal void--black)
  • Free fluid is white
  • Edema is white
  • Distinguishes fat from fluids/edema
  • Lesions more obvious when fat signal is removed



  • Fluid attenuated inversion recovery
  • Pure fluids are selectively suppressed 
    • Signal void (black)
  • Fat is gray/white
  • Free fluid is black
  • Edema is white
  • Distinguishes free fluid from edema

A image thumb