Alternate imaging Flashcards
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
Pixel
Picture element; tiny square, 2-d
Represents the average number of a voxel of tissue
Voxel
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
- X-rays are used to create an attenuation map of the patient
- X-ray tube rotates around the patient
- The x-rays are attenuated by a number of factors
- Transmitted beam intensity measured by detectors
- 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)
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)
Black
Hyperdense (hyperattenuating)
White
What are 5 reasons for CT-ing a thoracic mass?
- Additional imaging findings help with diagnoses
- More information–picks up subtle changes faster
- CT biopsy possible
- Prognostication
- Surgery planning
- 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
T/F: 3D reconstructions can be achieved with CT scanning
TRUE
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)?
- No imposition of overlying structures
- Multiple imaging planes (via reconstruction)
- Superior soft tissue differentiation
- Fluid vs. soft tissue for example
- Manipulation of grayscales allows optimal visualization of all tissues within the slice
- Improved lesion detection
- Ex: lung metastasis
- CT-guided biopsy possible
What are 3 advantages of CT (compared to MRI)?
- Since also based on x-ray technique, usually easier to interpret and more familiarity from the start
- Faster and cheaper
- CT-guided biopsy possible
What are 4 disadvantages of CT?
- General anesthesia or sedation
- Radiation (animal and humans)
- Limited to referral centers
- Image aquisition
- Image interpretation
- 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?
- Strong magnet (0.5-3.0 Tesla)
- Radio waves
- Patient (Hydrogen protons)
- Computer
What are the physics of an MRI (5)?
- Patient placed in a strong magnetic field
- Protons of Hydrogen atoms align with magnetic field
- RF pulse is applied and protons tip over
- Signal released as protons “relax”
- Signal is measured and converted to gray scale
T/F: Most legions result in increased tissue water (MRI)
TRUE
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)
What are T2 characteristics of MRIs?
Bone –> white matter –> gray matter –> fat –> CSF
What are 3 advantages of MRI?
- Best low contrast resolution
- No ionizing radiation used
- Direct multiplanar imaging can be obtained
What are 6 disadvantages of MRI?
- Based on very complex physics
- Does not resemble any other previously used principle in veterinary science
- Cost
- Time consuming
- Focal areas should only be imaged
- Emphasizing importance of proper clinical work-up
- No ferromagnetic metals can be used
- 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
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
FLAIR
- 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
