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MRI & Scintigraphy

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sihirlifil's version from 2017-11-03 16:49

MRI

Question Answer
MRI has revolutionized diagnostic imaging mainly for diagnosis of ___ and _ _ pathologyIntracranial & soft tissue
MRI has excellent ___ contrastSoft tissue
Main applications for MRIBrain (white & gray matter, CSF)
IVD, spinal cord, meninges, nerve roots, peripheral nerves
Inner ear, orbit, neck
Draining tracts, FB
MSK (larger joints, equine distal limb)
Abdominal organs
How are contrast studies usued?Highlight inflammatory & neoplastic lesions
Synoviography
Contrast medium?Gadolinium (passes breakdowns of BBB)
Main caveat of MRI?Ferrous material will be attracted to the magnet & may destroy it (gas cylinders, chairs, tables…)
Pacemakers may malfunction, credit card chips wiped, ferromagnetic FB, implants (aneurysm clips, skin staples)
NON-FERROUS MATERIALS ARE SAFE (Al, Cu, Pb, Ni, Sn, Zn, Au, Ag, Pt; alloys e.g. brass)
Components of MRIMagnet: creates high magnetic field
Gradient coils inside gantry
Radiofrequency coils on/around patient
Computer
Non-ferrous anesthetic equipment & positioning aids
Low field MRI = ? High?Low = 0.25 – 0.7 T
High = 1.5-3 T
What is the unit for MRI?Tesla = magnetic field strength; multitude of earth’s magnetic field
MRI image formation based on?Behavior of Hydrogen protons within the magnetic field
Types of MRI systems (4)Gantry vertical & horizontal
Low- and high-field
Standing MRI in horses
Mobile units
Size of the ___ limits size of the object imagedGantry! (same as CT)
Principles of MRI image formation: H+ are? Do what?Dipoles that spin around their own axis (like the earth), align with B0 (=magnetic field, which has direction/vector)
How is an MRI image formed?Hydrogen atoms (dipoles) are excited using radiofrequency pulses, flip at specific angle into another plane, then re-align when pulse is stopped. Radiofrequency signal is created when the protons return to their initial alignment with B0 (magnetic field) and original energy state. As they return they emit energy at radiofrequency which is recorded by the coils
Each H+ returns a signal reflecting its state, location, & surroundings; the intensity of this signal is the basis of image formation
What is needed to differentiate tissues?Contrast!
MRI is the preferred modality forBrain & vertebral column
Articulations (species-dependent)
Normal cortical bone is always ___ in MRIHypointense
Why is normal cortical bone hypointense in MRI?Low H+ content
Which MRI sequences are intrinsic to each tissue?T1 recovery
T2 decay
Proton density
How do T1 & T2 differ?Depending on sequence chosen, tissues appear brighter or darker on image
How is this image weighted? Why?
T1
CSF is dark = hypointense
How is this image weighted? Why?
T2
CSF is bright = hyperintense
T1 recovery occurs ___ in fat and ___ in waterRapid in fat = high signal
Slowly in water = low signal
T2 decay occurs ___ in fat and ___ in waterRapid in fat! = later, low signal
Slow in water = HIGH signal!!!
Different grades of brightness =Signal intensity
Signal void =Black
Hyperintense =White
Hypointense =Moderately dark; darker than _________
Isointense =Of similar intensity as _________
How are MRI sequences chosen?According to protocols, depending on the area imaged
MRI sequences (4)Spin echo/Fast spin echo (T1, T2)
Inversion recovery (STIR, FLAIR)
Gradient echo (T2*)
Proton density (PD)
STIR: suppresses what?Fat signal (stir fry) (1.5 T)
FLAIR: suppresses what?Fluid signal
Fat appears bright in which sequences?T1 AND T2
FLAIR sequences are used for?Differentiate areas that contain fluid e.g. CSF/edema/lesions from the surrounding neural tissue
STIR sequences used for?Differentiate fluid from surrounding fat e.g. edema from BM/fat surrounding spinal cord
STIR and FLAIR both have +/- the characteristics of which sequence?T2
How is this image weighted? Why?
FLAIR
Suppresses fluid signal
Which MRI sequence is this?
Proton density
Fluid higher signal than T1
Bone, tendons, ligaments hypointense
Fat signal high
Better to look at fluid vs soft tissue than T1
Which MRI sequence is this?
T1
Fluid: low signal
Cortical bone, tendons, ligaments hypointense
Fat signal high
Very good anatomical detail
Which MRI sequence is this?
T2
Fluid HYPERINTENSE!
Bone, tendons, ligaments hypointense
Fat intermediate to high signal
Good to differentiate fluid from surrounding ST but poorer anatomical detail
Which MRI sequence is this?
STIR
Fluid HYPERINTENSE! (synovial fluid white)
Specific sequence to suppress fat (bone marrow = dark), detect abnormal fluid in bone medulla & soft tissue
What’s going on here?
Bone marrow edema in equine distal limb (STIR really good for this!)
Arrows pointing at?
Hypophyseal adenoma in a dog, with gadolinium contrast (C) highlighting lesion
T1: Lesion visible more hypointense than adjacent neural tissue, strongly enhanced after contrast
T2: Hyperintense rim around mildly hyperechoic lesion may indicate presence of fluid
Dorsal T1 (bottom row): Lesion visible as in saggital T1
memorize

Scintigraphy

Question Answer
Clinical applications (9)Lameness investigations (bone scan)
Thyroid/parathyroid scans
Renal scans
PSS studies
CVS studies
Pulmonary ventilation & perfusion
Tumor diagnostic
Liver investigation
Pyrexia of unknown origin/invest. of sites of inflammation (white cell scan)
Radioactive elementTechnetium (99m Tc). t1/2 = 6 hours
Decay product of Molybdenum (produces metastable Tc & gamma rays)
What is the energy of a gamma ray?140 keV (similar to 140 kV in radiography)
How does Tc work for bone imaging? Other organs?Bound to carrier molecule (e.g. MDP) for bone, different molecule for other organs
(Bone: high cell turnover shows high uptake)
How is the radioactive substance (R) given?IV (SAFETY!!!)
How is R distributed? Excreted?Distribution depends on tissue perfusion
Excreted via kidneys! Urine is a potential contaminant, as is SWEAT in horse
Image description for scintigraphyDegree of radiopharmaceutical uptake (RU)
Void, hot spot, increased/decreased RU
Radiation safety aspects to keep in mind (5)Handling of radiopharmaceutical
URINE during scan and 48 hours after
Establish controlled areas and signal
Discharge of horses/dogs/cats
Contaminated bedding/kennels
How is the radiation captured?Gamma camera captures radiation emitted from patient (scintillating crystal activated when hit by gamma rays, emits light, translates signal into electrical)
When photographing limbs, extra consideration?Opposite limb needs to be shielded, otherwise it will appear in the image
What are these images of the equine skull showing?
R mandibular tooth root abscess with associated osteomyelitis
The darker areas indicate? Dx?
More active turnover
Tooth root abscess of PM4
Can confirm side of lesion, check if bilateral. Cheek teeth are where empty spaces are
How can small patients be positions?ON the tilted gamma camera
What are the dark areas in this image? (Lat view of thorax & cr abdomen, dog)
Top 3 arrows = leisions (bone infection, fungal disease, metastasis)
Bottom: URINARY BLADDER! Tc accumulates & excreted
What are these images of the thyroid in a cat showing?
Ectopic thyroid tissue located in caudal cervical area & cranial mediastinum
Thyroid scan of a cat: dx?
Left: Euthyreotic (normal)
Right: Unilateral Thyroid adenoma --> hyperthyroid
Functional renal scintigraphy used for?GFR calculation
Uses 99mTc-DTPA (diethylene triamine pentaacetic acid)
Why is scintigraphy often used to screen for skeletal metastasis in homo sapiensMore sensitive to bony changes before they are visible on rad/CT
memorize

 

From long notes
*Phase I: Vascular or pool phase. IV bolus passing through the vasculature and extravascular fluid in the area imaged. Very short phase, only one area can be imaged, best real-time. The camera should be placed over the region of interest prior to the injection.
*Phase II: Soft tissue phase distribution in the ECF of all body tissues, the bones are not clearly or not at all visible. Image acquisition during 1 to 15 minutes after injection.
*Phase III: Bone phase. Normal bones take up radiopharmaceutical, growing normal horses show a lot of RU in the growth plates (active growth and incorporation of the TcMDP), the subchondral bone always shows higher uptake than the adjacent bone. Abnormal RU is found in pathological increase in blood flow, bone pathology such as osteomyelitis , osteoarthritis, fractures, periosteal callus, bone cysts, neoplasia. Image acquisition during 2 to 4 hours after injection

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