Chapter 72 - Magnetic Ressonance Imaging Flashcards
Why is MRI preferred over CT for evaluating cartilage in equine patients?
MRI does not require intraarticular contrast injection for cartilage evaluation.
What fundamental property makes hydrogen protons ideal for generating MRI signals?
They are abundant in the body and have favorable magnetic properties for imaging.
What process do hydrogen protons undergo during an MRI exam?
They are excited by a radiofrequency pulse, gain energy, and then return to their base resting state, generating a signal.
What are the three processes through which a proton’s relaxation occurs in MRI?
T1 relaxation, T2 relaxation, and T2* decay.
Figure 72-1. Sagittal images of the distal limb with different pulse sequences. (A) Sagittal proton-dense image. Note the high signal of medullary fat and relatively high signal of joint fluid. (B) Sagittal T2-weighted image. Note the high signal of medullary fat and the very high signal of joint fluid. (C) Sagittal STIR image. Note that the signal of the medullary cavity of the bones is very low owing to suppression of fat. (D) Sagittal T1-weighted image. Joint fluid is of relatively low signal, but fat within the bones retains a high signal.
Describe what happens to hydrogen protons during MRI exam
the hydrogen protons are placed under the influence of a strong magnetic field. The protons are then excited by an applied radiofrequency (RF) pulse at exactly the proper frequency such that the protons gain energy through a process known as resonance (hence the name, magnetic resonance imaging). When the RF pulse is removed, the protons lose energy and return to their base resting state (a process called relaxation), and through this process generate a small signal that can be measured. By repeating this process over and over, the signal within an entire volume of tissue may be mapped to create the stack of cross-sectional images that comprise the examination.
What does the term ‘pulse sequence’ refer to in MRI?
programmed set of radiofrequency stimulation sequences resulting in images with specific characteristics.
describe what happens to protons in each pulse sequence (ie T, T2…)
T1 relaxation (or T1 recovery) refers to the release of energy from a proton into its surrounding molecular environment. T2 relaxation (or T2 decay) results from the energy transfer between the protons themselves. Finally, T2* (read T2 star) decay is energy loss owing to inhomogeneities within the magnetic field, which are inherent to all magnets but increased by some types of material (especially hemorrhage or metal). Protons are present in different densities and have different T1, T2, and T2* properties between types of tissue (e.g., fat, tendon), providing the fundamental property that results in contrast between tissue types on MRI.
Tissues emitting high-intensity signal will appear very white on the final image and are termed
hyperintense
Areas of low signal are termed
hypointense and will appear black on an image
areas of intermediate signal intensity will have some shade of
gray in between
What do T1-weighted pulse sequences emphasize in MRI images?
They produce images with good anatomic detail and contrast between tissues based on differences in T1 relaxation.
Why are T2-weighted pulse sequences commonly used in orthopedic imaging for horses?
They create greater signal intensity differences between adjacent tissues.
What is the purpose of Proton Density (PD) pulse sequences in MRI?
They emphasize the density of mobile hydrogen protons in tissues, useful for musculoskeletal imaging.
What is the advantage of fat-suppression technology in MRI?
It eliminates the signal from fat while retaining the high signal typical of many pathologies.
Figure 72-2. Sagittal images comparing two methods of fat suppression. (A) Sagittal fat-saturated proton-dense image. Owing to the inhomogeneity of the magnetic field, fat signal within the proximal anddistal phalanx are incompletely suppressed (arrows). (B) The STIR image has more homogeneous fat suppression.
What are the two fundamental classes of pulse sequences in MRI?
Spin echo (SE) and gradient recalled echo (GRE) pulse sequences.
What are the advantages of GRE sequences in MRI?
Shorter acquisition time and the potential for volumetric (three-dimensional) acquisition.
Why are GRE sequences often used in standing MRI systems?
To reduce motion artifacts and due to their relatively low signal-to-noise ratio.
What is the main disadvantage of GRE sequences compared to SE sequences?
GRE sequences have poorer contrast resolution due to increased susceptibility to field inhomogeneities.
what does it mean STIR
short tau inversion recovert
STIR is highly sensitive to what? fat or water?
STIR images are highly water-sensitive and the timing of teh pulse sequence used acts to suppress signal comming from fatty tissues - ONLY WATER britght
usually you compare a combination of T__ and STIR to determine the amount of fat or water
T1 and STIR
What is the difference between T1 and T2?
T1 highlight fat tissue
T2 highlight fat AND water
T1 images - 1 tissue type is bright - FAT (tendon ligament injury)
T2 images - 2 types are bright - FAT and WATER (bone edema and cystlike lesions)
GRE sequences are frequently used in standing or GA?
standing MRI systems where motion artifacts but on the other hand GRE have poorer contrast resolution compared to SE seuqeunces
what are the existing planes in the complte MRI protocols
A complete MRI protocol consists of a combination of pulse sequences in the transverse, sagittal, and dorsal planes, with additional oblique planes as desired
Figure 72-3. Sagittal T2-weighted images of the fetlock. (A) The image displays poor signal (graininess) owing to use of a receiver coil that had a large distance between the skin surface and the coil. (B) When the proper coil was used, which had little distance between the coil and skin surface, image signal was much improved.
Figure 72-4. (A) Transverse proton density image of the distal limb at the level of mid-second phalanx showing two vitamin E capsules (arrow) taped to the lateral aspect of the limb. (B) Transverse T2-weighted image of the distal limb, showing a commercially available marker (with a disclike shape) that can be used for both MRI and CT imaging, but cost approximately four dollars each.
What is the advantage of using fat saturation technique in MRI?
It can be applied selectively to T1, T2, or PD-weighted images, maintaining the original tissue contrast with the signal from fat subtracted.
What are the limitations of fat saturation techniques in MRI?
Suppression of fat is not always uniform across the image and is less effective in low-field systems.
Why are Fast (turbo) SE sequences often used for equine imaging in MRI?
They decrease acquisition times compared to traditional SE sequences but still provide good contrast resolution.
What is the importance of MRI contrast agents in equine imaging?
They enhance the contrast in T1-weighted images, particularly useful when gadolinium-based MRI contrast agents are employed.
Which magnetic resonance pulse sequences are commonly used to image the equine limb?
Fast spin echo (SE), short tau inversion recovery (STIR), and gradient recalled echo (GRE) sequences.
What makes fluid-sensitive sequences like T2 or PD valuable in equine MRI?
They are useful as diseased tissues often have increased water content, resulting in a high signal on these images.
How does the use of fat-saturation pulses or STIR sequences benefit MRI imaging on high-field magnets?
They help to eliminate the fat signal, making it easier to identify lesions.
In what orientations can the aforementioned pulse sequences be performed?
They can be performed in any orientation, including transverse, sagittal, and dorsal planes.
Why is time of acquisition a major consideration when designing an MRI protocol?
Longer acquisition times are needed for high-field magnets, and multiple scan planes increase the total scanning time.
How is the region of interest positioned for optimal imaging quality in MRI?
It should be positioned in the exact middle of the magnet, known as the isocenter.
What are ‘receiver coils’ used for in MRI examinations?
They are placed around the anatomic area of interest to receive the signal emitted by hydrogen protons.
Why must horseshoes and nails be removed before conducting an MRI study in horses?
To avoid artifacts in the MRI images.
What is the maximum diameter of the gantry available in commercial high-field MRI systems for equine use?
Up to 70 cm diameter gantry.
What anatomic regions can be imaged using high-field MRI systems in horses?
Distal limbs up to and including the carpus and tarsus, and the equine head.
Figure 72-5. (A) Dorsal gradient echo image (GRE) of the distal sesamoid bone. Note the fragment next to the distal margin of the distal sesamoid bone at the origin of the distal sesamoidean impar ligament (arrow). Also note the enlarged rounded distal synovial invaginations (arrowhead). (B) Sagittal short tau inversion recovery (STIR) image of the foot showing the plane image from which (A) was obtained.
LEFT Figure 72-6. Sagittal short tau inversion recovery (STIR) image. The distal sesamoid bone is diffusely hyperintense. In addition, a more focal hyperintensity is present in the region of the enlarged synovial invaginations (arrow). RIGHT Figure 72-7. Sagittal T2-weighted image of the same foot as Figure 72-6, showing diffusely decreased signal within the distal sesamoid bone.
Figure 72-8. (A) Transverse T2 weighted image at the level of the insertion of the deep digital flexor tendon onto the distal phalanx. Note the focal high signal intensity lesion (arrow) at the insertion of the deep digital flexor tendon onto the distal phalanx. (B) Sagittal T2 image showing the same lesion. This lesion is not caused by magic angle effect given that this image is obtained using a long echo time (TE).
Figure 72-9. (A) A normal contralateral limb is shown for comparison. (B) Transverse proton density image of the distal limb at the level of the proximal aspect of the second phalanx. Note the high signal intensity parasagittal split through the lobe of the deep digital flexor tendon (arrow).
Figure 72-10. Transverse proton density image of the distal limb at the level of mid-middle phalanx. Note the irregularities of the dorsal margin of both lobes of the digital flexor tendon (arrows).
What is the primary purpose of MRI contrast media?
To enhance the ability to differentiate two tissues that would otherwise have similar MRI characteristics by increasing their signal.
What are the most common forms of MRI contrast media?
They are useful as diseased tissues often have increased water content, resulting in a high signal on these images.
How does Gadolinium enhance T1 relaxation times in MRI?
Gadolinium in tissue significantly enhances T1 relaxation times, making tissues containing more contrast agent appear more hyperintense on T1-weighted images.
What is the typical administration route of MRI contrast agents for brain imaging?
Intravenous administration.
Why do inflammatory and neoplastic lesions in the brain typically show significant enhancement post-contrast injection?
Due to their increased vascularity and effect on the blood-brain barrier.
What structures are commonly affected in navicular syndrome as identified by MRI?
The distal sesamoid (navicular) bone, podotrochlear bursa, deep digital flexor tendon (DDFT), distal sesamoidean impar ligament, and collateral sesamoidean ligaments.
What advantage does MRI offer over radiography in detecting distal sesamoid bone pathology?
MRI typically reveals abnormalities of the distal sesamoid bone not detectable with radiography.
What types of lesions in the distal sesamoid bone are often seen concurrently with MRI?
Lesions of the DDFT, collateral sesamoidean ligament, and distal sesamoidean impar ligament.
How is fluid within the podotrochlear bursa characterized on MRI?
It has a high signal intensity on T2 and PD images.
What is indicated by an increased amount of fluid within the podotrochlear bursa on MRI?
Podotrochlear bursitis characterized by fluid distention with or without proliferation of soft tissues.
How does the distal recess of the podotrochlear bursa behave in response to fluid accumulation as compared to the proximal recess?
The distal recess does not distend as much as the proximal recess, and larger amounts of fluid must be present to separate the distal sesamoid bone from the DDFT.
What unique appearance might the DDFT show on T1 images?
A stippled appearance due to intratendinous foci of normal loose connective tissue septa.
How is the normal DDFT typically visualized on MRI?
It has regular margins and uniform low signal intensity on standard sequences.
How does the DDFT appear in the distal limb on MRI?
Formed by two symmetric rounded lobes separated by a high signal intensity septum.
What shape does the DDFT take at its insertion point?
crescent-shaped with minimal lobe separation.
What is the ‘magic angle effect’ and how does it relate to the DDFT on MRI?
It’s a phenomenon causing high signal intensity in the distal aspect of the DDFT, seen as a normal finding in some sequences.
How are DDFT lesions typically identified on MRI?
By the loss of symmetry in shape, size, or signal between the two lobes of the tendon.
What MRI image characteristics indicate DDFT lesions?
Increased signal intensity on T2 and PD images.
What are the common types of DDFT lesions?
Core lesions, parasagittal splits, and dorsal irregularities or fibrillations.
Where do lesions of the distal DDFT typically occur?
Proximal to the DSB, at the level of the collateral sesamoidean ligament and proximal recess of the podotrochlear bursa, and at the level of and distal to the DSB.
What additional foot structure should be inspected in cases of DDFT lesions?
he flexor surface of the distal phalanx for resorptive lesions.
What term is considered more appropriate for describing DDFT lesions localized to the foot?
Tendinopathy, as opposed to tendonitis.
What is the significance of adhesions of the DDFT on the collateral sesamoidean ligament and distal sesamoid bone?
They have important treatment and prognostic considerations and may benefit from débridement during bursoscopy.
How is the distal sesamoidean impar ligament visualized on high-field MRI?
Easily seen due to the high signal intensity of the synovial fluid within the adjacent DIP joint and distal recess of the podotrochlear bursa.
What challenge exists in evaluating the distal sesamoidean impar ligament on low-field MR images?
Typically, only one slice on each sequence clearly shows the ligament due to thicker slice acquisition.
What should be considered when interpreting increased signal intensity within the distal sesamoidean impar ligament?
Interpret with caution due to its fan shape and synovial in-pouchings between fiber bundles.
What MRI finding is associated with the origin of the distal sesamoidean impar ligament?
Fragmentation, seen clearly on thin-slice planes oriented over the DSB.
How does the distal recess of the podotrochlear bursa respond to fluid accumulation?
It does not distend as much as the proximal recess, requiring larger amounts of fluid to separate the DSB from the DDFT.
What is the clinical significance of fragmentation at the origin of the distal sesamoidean impar ligament?
It is associated with other pathological MRI abnormalities of the DSB, but its contribution to pain and lameness is unclear.
What orientation makes the collateral sesamoidean ligament challenging to evaluate in MRI?
Its oblique orientation.
How does the collateral sesamoidean ligament appear on transverse MRI images?
It is formed by two thin branches (lateral and medial) originating from the dorsal distal aspect of the proximal phalanx.
What changes occur in the collateral sesamoidean ligament proximal to the DSB?
The branches merge with the ligament’s body, forming a narrower central body with symmetric lateral and medial branches.
What is a common normal finding within the collateral sesamoidean ligament’s branches on MRI?
Ill-defined symmetric increased signal intensity just proximal to where the branches merge with the ligament’s body.
What additional pathologies are often associated with lesions of the collateral sesamoidean ligament?
DSB and/or DDFT lesions.
How is injury to the collateral sesamoidean ligament typically manifested on MRI?
Thickening of its body and/or branches, diffuse or focal increased signal intensity, and loss of separation from the DDFT.
Figure 72-11. (A) Sagittal proton density (PD) image of the foot. The collateral sesamoidean ligament is thickened with high signal intensity (arrow) close to its insertion onto the distal sesamoid bone. (B) Transverse short tau inversion recovery (STIR) image at the level of mid-second phalanx. The collateral sesamoidean ligament is thickened (arrow) with increased signal intensity consistent with desmopathy. Sagittal PD (C) and transverse STIR (D) images of the contralateral foot of the same horse as (A) and (B), showing a normal collateral sesamoidean ligament.
Figure 72-12. (A) Sagittal short tau inversion recovery image of the foot. Note the focal high signal intensity in the subchondral bone of distal second phalanx consistent with an osseous cystlike lesion (arrow). (B) Sagittal proton density image shows a rim of hypointensity surrounding the subchondral bone lesion consistent with sclerosis (arrow).
Figure 72-13. (A) Sagittal short tau inversion recovery image of the foot, showing a round signal void (black) surrounded by a high signal intensity (white) rim in the region of the distal phalanx. This appearance is caused by severe magnetic susceptibility artifact because of an incompletely removed shoe nail. (B) Transverse T2-weighted image of the same foot, at the level of the distal phalanx. Note the anatomic distortion of the distal phalanx because of a magnetic susceptibility artifact (left side).
What can be observed at the ligament’s insertion onto the DSB in case of injury?
They have mildly heterogeneous low signal intensity.
What are common MRI findings in lame horses related to the collateral ligaments of the DIP joint?
Focal or diffuse increased signal intensity, poorly defined margins, and osseous changes at the attachment sites.
What term is considered more accurate to describe MRI changes of the collateral ligaments of the DIP joint?
Desmopathy, as opposed to desmitis, due to the primarily degenerative changes observed.
What are the criteria for considering the distal digital annular ligament abnormal on MRI?
Focal or diffuse thickness greater than 2 mm and heterogeneous signal intensity.
What can be observed with high-field MRI in relation to the distal annular ligament?
Adhesions between this ligament and the DDFT.
What is a potential treatment for horses with distal annular ligament desmopathy detected by MRI?
Tenoscopic surgical transection of the ligament.
Figure 72-14. (A) Transverse gradient recalled echo (GRE) sequence with susceptibility artifact caused by metallic debris along the hoof. Note this artifact results in false increased signal within the distal phalanx (arrow), with an adjacent area of decreased signal (arrowhead). (B) Transverse STIR image at the same level. This sequence displays less susceptibility artifact. Signal within the distal phalanx is homogeneous, confirming the increased signal on the GRE image as artifactual rather than pathology.
Figure 72-15. (A) Sagittal proton density image of the foot. Note the increased signal intensity within the deep digital flexor tendon (DDFT) (arrow) distal to the distal sesamoid bone because of magic angle effect. (B) Sagittal T2-weighted image of the same foot. Because of the long echo time of the T2 sequence, the hyperintense region in the distal DDFT is no longer seen.
Figure 72-16. Transverse T2-weighted image of the distal third phalanx, at the insertion of the deep digital flexor tendon. Note the small (pinpoint) high signal intensity dots oriented vertically in the image because
of zipper artifact (arrows). This artifact is caused by extraneous radiofrequencies entering the magnet’s room when the room’s door is opened during scanning.
Figure 72-17. Sagittal T2-weighted image of the foot. Note the repeated hypointense curved lines (arrows) that repeat themselves through the first and second phalanges. These lines are caused by motion artifact.
Figure 72-18. Transverse STIR images of the foot. Note the ghosting (arrows) of the palmar vasculature is in the horizontal direction on the first image (A), and then in the vertical direction on the second set of images (B).
How are the medial and lateral distal oblique sesamoidean ligaments usually visualized on MRI?
As symmetric structures with low signal intensity and thin linear intermediate-to-high signal areas.
What is a normal finding between the suspensory ligament and the second metacarpal bone on MRI?
A small space of intermediate-to-high signal intensity, indicating loose connective tissue.
What treatment might benefit horses with adhesions between the suspensory and the splint bone?
Surgical release of adhesions and partial splint bone ostectomy.
What should be considered in interpreting the low signal intensity between the suspensory ligament and the second metacarpal bone?
It should be interpreted with caution as osseous proliferation and an enlarged suspensory ligament could give a similar appearance.
What is important in assessing the accessory ligament of the DDFT on MRI?
Comparing with the contralateral limb to avoid overinterpretation.
What MRI findings suggest articular cartilage injury?
Loss of the normal high signal intensity, corresponding to defects or thinning.
How is articular cartilage typically represented on MRI?
Articular cartilage appears with uniform, relatively high signal intensity adjacent to the low signal intensity of subchondral bone.
Why are three-dimensional GRE sequences advantageous for evaluating cartilage damage?
They allow for thin slices to be obtained, providing better spatial resolution of the relatively thin cartilage layer.
What makes confidently identifying cartilage lesions challenging on MRI?
The relatively thin cartilage layer can average its signal with adjacent bone or joint fluid, complicating lesion identification.
What advanced technique has been investigated for imaging articular cartilage?
Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC)
How does dGEMRIC work?
It uses a gadolinium-based contrast agent and measures differences in cartilage signal after a delay, correlating with degeneration.
Limitation: A delay of 60 minutes or more is needed for contrast penetration, and there are technical challenges with thin cartilage in some joints.
What causes a Magnetic Susceptibility Artifact in MRI?
A. Tissue movement
B. Magnetization of tissues
C. Software glitches
D. Electrical interference
B. Magnetization of tissues
What does a Susceptibility Artifact typically look like on MRI?
A. A bright area
B. A hypointense signal void
C. A blurred image
D. A repeated image pattern
B. A hypointense signal void
Which MRI sequences are more prone to severe susceptibility artifacts?
A. SE sequences
B. GRE sequences
C. T2-weighted sequences
D. T1-weighted sequences
B. GRE sequences
Why is air/tissue interface a concern in MRI artifacts?
A. It increases the scan time
B. It changes the color of the image
C. It causes signal distortion due to different susceptibilities
D. It reduces image clarity
C. It causes signal distortion due to different susceptibilities
What is the ‘Magic Angle Effect’ in MRI?
A. A software malfunction
B. Increased signal intensity due to specific ligament alignment
C. Distortion caused by the patient’s movement
D. An error in the MRI machine’s magnetic field
B. Increased signal intensity due to specific ligament alignment
How does the ‘Magic Angle Effect’ appear in ligaments and tendons?
A. As low signal intensity
B. As hypointense regions
C. As high signal intensity
D. As blurred areas
C. As high signal intensity
What does the ‘Zipper Artifact’ in MRI typically result from?
A. Movement of the patient
B. Electromagnetic energy leakage
C. Incorrect positioning of the patient
D. The use of contrast agents
B. Electromagnetic energy leakage
What is the appearance of a Zipper Artifact in MRI images?
A. Lines of white and black dots
B. Brightly colored areas
C. Blurred sections
D. Repeated patterns of the image
A. Lines of white and black dots
Which artifact is most sensitive to subject motion in MRI?
A. Motion Artifact
B. Zipper Artifact
C. Magnetic Susceptibility Artifact
D. Magic Angle Effect
A. Motion Artifact
How does a Motion Artifact manifest in MRI images?
A. As signal voids
B. As bright spots
C. As blurring and ghosting
D. As distorted colors
C. As blurring and ghosting
What is a common cause of Motion Artifact in MRI of horses?
A. Metal objects
B. Sedation level
C. Breathing of the horse
D. The type of MRI machine
C. Breathing of the horse
What is the primary effect of ‘flow artifact’ from blood vessels in MRI?
A. Increased signal intensity
B. Decreased signal intensity
C. Change in blood vessel diameter
D. Distortion in image shape
C. Change in blood vessel diameter
What is a method to minimize respiration-related motion in MRI?
A. Using a faster pulse sequence
B. Employing motion-correction software
C. Placing sandbags on the limbs
D. Applying a higher field strength
C. Placing sandbags on the limbs
How can ‘flow artifact’ be addressed in MRI?
A. Changing the patient’s position
B. Using a different pulse sequence
C. Reperforming the image series with altered settings
C. Reperforming the image series with altered settings
What is the purpose of ‘navigator echo’ in MRI?
A. To enhance image quality
B. To track motion during an examination
C. To decrease the scan tim
eD. To increase signal intensity
B. To track motion during an examination
What does ‘Volume Averaging Artifact’ result from in MRI?
A. The thickness of each MRI image slice
B. Incorrect positioning of the patient
C. Interference from external radiofrequency
D. Movement of the patient during scanning
A. The thickness of each MRI image slice
How does Volume Averaging Artifact typically affect MRI images?
A. Causes blurring
B. Results in repeated images
C. Leads to the appearance of false lesions
D. Creates a zipper-like pattern
C. Leads to the appearance of false lesions
What increases the likelihood of Volume Averaging Artifact?
A. Using a high-field magnet
B. Shorter scan times
C. Thicker slice thickness
D. Lower patient motion
C. Thicker slice thickness
