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.
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).
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).
