The Long Explaination...

There are many terms that are used in MRI and below is a brief bulleted overview of important terms as they relate to equine MRI.

  • A pulse sequence is a measurement technique by which an MR image is generated.
  • It is the software which contains hardware instructions necessary to acquire data in desired manner
  • A large number of possible pulse sequences available.
  • The pulse sequence chosen determines the type of contrast observed in the resultant MR image.
  • The same tissue can appear very different depending on the pulse sequence selected.
  • The way tissue appears on many sequences helps one assess the type of tissue imaged and in some cases the likely disease process.
  • Similar sequences often have different names depending on MR manufacturer.
  • T1 and T2 are measurable parameters of tissue that indicate how a particular tissue ‘behaves’ in response to the magnetic field and the applied RF pulse. 
  • Image contrast is markedly influenced by the T1, T2 and proton density characteristics of the tissue.
  • Weighting refers to the general configuration of the sequence and is done to accentuate one of the MR recovery characteristics of tissue.
  • Proton Density refers to the density of hydrogen protons/voxel and the image appearance is independent of T1 and T2 effects.
  • The most commonly used pulse sequences in clinical imaging are fast spin echo (FSE), inversion recovery (IR), and gradient recalled echo (GRE).
  • Each pulse sequence may take many minutes to acquire, (less on high field magnets) and sequences must be acquired consecutively (except PD/T2)
  • Proton Density and T2 weighted FSE images can be acquired concurrently, so called Dual or Multi echo.
  • 2 major measurement parameters determine whether a sequence is T1 weighted or T2 weighted. TR = repetition time – the time interval between each new RF pulse cycle and TE = time delay between termination of the RF pulse and collection of the RF signal emitted by the patient.

Common Pulse Sequences

  • Spin Echo
    • Spin Echo (SE) or ‘Conventional’ SE is the simplistic clinical imaging sequence and is the basis for many of the commonly sequences.
  • Spin echo weighing
    • SE sequences can be modified to emphasize T1 differences (T1 weighting), T2 differences (T2 weighting), or proton density differences.
  • Fast spin echo
    • Fast spin echo (FSE) techniques are essentially a modified Conventional Spin Echo. The ‘fast’ technology markedly reduces image acquisition time without causing a significant negative impact on the appearance of the image
    • Because of the time saving Fast Spin Echo (aka Turbo) has largely replaced conventional spin echo sequences.

Inversion recovery

  • Inversion recovery (IR) is a method by which the signal from certain tissue types can be nulled out (ie appears black on image)

  • Fat and Fluid are the two tissue types commonly nulled by this technique.

  • STIR  A T2 weighted sequence but where fat, which would normally appear bright has been nulled, and now tissue comprising fat, appears dark.

  • FLAIR is another T2 weighted inversion sequence in which the signal from free fluid is nulled. Used primarily in neuroimaging to allow critical evaluation of the periventricular tissues of the brain in a T2 weighted image. FLAIR improves visualization of difficult to see subtle high signal pathology adjacent to regions of normal CSF. Not normally used in equine distal limb.

Gradient recalled echoes (gradient echo imaging)

  • Gradient recalled echo techniques make use of short TRs, short TE’s, low flip angles and other acquisition modifications.

  • This combination permits fast acquisition times and also allows three-dimensional imaging within reasonable times.

  • Short TE values emphasize T1 differences between tissues.

  • Examples of fast imaging include fast low-angle shot (FLASH), and fast imaging with steady state precession (FISP) (Siemens)

  • Parameters can be adjusted to emphasize blood flow and, therefore, angiographic images can be created.

Three-dimensional imaging

  • Image data is typically acquired as slice data and usually images in 3 planes are generated

  • In 3D imaging, data is acquired from an entire volume in one sequence. Post processing can be done to allow image generation in any selected plane.

  • These sequences typically take longer to acquire.

  • Requires a compliant/anesthetized patient as movement during acquisition usually disrupts the whole study

  • A main advantage of 3D sequences include the high resolution in all three orientations and the availability of contiguous sections.

General Principles of Image Interpretation

  • The prettiest images may not necessarily be the most informative. The objective is to image in such a way as to generate CONTRAST between pathology and normal tissue.

  • Knowledge of normal anatomy and ‘normal’ variations is mandatory.

  • The clinical significance of many ‘lesions’ found on MR has yet to be established.

  • Cannot confirm the presence of a lesion in one plane if it cannot be verified by visualization in the orthogonal plane.

  • Generally it is easier to see bright pathology against a dark background of normal tissue.

  • Sequence acquisition is done consecutively. There is a limit to the # of sequences that can be done, - due to patient compliance (standing) issues and potential anesthesia complications (recumbent).

  • Generally, pathologic tissue has more hydration than normal tissue and this results in an increase in signal on T2 weighted images and a decreased signal (often hard to detect) on T1 weighted images.

Basic appearance of tissue relative to sequence weighting (1.5 T)

  • Tissues with low numbers of protons appear dark on all image sequences

    • Cortical bone

    • Normal ligaments and tendons

    • Non-articular cartilage (Meniscus)

    • Areas of mature bone sclerosis

    • Normal lung

    • Insensitive lamellae and outer hoof

  • T1 weighted Images

    • Fat is bright

    • Fluid is dark

    • Active pathology (edema) is usually darker than surrounding normal tissue and can be difficult to see.

    • Fat infiltration and chronic fibrous tissue usually brighter than surrounding normal tissue

    • T1 weighted images are good for anatomic detail but less useful for detecting subtle active pathology (1.5T), unless combined with intravenous/cavitary contrast medium administration.

    • More useful in low field imaging 

    • Bone sclerosis appears dark (decreased hydration of bone)

    • T1-weighted images are obtained with a short TR (less than 600 ms) and a short TE (less than 20 ms).

  • T2 weighted images

    • Fat is bright on T2 weighted TURBO or FAST sequences.

    • Free fluid is bright.

    • Most active pathology is brighter than adjacent normal tissue (eg: bone bruising, ligament and tendon edema, joint effusion).

    • Bone sclerosis appears dark (decreased hydration of bone).

    • Connective tissue and chronic scar tissue is intermediate in signal intensity.

    • T2-weighted images are obtained with a long TR (more than 2000 ms) and a long TE (more than 90 ms).

  • Proton Density weighted images

    • Fat and fluid are relatively bright but not as bright as in T1 and T2 images respectively

    • Cartilage is usually brighter c/f T2. 

    • Active inflammation usually bright but not as bright as T2 and STIR.

    • Connective tissue intermediate in signal intensity.

    • Have generally less range of contrast compared to T1 and T2 weighted.

    • Best for cartilage of the Conventional SE and Fast SE sequences.

    • Proton density weighted images are obtained with a long TR (more than 2000 ms) to minimize T1 differences and a short TE (less than 20 ms) to minimize T2. differences. The resulting signal intensity is a reflection of the H+ proton density /voxel.

  • STIR (turbo T2 weighted)

    • All tissues characteristics as for T2 except fat is nulled and therefore has no signal appearing dark.

    • Allows critical evaluation of subchondral trabecular bone with respect to bruising and cyst formation.

    • Without fat being nulled the high signal associated with bone edema and normal fat would be superimposed. With STIR the bruised marrow/trabecular bone appears bright against a background of normal (dark) marrow.

    • STIR is an excellent screening sequence.

    • In many situations considered more sensitive than bone scintigraphy.

  • Differences exist in tissue appearance in images generated in low field (0.3T) vs high field (1.5T) magnets.

Standing Magnet and Low Field Imaging

  • At low field T1 is shorter than at high field, and there is a greater difference in T1 values between different tissues, and between normal and pathological tissue. 

  • T1 weighted images also used as can be acquired in a shorter time compared to other sequences.

  • T1 weighted sequence is relatively more useful at low field, giving both a high SNR and a high contrast to noise, particularly for bone pathology. (PD weighted supersedes this sequence on high field magnets). 

  • Ligament or tendon damage usually results in an increase in signal in low field T1 images – tendon changes from black to gray. Chronic fibrous tissue and non active lesions may be gray in T1 weighted image and show no increase in signal on T2 weighted images, (T2 and STIR) as  no active edema. 

  • Acutely damaged tendons or ligaments may appear gray on T1 and show high signal on T2 weighted images (due to active edema).

  • To reduce potential for movement artifact dual echo sequences not run concurrently as there is a slight time penalty and therefore potential for movement artifact.

  • Use more gradient echo T1 and T2* (star) weighted sequences, and 3D sequences, as less acquisition time. 

  • Motion correction software.

  • STIRand FSE available.

Slice Positioning

  • Accurate slice positioning critical.

  • Symmetry plays an important role in interpretation.

  • Optimal position can change depending on pathology being imaged, but ..

  • Limited by time so must select the most useful imaging planes.

  • Usually only 3 plane imaging sagittal, transverse and dorsal.

  • Volume acquisition with isotropic pixels allows post processing reconstruction in any plane.

Magnetic Resonance Artifacts

  • Artifacts are areas of high or low signal intensity or distortion in the image that can simulate or mask anatomic structures or pathologic.

  • To minimize or overcome MR artifacts, their origin must be understood.

  • It is important to able to differentiate MR artifacts from both normal anatomy and pathologic conditions.

Common artifacts in equine distal limb imaging

  • Patient motion is common because of the long image acquisition times.

  • Patient motion results in ghost images that normally appear in the phase-encoding direction due to mismapping of measured signals.

  • Pulsing vessels and flowing blood result in MR image artifacts, also in the phase encoding direction. Phase and frequency encoding directions can be changed to negate a pulsatile artifact if it is a problem.

  • Inhomogeneity in the main magnetic field has a significant impact on GRE and most other fast imaging techniques. Affects FSE less.

  • Metallic (especially ferromagnetic) susceptibility artifact appears as a black hole and local image distortion as a result of disruption (inhomogeneity) of the local magnetic field. Rust or remnants of nails in the hoof will result in metallic susceptibility artifact.

  • Magic angle artifact is a change (increase) in signal that occurs in tissues with internal architecture at 550 to the main magnetic field. It is seen only in images generated using a low TE.  Most commonly seen as an increase in signal in the distal aspect of the deep digital flexor tendon on T1 weighted and Proton Density (PD) images. Assess the same structure in a T2 weighted image to differentiate.

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