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. 2021 Sep 30;22:101616. doi: 10.1016/j.jcot.2021.101616

Table 1.

Summary of the recent advances, applications and drawbacks of CT Scan and MRI in postoperative imaging in musculoskeletal surgery.

Imaging modality Advances Applications in post operative imaging Drawbacks/Disadvantages
CT Scan: CT is readily available, easy to perform and less expensive modality used in post operative period. It is popular imaging tool used mainly in revision arthroplasty to look for bone loss and complex anatomies before revision surgeries. Although soft tissue details pertaining to collections, infection can also be evaluated by CT scan, MRI is preferred for such details.
  • 1.

    Multi detector CT scan (MDCT): will use wide Xray beam to cover to multiple detector rows, hence multiple sections at same slice can be obtained which can be reconstructed to form single image without increasing the dose.
  • 1.

    Isotropic volumetric acquisition with multiplanar reformatting capability.
  • 1.

    Radiation exposure
  • 2.

    Excellent resolution of bony details and implants
  • 2.

    Limited soft tissue contrast
  • 3.

    Faster acquisition time
  • 3.

    Metal artifacts.
  • 2.

    Advances in metal artifact reduction: Routine techniques like Image acquisition with higher kilovoltage and increase in mAs to increase the x ray penetration, Decrease in collimation, avoiding the imaging plane parallel to thickest portion of the metal, gantry tilt, reduced pitch can be used to reduce the artifacts Advancement in post processing techniques as below results in great deal of artifact reduction.
  • 1.

    Routine techniques mentioned here are greatly helpful even with low configuration scanners and reduces the metal artifacts and technicians can easily follow these procedures.
  • 1.

    These routine techniques to reduce metal artifact may not feasible in all the situations for example gantry tilt and change in imaging plane may not be possible for hip implants. Increasing the X Ray energy can result in increased dose.
  • a)

    Multiplanar reconstruction
  • 1.

    MPR images and using raw data to obtain images with thicker slides is possible with MDCT and is useful in good metal artifact reduction.
  • 1.

    Although present scanners have inbuilt advanced post processing techniques these are not widely available.
  • b)

    Reconstruction of images from the acquired raw data with thicker slices.

  • c)

    Iterative image reconstruction instead of filtered backprojection
  • 2.

    Combinations of various methods of image reconstruction methods and algorithms reduce the metal artifact without additional radiation dose.
  • 2.

    In spite of great deal of soft ware advancement, metal artifact reduction is not completely overcome in most of the cases. There is also possibility of image blurring with MAR algorithms.
  • d)

    MAR algorithms (sinogram inpainting techniques). Here corrupt X-ray projections that traversed the metal hardware are removed and replaced with interpolation from adjacent unaffected projections to obtain artifact free image.

  • e)

    Combination of MAR and Iterative software
  • 3.

    Additionally, using soft reconstruction kernel instead of bone kernel which is routinely used for MSK imaging and wider window setting
  • 3

    Dual energy CT scan: also known as spectral CT, is a computed tomography technique that uses two separate x-ray photon energy spectra, allowing the interrogation of materials that have different attenuation properties at different energies. Whereas conventional single energy CT produces a single image set, dual energy data (attenuation values at two energy spectra) can be used to reconstruct numerous image types.
  • 1.

    Knowing how a substance behaves at two different energies can provide information about tissue composition beyond that obtainable with single-energy techniques.
  • 1.

    Not widely available, expensive and requires a learning curve to interpret various image sets at different energy settings.

  • 2. Early detection of subtle fractures.
  • 2

    Bone marrow edema detection is only suitable for yellow marrow.
  • 3.

    DECT provides an additional option for metal artifact reduction, particularly the beam-hardening component, without any radiation dose penalty. Virtual monoenergetic imaging simulates images acquired with a monoenergetic x-ray beam and can decrease beam hardening. This is helpful in reduction of the metallic artifact and optimal visualization of periprosthetic soft tissues.
  • 3.

    The Virtual monoenergetic metal artifact reduction techniques unique to DECT may not be applicable for larger and denser metals.
Magnetic resonance imaging (MRI): Due to its superior soft tissue resolution MRI is increasingly used as a problem-solving modality in various complications like soft tissue collections around the implant, adjacent muscle and tendon pathologies, tumor surveillance. It is well known that ferromagnetic materials will be attracted to the main magnetic field of an MR scanner; however, it is safe to scan patients with orthopedic hardware such as intramedullary nails, screws, malleable plates, and joint prostheses, even in the immediate postoperative period.
  • 1

    Metal artifact reduction techniques: Generally, artifacts less severe in 1.5T machines as compared to 3T machines. Spin echo and using higher bandwidth radio frequency waves for excitation as well as for receiving the signals will help to reduce incoherence and Signal loss. Spatial misregistration can be overcome by swapping the direction of slice selection and readout gradients.
  • 1.

    All these parameter changes are inbuilt in to the system as metal artifact reduction sequence in most of the scanners and help in reduction of the artifacts.
  • 1

    These are time consuming and result in increase in heat deposition
  • 2.

    Short tau inversion recovery (Stir) is preferred technique since its immune to metal induced magnetic field changes.
  • 2.

    Stir sequences are immune to metal induce magnetic field changes.
  • 2.

    Stir sequences should be avoided when gadolinium contrast is used. Dixon sequences is preferred when contrast is necessary.

  • Advanced techniques for metal artifact reduction:

  • a.

    View angle tilting (VAT): spatial misregistration occurring during slice selection and signal readout is reduced by replaying a gradient with an angle or a tilt to bring back the signals in coherence
  • a.

    mainly circumvents in plane image distortions
  • a.

    Unless VAT is combined with isotropic three-dimensional fast spin echo sequences (eg: space sequences in Siemens)) there will be a degree of blurring hence of no added value. These sequences can be also challenging in area with large surface area like hip joint where 3D imaging cannot be applied
  • b.

    Slice encoding metal artifact correction (SEMAC): this is a two-dimensional (2D) imaging method adds multiple spatial partitions to the (turbo) spin echo pulse sequence to mitigate the through plane artifacts. Simply put every slice is reconstructed by several images from multiple partitions which are reconstructed separately. These are then combined together with summation techniques to form a final composite image.
  • b.

    Combining SEMAC with VAT will give additional benefit of reducing in-plane and through plane distortions
  • c.

    Multi –acquisition with variable –resonance image combination (MAVRIC):MAVRIC the resonance frequency or RF pulse is split into multiple frequency bins or spectral bins. These frequencies are capable of exciting different set of protons in inhomogeneous B0. For each frequency bin, a sub image is acquired corresponding to a particular subset of protons which are then added up to form a final image.
  • c.

    Like SEMAC, MAVRIC has better resolution with isotropic 3D acquisition.
  • d.

    Both SEMAC and MAVRIC are time consuming due to spatial partitions and additional spectral bins respectively. Various acceleration algorithms have been developed and studied. However, these are in research applications.
  • 2.

    Diffusion Weighted Imaging: Diffusion weighted imaging which is routinely used to study brain perfusion is also available for MSK applications.
  • 2

    Helpful tool to detect abscess. Useful tool to differentiate tumor recurrence vs post treatment changes.
  • 2

    May be challenging in presence of metal implants. The quantitative values are not standardized.
  • 3.

    Dynamic Contrast Imaging: here the post contrast images are acquired in arterial, venous and late phases.
  • 3.

    useful for post tumor excision follow up studies. Helpful in targeted biopsy in tumor recurrence
  • 3.

    Can be equivocal in some cases due to post-operative soft tissue changes. Contrast cannot not be used in renal failure patients
  • 4.

    US-MR fusion: These are recent advanced software systems available where MRI image can be fused while performing guided biopsy using USG.
  • 4.

    helpful in targeted biopsy of the tumour.
  • 4

    Limited availability
  • 5.

    MRI wire localization: MRI comparable wire is used for localising the tumor
  • 5.

    helpful in selective excision of tumour
  • 5.

    Limited availability