Technical Considerations & Artifacts



Technical Considerations & Artifacts


Peter Stanwell, PhD

Robyn L. Birdwell, MD, FACR










Sagittal GRE MR 3-plane localizer shows good positioning (left). The right breast is too cephalad within the coil with signal loss superiorly image and inclusion of abdominal fat inferiorly image.






Axial T1WI C+ FS MR shows random noise artifacts image causingSNR with image degradation. The “snowstorm” appearance throughout the image is caused by improper tuning of the RF transmitter.


TERMINOLOGY


Definitions



  • Factors leading to nondiagnostic &/or confounding features seen on breast MR


  • Artifacts are image features that do not faithfully reproduce actual anatomic structures due to distortion, addition, or deletion of information


  • May be patient, sequence, or equipment related



    • Motion = patient


    • Susceptibility = patient or sequence



      • Metal; inhomogeneous fat saturation


    • Aliasing = sequence


    • Chemical shift = sequence


    • Parallel imaging related = sequence


    • Radiofrequency issues = equipment



      • Zipper due to RF leak, random noise, faulty board


Magnet Strength



  • Translating imaging protocols from 1.5 to 3T requires parameter changes



    • Parameter differences between different magnetic field strengths (e.g., opposed phase TE = 6.7 ms at 1.5T and 3.4 ms at 3T)


  • Increased RF heating: Specific absorption rate (SAR) increases with field strengths


  • Greater concern for safety of metallic implants at higher magnetic field



    • IUDs, cardiac stents, neuro stimulators, otologic implants, orthopedic hardware


  • Larger artifacts secondary to metallic post core biopsy markers and surgical clips are seen at higher field strengths


  • Inhomogeneous B1 electromagnetic field can affect signal intensity



    • Deviations in transmitted RF flip angle that are spatially dependent lead to signal loss



      • Severity scales with flip angle and technique (e.g., 2D worse than 3D, high flip angle > lower flip angle)


  • RF interferences (dielectric resonances)



    • Wavelength of RF field approximates geometric dimensions of imaged object


    • Constructive or destructive interferences appear as central brightening or local signal loss


  • Siting issues



    • Greater potential for interactions with surrounding environment at higher field strength



      • External power sources


    • 1.5T and 3T systems can be sited together as they operate at different frequencies


Coil and Patient Positioning



  • Surface coil = potent determinant of signal to noise ratio (SNR)


  • Dedicated breast coils are prerequisite, preferably with biopsy capability


  • Careful breast positioning = optimal localized homogeneity


  • Efficacy of frequency: Selective fat-saturation pulses can be diminished with magnetic field inhomogeneities



    • Effects are more pronounced the further the image slice is from isocenter of magnet


    • Correct landmarking of breast in superoinferior (Z) direction is required to reduce localized inhomogeneity



      • Optimizing placement of breast at isocenter of magnet


    • Field homogeneity may be improved by setting more specific shim volume


CLINICAL IMPLICATIONS


Clinical Importance



  • Once artifacts are recognized, many can be limited or corrected and their clinical impact reduced



PATIENT RELATED


Periodic Movement



  • Coherent ghosts are caused by blood flow, cardiac, or respiratory motion


  • Limit cardiac/pulsation artifacts by selecting appropriate phase-encoding direction


  • Limit flow-related artifacts by placing presaturation pulses


Voluntary Movements



  • Coughing or limb movements result in image smearing or blurring


  • Ensure patient comfort; premedicate for severe claustrophobia


Body Habitus



  • Coil size may be limiting factor for larger breasts


  • Tissue outside or touching coil → decoupling of coil → localized bright signal artifacts


Metallic Artifacts



  • Local magnetic field associated with metal affects precessional frequencies of adjacent tissue = stronger dephasing of spins → localized magnetic field inhomogeneity



    • Artifacts seen as localized bright and dark areas with varying degrees of spatial distortion of surrounding anatomy


    • Severity of distortion is dependent of B0 strength and type of metal


    • Severity increases with longer echo times and gradient echo sequences


    • Limited by using FSE sequences, shortening echo times, ↑ receiver bandwidth, ↑ frequency matrix, &/or by ↓ slice thickness


Susceptibility Artifacts



  • Magnetic susceptibility: Extent that material placed in magnetic field becomes magnetized


  • Susceptibility varies substantially for different materials (e.g., air, different biologic tissues, metals)


  • Differing magnetic fields = induced superimposition over original magnetic field (B0) to create inhomogeneities that can limit overall field homogeneity


  • Scales with magnetic field strength


Inhomogeneous Fat Suppression



  • Achieving good fat suppression



    • Active suppression during sequence selection (frequency-selected fat suppression)


    • Requires homogeneous magnetic field (B0)


    • Optimize local magnetic field (shimming)


    • Higher field strength = better fat, water peak separation


    • Inhomogeneously suppressed images continue to demonstrate high signal of fat, limiting recognition of enhancing lesions


    • Subtraction



      • May fail in cases of patient movement


      • May be worse at higher spatial resolution


SEQUENCE-RELATED


Aliasing or Wrap Around Artifacts



  • When any part of body extends outside field of view (FOV), producing signal that reaches receiver coil


  • Extended body part is visualized (wrapped) on opposite side of image


  • Eliminate in frequency-encoding direction



    • Apply low-pass filter to received signal attenuating higher frequencies outside FOV


    • Oversampling in frequency-encoding direction


  • Eliminate in phase-encoding direction



    • Measure larger FOV to cover all signal-producing tissues


    • Increasing sampling rate = “phase oversampling”


    • Decrease signal from structures outside FOV



      • Apply spatially located presaturation pulses


      • Use local surface coils


  • 3D specific



    • Slice is superimposed from out of FOV into image stack in Z-axis




      • May eliminate through “phase oversampling” and spatially located presaturation pulses


  • Moire fringes (zebra artifacts)



    • Aliasing artifacts + field inhomogeneities = interference patterns from 1 side of body, superimposed on other side


    • Interference patterns alternatively add and subtract to generate artifacts


    • Particular to gradient echo sequences


Chemical Shift



  • 1st degree: Different resonance frequencies of hydrogen vs. water in fat



    • Seen as bands in frequency-encoding direction with dark border at 1 fat-water interface and bright border at opposite interface


    • Severity depends on pixel size and bandwidth per pixel of imaging sequence


  • 2nd degree: Exclusively in gradient echo imaging



    • Signals from water and fat either cancel out or amplify each other


    • Dark rim at interface between fat and water-containing tissue


    • Limited by ↑ pixel, ↑ bandwidth, active fat suppression, appropriate echo time (in phase), swapping phase and frequency-encoding directions


Parallel Imaging



  • These techniques use local coil sensitivities to provide spatial information to reduce phase-encoding steps required for image reconstruction (reduced scan time)


  • Imaged object larger than FOV = aliasing artifacts + possible additional central image artifacts


  • Location of artifacts is dependent on acceleration factor used



    • If acceleration factor is 0, then artifact transforms into traditional wrap around artifact


    • Overly aggressive acceleration factors increase severity of artifacts


EQUIPMENT-RELATED


Zipper Artifact



  • Common; secondary to hardware problems



    • Extrinsic signals in scan room (e.g., scan room door does not close completely)


    • Non-MR-compatible devices (e.g., monitoring or biopsy equipment)


    • Defective light bulbs; static electricity


  • Appear as discrete lines of noise or alternating bright & dark pixels across image in phase-encoding direction


  • Width and position depend on frequency and bandwidth of interfering signal


Random Noise



  • Appears as low SNR or noisy static on images


  • May have many sources including



    • Electronic noise from failed hardware components


    • Unshielded devices in MR scan room



REFERENCES

1. Genson CC et al: Effects on breast MRI of artifacts caused by metallic tissue marker clips. AJR Am J Roentgenol. 188(2):372-6, 2007

2. Harvey JA et al: Breast MR imaging artifacts: how to recognize and fix them. Radiographics. 27 Suppl 1:S131-45, 2007

3. Ojeda-Fournier H et al: Recognizing and interpreting artifacts and pitfalls in MR imaging of the breast. Radiographics. 27 Suppl 1:S147-64, 2007

4. Bernstein, MA et al; Imaging artifacts at 3.0 T. JMRI 24(4):735-46, 2006

5. Rausch DR et al: How to optimize clinical breast MR imaging practices and techniques on Your 1.5-T system. Radiographics. 26(5):1469-84, 2006

6. Zhuo J et al: AAPM/RSNA physics tutorial for residents: MR artifacts, safety, and quality control. Radiographics. 26(1):275-97, 2006

7. Coulthard A et al: Pitfalls of breast MRI. Br J Radiol. 73(870):665-71, 2000

8. Arena L et al: MR imaging artifacts that simulate disease: how to recognize and eliminate them. Radiographics. 15(6):1373-94, 1995






Image Gallery









Axial FSE MR shows a left-to-right wrap around artifact image from the patient’s arms, which in this case were positioned by her side.






Sagittal T1WI C+ FS MR shows a bright signal artifact image in the inferior breast where the inferior posterior tissue is touching the coil.






(Left) Axial GRE FS MR shows homogeneous fat suppression associated with optimal localized shim and position within the breast coil. (Right) Axial GRE FS MR of a different patient shows inhomogeneous fat suppression image associated with suboptimal localized shim and breast position within the coil.

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Sep 18, 2016 | Posted by in OBSTETRICS & GYNAECOLOGY IMAGING | Comments Off on Technical Considerations & Artifacts
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