Multi-detector CT imaging in the postoperative orthopedic patient with metal hardware

Multi-detector CT imaging (MDCT) becomes routine imaging modality in the assessment of the postoperative orthopedic patients with metallic instrumentation that degrades image quality at MR imaging. This article reviews the physical basis and CT appearance of such metal-related artifacts. It also addresses the clinical value of MDCT in postoperative orthopedic patients with emphasis on fracture healing, spinal fusion or arthrodesis, and joint replacement. MDCT imaging shows limitations in the assessment of the bone marrow cavity and of the soft tissues for which MR imaging remains the imaging modality of choice despite metal-related anatomic distortions and signal alteration.     

Magnetic resonance imaging artifacts: mechanism and clinical significance

Many types of artifacts may occur in magnetic resonance imaging. These artifacts may be related to extrinsic factors such as patient motion or metallic artifacts; they may be due specifically to the MR system such as power gradient drop off and chemical shift artifacts; they may occur as a consequence of general image processing techniques, as in the case of truncation artifacts and aliasing. Change in patient position, pulse sequence, or other imaging variables may improve some artifacts. Although reduction of some artifacts may require a service engineer, the radiologist has the responsibility to recognize MR imaging problems. The radiologist's knowledge of MR imaging artifacts is important to the continued maintenance of high image quality and is essential if one is to avoid confusing artifactual appearances with pathology.     

MR imaging and metallic implants for anterior cruciate ligament reconstruction: assessment of ferromagnetism and artifact.

Magnetic resonance (MR) imaging is contraindicated for patients with certain ferromagnetic implants, primarily because of potential risks related to movement or dislodgment of the devices. An additional problem with metallic implants is the potential image distortion that may affect the interpretation of the MR study. Since MR imaging is frequently useful for the evaluation of postoperative anterior cruciate ligament (ACL) reconstruction, the ferromagnetic qualities and artifacts associated with MR imaging were determined for five metallic orthopedic implants commonly used for this surgery. Only the Perfix interference screw displayed a substantial deflection force and caused extensive signal loss. Images of the knee of one patient with two Perfix screws in place were not interpretable because of the image distortion caused by these implants. Therefore, alternative nonferromagnetic implants should be considered for reconstruction of the ACL.     

Artifacts in MR imaging after surgical intervention

Artifacts are encountered repeatedly in magnetic resonance (MR) imaging after neurological orthopedic surgery, although there is no evidence of metallic foreign bodies on radiography and CT. Experiments demonstrated that surgical filling materials, such as bone cement and bone transplants are not the cause of the artifacts. On the other hand, we have been able to show that short-term contact between a diamond drill and untempered operating instruments can cause abrasions and set free minute metallic fragments. Those particles are missed by X-ray procedures including CT but cause MR artifacts by local disturbance of the homogeneity of the magnetic field. Therefore, MR represents a highly sensitive method to trace extremely small amounts of magnetic substances. Postsurgical MR, however, may be of limited value because of the disturbing artifacts.     

MR imaging of large nonferromagnetic metallic implants at 1.5 T

To evaluate the feasibility of magnetic resonance (MR) imaging in patients with nonferromagnetic metallic implants, we imaged implants in vitro and in 15 patients. Image artifacts both in vitro and in vivo occurred only locally and consisted of image distortions, areas of total signal loss, and lines of high signal intensity. The artifacts were most prominent in areas where the implants exhibited edges or points, but overall image quality was good except for regions lying very close to the implants. The implants themselves appeared as structures without signal in all patients. We conclude that in patients with femoral head prostheses or osteosynthetic plates that are nonferromagnetic MR may be preferable to CT, where beam hardening artifacts usually degrade image quality severely in the entire field of view.     

Magnetic resonance imaging of in vivo kinematics after total knee arthroplasty

PURPOSE: To improve the quality of magnetic resonance imaging (MRI) on knees after total knee arthroplasty (TKA) by minimizing image artifacts caused by metallic implants, and to establish a method determining in vivo kinematics of TKA knees using MRI. MATERIALS AND METHODS: Two knee implants made of cobalt-chrome and oxidized zirconium were tested with different pulse sequences and imaging parameters. Then, in vivo kinematic MRI was performed on five well-functioning TKAs under simulated weight-bearing conditions. Kinematic measurements were made and a linear correlation test was run between the tibio- and patellofemoral measurements. RESULTS: The best images with minimum metallic artifacts were observed using oxidized zirconium implants, a fast spin echo sequence (FSE), thin slice thickness, and high readout gradient. TKA kinematics exhibited a large deviation from the normal kinematics and considerable patient-to-patient variability. However, significant linear correlations between tibiofemoral and patellofemoral kinematics were observed (R = -0.96, 0.92, 0.88). CONCLUSION: Metallic artifacts due to orthopedic implants can be reduced in MR images for some materials, appropriate pulse sequence, and imaging parameters selection, enabling MR quantification of knee kinematics. Tibiofemoral kinematics appears to affect patellofemoral position after total knee arthroplasty.     

Optimizing imaging techniques in the postoperative patient

Postoperative patients may develop complications requiring imaging. Although any imaging technique can be used to investigate these patients, the presence of metal hardware in the region of interest may distort the image and interfere with diagnosis. It is important to understand why this distortion occurs and how to compensate for it. Because some of the most common cross-sectional imaging methods used to image this patient population are computed tomography (CT) and magnetic resonance imaging (MRI), this article focuses on these imaging methods. Metal-related artifacts on CT depend on the hardware alloy, the geometry of the hardware, and the location of the hardware relative to the region of interest. The artifacts may be reduced or eliminated by altering the scan technique, changing the patient position, selecting a smoother CT reconstruction algorithm, and by creating thicker slice multiplanar reformations. Like CT, metal artifacts at MR imaging depend on the type of hardware alloy. Hardware-related artifacts at MR imaging can be reduced by using appropriate pulse sequences, such as fast or turbo spin echo and inversion recovery. Additionally, important pulse sequence modifications that are addressed here include manipulation of the receiver bandwidth and orientation of the frequency encode axis.     

Managing postoperative artifacts on computed tomography and magnetic resonance imaging

Orthopedic hardware should not be considered a contraindication to computed tomography (CT) or magnetic resonance (MR) imaging. The hardware alloy, the geometry of the hardware, and the orientation of the hardware all affect the magnitude of image artifacts. For commonly encountered alloys, the severity of image artifacts is similar for CT and MR. Cobalt chrome or stainless steel hardware produces the most artifacts; titanium hardware produces the least. In general, image artifacts are most severe adjacent to the hardware. CT image artifacts are related to incomplete X-ray projection data resulting in streaks. These can be mitigated by increasing scan technique and using a smoother reconstruction filter. Hardware with a rectangular cross-sectional shape such as a fixation plate will cause more artifacts than a radially symmetrical device such as an intramedullary nail. Image artifacts at MR are caused by the hardware magnetic susceptibility and the induction of eddy currents within the metal. A turbo spin-echo sequence yields the best results. The use of larger image matrices, thinner slices, and a wide receiver bandwidth are recommended parameter adjustments when imaging patients with hardware. This article discusses how hardware-related artifacts can be minimized by altering scan technique and image reconstruction.     

Contrast-enhanced T1-weighted fluid-attenuated inversion-recovery BLADE magnetic resonance imaging of the brain: an alternative to spin-echo technique for detection of brain lesions in the unsedated pediatric patient?

RATIONALE AND OBJECTIVES: We compared contrast-enhanced T1-weighted magnetic resonance (MR) imaging of the brain using different types of data acquisition techniques: periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER, BLADE) imaging versus standard k-space sampling (conventional spin-echo pulse sequence) in the unsedated pediatric patient with focus on artifact reduction, overall image quality, and lesion detectability. MATERIALS AND METHODS: Forty-eight pediatric patients (aged 3 months to 18 years) were scanned with a clinical 1.5-T whole body MR scanner. Cross-sectional contrast-enhanced T1-weighted spin-echo sequence was compared to a T1-weighted dark-fluid fluid-attenuated inversion-recovery (FLAIR) BLADE sequence for qualitative and quantitative criteria (image artifacts, image quality, lesion detectability) by two experienced radiologists. Imaging protocols were matched for imaging parameters. Reader agreement was assessed using the exact Bowker test. RESULTS: BLADE images showed significantly less pulsation and motion artifacts than the standard T1-weighted spin-echo sequence scan. BLADE images showed statistically significant lower signal-to-noise ratio but higher contrast-to-noise ratios with superior gray-white matter contrast. All lesions were demonstrated on FLAIR BLADE imaging, and one false-positive lesion was visible in spin-echo sequence images. CONCLUSION: BLADE MR imaging at 1.5 T is applicable for central nervous system imaging of the unsedated pediatric patient, reduces motion and pulsation artifacts, and minimizes the need for sedation or general anesthesia without loss of relevant diagnostic information.     

Signal intensity artifacts in clinical MR imaging.

Signal intensity artifacts are often encountered during magnetic resonance (MR) imaging. Occasionally, these artifacts are severe enough to degrade image quality and interfere with interpretation. Signal intensity artifacts inherent in local coil imaging include intensity gradients and local intensity shift artifact. The latter can be minimized but not eliminated with optimal coil design and tuning. Improper coil or patient positioning can produce subtle or, in some cases, severe signal intensity artifacts, and each is easily corrected. Signal intensity artifacts and image degradation can also occur in a perfectly functioning coil if protocols are not optimized. Failure of decoupling mechanisms can produce signal intensity artifacts that will not respond to protocol optimization and will worsen with gradient imaging. Improper coil tuning manifests as a shading artifact that can mimic other findings. Signal-degrading artifacts may be caused by a ferromagnetic foreign body in the imager. Signal intensity artifacts can also result from performing ultrafast imaging with coils that were not designed for this type of imaging or from MR imaging system malfunction. Familiarity with the various causes of signal intensity artifacts is necessary to maintain optimal image quality and should be required as part of any MR imaging quality assurance program.     

Forty-millisecond MR imaging of the abdomen at 2.0 T

The ability of an ultrafast magnetic resonance (MR) imaging technique to provide abdominal MR images free of motion artifacts was studied. Individual T2-weighted transverse MR images were acquired in as little as 40 msec on a whole-body system operating at 2.0 T. Clinical evaluation was undertaken with fat-suppressed images in which only protons of water molecules contributed to image signal intensity. The ultrafast MR images were compared with conventional MR images obtained at 0.6 T. In 22 patients and two healthy volunteers, ultrafast MR images were of diagnostic quality and free of motion artifacts. Images obtained at an echo time (TE) of 30 msec (imaging time, 40 msec) had liver signal-to-noise ratios of 56.3 +/- 22.6 (n = 19). Because of a smaller data matrix, ultrafast MR images had soft-tissue interfaces that were less sharp than those of the highest-quality conventional MR images in which no motion artifacts were present. However, ultrafast MR images demonstrated high T2-dependent soft-tissue contrast, and pathologic and normal anatomies were readily detected with both imaging techniques. This ultrafast imaging technique has significant promise in whole-body MR imaging, in which motion artifacts often degrade image quality.     

Biopsy needle tip artifact in MR-guided neurosurgery

A thorough understanding of both the appearance and origin of metallic biopsy needle tip artifact in magnetic resonance imaging (MRI) as well as its interaction with various magnetic resonance (MR) sequence parameters is beneficial for its application in today's MR-guided therapeutic procedures. In a more practical setting, this investigation has focused on the characteristics of MR image artifacts associated with a finite-length metallic needle, specifically at the tip of a biopsy needle when it is approximately parallel to the main magnetic field. The image artifact at needle tip, which exhibits as a blooming ball-shaped signal void, was demonstrated and studied using MR imaging and numerical simulation employing the finite difference method (FDM). In order to understand the origin of this image artifact, a numerical model or simulation software based on the FDM has been developed specifically to solve for the field disturbance to a uniform magnetic field due to a finite-length metallic needle. The solution for magnetic field shows that the field disturbance is spatially localized at the needle tip. From the numerical results, simulated images were generated which were in a very satisfactory agreement MR imaging experiment. Results showed that the MR image artifacts associated with MR-compatible metallic biopsy needles are not only present due to the magnetic susceptibility difference between the needle and its surrounding tissue, but also predictable in routine MR-guided procedures, and the size of the image artifacts could be reduced if optimal imaging parameters were used. J. Magn. Reson. Imaging 2001;13:16-22.     

Radial turbo spin echo imaging

Fast MR imaging methods should provide a familiar contrast behavior at a reduced scan time. The multi-spin echo approach (TSE) is one of the most promising techniques satisfying this condition. Although the data acquisition time is significantly reduced, image quality may still suffer from artifacts due to patient motion and flow. The radial turbo spin echo (rTSE) approach combines TSE methods and projection reconstruction (PR) techniques. In PR images, artifacts induced by patient motion or flow are known to have a different appearance with lower level of intensity. The contrast and artifact behavior of the rTSE approach has been investigated. The new technique has been applied to abdominal imaging with acquisition times shorter than 30 s and to heart imaging in combination with cardiac triggering.     

Gradient field switching as a source for artifacts in MR imaging of metallic stents.

Metallic implants, such as stents, have long been a concern in magnetic resonance imaging (MRI). In addition to safety issues, they are commonly associated with image artifacts. The mechanisms of radiofrequency- (RF) and susceptibility-induced artifacts have been thoroughly investigated. However, gradient-switching-induced artifacts have not been analyzed. In this study it was demonstrated that gradient switching may be a source of artifacts in metallic stent MR imaging. These artifacts differ from those caused by the RF pulse. A theoretical explanation is provided as well.     

PET/CT imaging artifacts

The purpose of this paper is to introduce the principles of PET/CT imaging and describe the artifacts associated with it. PET/CT is a new imaging modality that integrates functional (PET) and structural (CT) information into a single scanning session, allowing excellent fusion of the PET and CT images and thus improving lesion localization and interpretation accuracy. Moreover, the CT data can also be used for attenuation correction, ultimately leading to high patient throughput. These combined advantages have rendered PET/CT a preferred imaging modality over dedicated PET. Although PET/CT imaging offers many advantages, this dual-modality imaging also poses some challenges. CT-based attenuation correction can induce artifacts and quantitative errors that can affect the PET emission images. For instance, the use of contrast medium and the presence of metallic implants can be associated with focal radiotracer uptake. Furthermore, the patient's breathing can introduce mismatches between the CT attenuation map and the PET emission data, and the discrepancy between the CT and PET fields of view can lead to truncation artifacts. After reading this article, the technologist should be able to describe the principles of PET/CT imaging, identify at least 3 types of image artifacts, and describe the differences between PET/CT artifacts of different causes: metallic implants, respiratory motion, contrast medium, and truncation.     

Fast magnetic resonance imaging of liver.

Recent magnetic resonance (MR) units with a stronger gradient system have allowed various fast MR imaging techniques to develop. These fast scan techniques have easily realized breath-holding acquisition in the liver and the image quality has been greatly improved without sacrificing spatial resolution. The majority of the fast imaging techniques have been devoted to T2-weighted imaging to obtain useful T2-weighted images in the shortest possible time. Among the fast sequences, fast spin-echo (FSE) sequence is the most promising technique and allows high-quality T2-weighted images with reduced motion artifacts. However, FSE sequences using multiple refocused pulses may essentially realize only poor soft-tissue contrast due to magnetization transfer and T2-filtering effects, and therefore, echo-planar (EP) imaging is expected to provide high image contrast. In addition, single-shot EP imaging allows even diffusion-weighted (DW) and perfusion-weighted (PW) imaging in the liver due to its short scanning time. Recent development of fast gadolinium-enhanced 3D MR angiography has also impacted liver imaging. Combined with such gadolinium-enhanced 3D-MRA sequences and zerofilling image interpolation technique, biphasic gadolinium-enhanced 3D-MRA (whole-liver dynamic MR imaging in the arterial phase and MR portography in the portal phase) can be obtained.     

MR imaging of claustrophobic patients in an open 1.0T scanner: motion artifacts and patient acceptability compared with closed bore magnets

OBJECTIVE: To evaluate motion artifacts and patient acceptability of MR imaging of claustrophobic patients in an open 1.0T scanner. SUBJECTS AND METHODS: Thirty six claustrophobic patients were enrolled prospectively, 34 of which had previous MR examinations in closed bore magnets. Anxiety and pain during MR examination in an open 1.0T scanner were evaluated by visual analogue scales and various tests. Influence of motion artifacts on image quality was evaluated by two radiologists independently using a five-point scale. Additionally, 36 non-claustrophobic patients delivered a reference value of a non-claustrophobic population for the visual analogue anxiety scale. RESULTS: Termination rate of MR imaging of highly claustrophobic patients decreased from 58.3% (n=21) in closed bore magnets to 8.3% (n=3) in the open scanner (p相似文献   

Metallic artifact in MRI after removal of orthopedic implants

Objective

The aim of the present study was to evaluate the metallic artifacts in MRI of the orthopedic patients after removal of metallic implants.

Subjects and methods

From March to August 2009, 40 orthopedic patients operated for removal of orthopedic metallic implants were studied by post-operative MRI from the site of removal of implants. A grading scale of 0–3 was assigned for artifact in MR images whereby 0 was considered no artifact; and I–III were considered mild, moderate, and severe metallic artifacts, respectively. These grading records were correlated with other variables including the type, size, number, and composition of metallic devices; and the site and duration of orthopedic devices stay in the body.

Results

Metallic susceptibly artifacts were detected in MRI of 18 of 40 cases (45%). Screws and pins in removed hardware were the most important factors for causing artifacts in MRI. The artifacts were found more frequently in the patients who had more screws and pins in the removed implants.Gender, age, site of implantation of the device, length of the hardware, composition of the metallic implants (stainless steel versus titanium), and duration of implantation of the hardware exerted no effect in producing metallic artifacts after removal of implants. Short TE sequences of MRI (such as T1 weighted) showed fewer artifacts.

Conclusion

Susceptibility of metallic artifacts is a frequent phenomenon in MRI of patients upon removal of metallic orthopedic implants.     

Metal-induced artifacts in MRI

OBJECTIVE: The purpose of this article is to review some of the basic principles of imaging and how metal-induced susceptibility artifacts originate in MR images. We will describe common ways to reduce or modify artifacts using readily available imaging techniques, and we will discuss some advanced methods to correct readout-direction and slice-direction artifacts. CONCLUSION: The presence of metallic implants in MRI can cause substantial image artifacts, including signal loss, failure of fat suppression, geometric distortion, and bright pile-up artifacts. These cause large resonant frequency changes and failure of many MRI mechanisms. Careful parameter and pulse sequence selections can avoid or reduce artifacts, although more advanced imaging methods offer further imaging improvements.     

口腔内金属材料和磁共振检查伪影

检测口腔内常用金属材料在磁共振检查时否为影和伪影的严重程度;并比较不同检查序列对伪影的影响。材料与方法：对２２种２５件口腔内常用金属材料做了磁共振成像测试,磁共振仪磁场强度为１．５Ｔ,所用序列是梯度回波,部分材料加做自旋回波和快速自旋回波。