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Case 47 - Inappropriate inversion time selection for late gadolinium enhancement imaging
- from Section 6 - Cardiovascular MRI artifacts
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- By David Bonekamp, University Hospital Heidelberg, Stefan L. Zimmerman, Johns Hopkins Medical Centre
- Edited by Stefan L. Zimmerman, Elliot K. Fishman
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- Book:
- Pearls and Pitfalls in Cardiovascular Imaging
- Published online:
- 05 June 2015
- Print publication:
- 21 May 2015, pp 146-149
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Summary
Imaging description
Inappropriate selection of the inversion time (TI) in late gadolinium enhancement (LGE) cardiac MRI (CMR) examinations results in incomplete suppression of the myocardium. Most frequently encountered is the selection of a TI that is slightly too short, resulting in a subendocardial “ring of hypointensity” and a mid-myocardial zone of hyperintensity (Figures 47.1 and 47.2). These artifacts may mimic true mid-myocardial delayed enhancement that can be seen in pathologic conditions such as sarcoidosis or dilated cardiomyopathy (Figure 47.3). In a less commonly encountered clinical scenario, suppression of abnormal myocardium (amyloidosis is the prototypical example) will cause hypointensity of abnormal myocardium. This will result in the poor quality of delayed enhancement images that is a hallmark of patients with amyloidosis (Figure 47.4).
Importance
Two potential pitfalls result from incorrect inversion time selection. First, incorrect nulling of the myocardium reduces the conspicuity of true myocardial delayed enhancement, potentially hiding underlying pathology, resulting in a falsenegative result. Second, incomplete nulling due to a short inversion time, if not recognized as artifact, can be erroneously interpreted as diffuse mid-myocardial LGE, leading to a falsepositive result. The interpreting radiologist must be familiar with the appearance of deviations of the TI selection, and be aware of underlying conditions, particularly amyloidosis, that can cause difficulty in selecting the correct TI time.
Typical clinical scenario
TI is selected to provide maximal contrast between normal and abnormal myocardium by completely nulling any signal from normal myocardium. TI selection determines the sensitivity for the detection of myocardial damage. The actual TI time needed for normal myocardial suppression depends on multiple factors, including the time after contrast injection, renal elimination of contrast, patient size, cardiac output, contrast dose and contrast agent. A typical approach for TI selection is the use of a “TI scout” sequence, which is typically an ECG-gated post-contrast sequence with images acquired at multiple delay times after an initial 180 degree inversion pulse, resulting in different TI contrasts throughout the cardiac cycle.
Case 52 - Pseudostenosis on time-of-flight magnetic resonance angiography
- from Section 6 - Cardiovascular MRI artifacts
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- By David Bonekamp, University Hospital Heidelberg, Stefan L. Zimmerman, Johns Hopkins University
- Edited by Stefan L. Zimmerman, Elliot K. Fishman
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- Book:
- Pearls and Pitfalls in Cardiovascular Imaging
- Published online:
- 05 June 2015
- Print publication:
- 21 May 2015, pp 165-167
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Summary
Imaging description
Time-of-flight (TOF) magnetic resonance angiography (MRA) images are prone to several artifacts that may suggest stenosis or occlusion of vascular segments. TOF MRA is based on the acquisition of 2D or 3D gradient echo images which are optimized to saturate protons in stationary tissues while maximizing flow-related enhancement from inflowing blood protons that enter the slice during the acquisition. Saturation bands are added to selectively null signal from venous or arterial blood that is flowing in the opposite direction of the vessel of interest. For example, if an image of the abdominal aorta is desired, a saturation band below the plane of interest is used to null inflowing venous blood from the inferior vena cava. Several well-known artifacts occur with TOF imaging. In-plane flow can result in pseudostenosis or artifactual occlusion because blood protons flowing within a vessel parallel to the imaging plane will become saturated during the acquisition (Figure 52.1). Slow-flowing blood may also become saturated before it reaches the end of the TOF slab, such that the distal vessel appears attenuated in luminal diameter and signal. Reversal of flow, which can occur due to retrograde filling with collateral arteries, will be undetectable with TOF techniques due to the use of saturation bands to suppress venous contamination (Figure 52.2). Susceptibility artifacts from surgical clips or adjacent hardware may attenuate the MR signal, and the gradient echo (GRE) sequences used for TOF are especially sensitive for this type of artifact. Finally, dephasing of protons that occurs due to turbulent flow at vessel bifurcations may mimic stenoses, while accelerated and turbulent flow at existing stenoses may lead to overestimation of the degree of stenosis.
Importance
TOF MRA is a widely used unenhanced MRA method. Knowledge of artifacts that may mimic vascular stenosis or occlusion is essential for accurate interpretation. Misinterpretation of the absence or presence of vascular stenosis or occlusion can lead to unnecessary intervention or surgery, or fail to diagnose a treatable cause for the patient's symptoms.
Case 49 - Gibbs ringing artifact
- from Section 6 - Cardiovascular MRI artifacts
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- By David Bonekamp, University Hospital Heidelberg, Stefan L. Zimmerman, Johns Hopkins Medical Centre
- Edited by Stefan L. Zimmerman, Elliot K. Fishman
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- Book:
- Pearls and Pitfalls in Cardiovascular Imaging
- Published online:
- 05 June 2015
- Print publication:
- 21 May 2015, pp 154-158
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Summary
Imaging description
The Gibbs ringing artifact (truncation artifact) results from the limited fidelity of the superimposition of a finite number of sine and cosine functions to exactly reproduce a sharp border. In cardiac MR, it usually occurs as dark and bright periodic rings with decreasing amplitude at increased distance from the blood–myocardial border. It is the more noticeable the higher the contrast between blood and myocardium, and thus is especially prominent in first-pass perfusion studies during the early frames, where high ventricular contrast concentrations meet essentially unenhanced myocardium (Figure 49.1).
Importance
The Gibbs artifact causes oscillations around the sharp highcontrast blood/myocardial border. It results from the mathematical properties of the representation of MR images as a Fourier series, which can only approximate sharp borders. It can be made arbitrarily small at the cost of imaging time by increasing spatial resolution; however, it can never be fully avoided. The resulting image will show edges with adjacent ringing in the form of a sinc function (sin x/x). The artifact is present in both the frequency and phase directions. Mathematically, it can be shown that the positive and negative side lobes of the Fourier series have approximately 9% deviation from the baseline, thus resulting in a possible 18% deviation of signal in the subendocardium on perfusion studies if pixels are reconstructed exactly in the hills and valleys of these oscillations. This is in the range of mild perfusion defects and has important clinical consequences. These artifacts may either mask true underlying early perfusion defects or mimic such defects, leading either to decreased sensitivity for perfusion abnormalities or inappropriate diagnosis of a perfusion defect. Both Gibbs artifact and ischemia affect the subendocardial myocardium preferentially compared to the mid- or subepicardial myocardium (Figure 49.2). Thus, it can be a challenge distinguishing these from each other; and MR sequences should be optimized to minimize Gibbs artifact. By the same mechanism, Gibbs ringing can lead to simulation of a syrinx in the spinal cord or cause ringing artifact around vessels in MR angiographic examinations.
Case 53 - Maki effect
- from Section 6 - Cardiovascular MRI artifacts
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- By David Bonekamp, University Hospital Heidelberg, Stefan L. Zimmerman, Johns Hopkins University
- Edited by Stefan L. Zimmerman, Elliot K. Fishman
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- Book:
- Pearls and Pitfalls in Cardiovascular Imaging
- Published online:
- 05 June 2015
- Print publication:
- 21 May 2015, pp 168-170
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Summary
Imaging description
“Maki” artifact or effect is named after Jeffrey H. Maki, MD, PhD, who first comprehensively described this as a “ringing” or “banding” artifact occurring in MRA studies (Figure 53.1). It manifests as lack of central arterial enhancement with parallel bands of decreasing hyperintensity in the location of the arterial walls. The proximal margin and the transition to normal arterial enhancement are gradual, without a sharply defined border. The effect is often most pronounced on subtracted images.
Importance
“Maki” artifact may be incorrectly diagnosed as arterial thrombosis, leading to inappropriate referral to invasive angiography. The artery exhibiting the artifact is not adequately imaged and underlying pathology such as thrombosis or dissection is not evaluated. Recognition of the artifact is important to adjust the clinical protocol to avoid too early acquisition of MRA images after bolus injection.
Typical clinical scenario
This artifact is observed in MR angiography studies if the center of k-space is acquired before the peak arterial contrast concentration has been reached. The leading edge of the bolus is typically narrow and enhancement is brisk, while the exact delay between injection and bolus arrival is influenced by multiple factors. These include cardiac output, the distance of the artery from the heart, vascular shunts, and stenoses. Optimal arterial enhancement is only achieved if the acquisition of the center of k-space occurs at the time of maximal arterial enhancement. Imaging too early will result in the Maki artifact, whereas imaging after the peak will decrease the signal-to-noise ratio and venous contamination will be more pronounced. The challenge of correct bolus timing is increased when a shorter contrast bolus is used to minimize the amount of contrast. Centric k-space encoding acquires the center of k-space first, allowing for the most precise bolus timing; however, it renders the MRA technique more prone to artifacts.
1 - Imaging of flow: basic principles
- from Section 1 - Techniques
- Edited by Peter B. Barker, The Johns Hopkins University School of Medicine, Xavier Golay, Gregory Zaharchuk
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- Book:
- Clinical Perfusion MRI
- Published online:
- 05 May 2013
- Print publication:
- 16 May 2013, pp 1-15
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Summary
Introduction
The flow of blood to an organ is a fundamental physiological factor affecting tissue health, growth, and repair. Blood flow and volume are perturbed in many disease conditions, most notably in vascular disease and in tumors. The ability to determine non-invasively blood flow and blood volume using imaging methods therefore has important diagnostic and therapeutic implications. Since the early days of radiological imaging, scientists and physicians have been searching for methods that can accurately and non-invasively depict the major blood vessels of the body, and measure blood flow in tissue. For instance, X-ray projection imaging of blood vessels (angiography) was first demonstrated in 1927 by Moniz [1], using iodinated contrast agents injected intravascularly, while early measurements of tissue blood flow were based on the inhalation of freely diffusible tracers (e.g., nitrous oxide [N2O] [2], or radioactive xenon or krypton [3]). Subsequently, stable (i.e., non-radioactive) xenon was used in conjunction with X-ray computed tomography (CT) to image cerebral blood flow (CBF) [4], while other methods such as single-photon emission CT (SPECT) [5, 6] and positron emission tomography (PET) [7, 8] imaging using a variety of radiotracers also became available. More recently, dynamic CT perfusion imaging using bolus injection of iodinated contrast agents has been growing in popularity [9], particularly as fast multi-slice CT scanners have become widely available.