Rationale and Objectives
The magnitude of iron-induced susceptibility changes in gradient echo T2*-weighted magnet resonance imaging (T2* MRI) increases with the field strength and should increase the sensitivity for detection of cerebral microbleeds (CMBs) at 3.0 T. To test these hypotheses, we prospectively examined individuals with documented CMBs at 1.5 and 3.0 T.
Materials and Methods
Five hundred fifty elderly individuals, who participated in an interdisciplinary study of healthy aging, were examined at 3.0 T using T2* MRI sequences (repetition time [TR]/echo time [TE]/flip angle [FA] = 573 ms/16 ms/18°). Individuals positive for CMBs were asked to undergo an additional examination at 1.5 T (TR/TE/FA = 663 ms/23 ms/18°). Images were analyzed independently by two observers. CMBs were counted throughout the brain and were qualitatively analyzed comparing the degree of visible hypointensity on a 5-point scale from 1 (complete signal loss) to 5 (no detection) for both field strengths. Contrast-to-noise ratio of CMBs to surrounding brain tissue was calculated.
Results
At 3.0 T, CMBs were detected in 45 of 550 individuals; 25 agreed to an additional examination at 1.5 T. In this group ( n = 25), a total of 53 CMBs were detected at 3.0 T, compared to 41 CMBs at 1.5 T. The mean contrast-to-noise ratio of CMBs was significantly increased at 3.0 T compared to 1.5 T (27.4 ± 8.2 vs. 17.4 ± 8.0; p < .001). On qualitative analysis, visibility of CMBs was ranked significantly higher at 3.0 T (1.3 ± 0.4 vs. 2.9 ± 1.1; p < .001).
Conclusion
Evidence of past microbleeds may even be found in neurologically normal elderly individuals by MRI. Detection rate and visibility of CMBs benefit from the higher field strength, resulting in a significantly improved depiction of iron-containing brain structures (CMBs) at 3.0 T with potential clinical relevance.
Cerebral microbleeds (CMBs) are discrete or isolated punctuate hypointense lesions less than 5 mm in size ( ). They are mainly located in the deep gray matter and subcortical white matter and persist indefinitely after initial detection. The best demonstration of CMB can be archived by gradient-echo T2*-weighted magnetic resonance imaging (MRI) sequences. Hemosiderin remains stored in macrophages and leads to focal dephasing of the MRI signal. This causes areas of past bleeding to appear dark on T2*-weighted images ( ). CMBs are generally considered clinically silent but are frequently associated wih small vessel disease, cerebral amyloid angiopathy, chronic hypertension, and lacunar stroke and may be a marker for increased risk for future intracerebral hemorrhage ( ). Further studies are needed to confirm these associations, and investigate other potential risk factors for the occurrence of CMB and their possible implications for antithrombotic treatment ( ).
Signal intensity and contrast of MRI are determined by a variety of factors, including relaxation times, proton density, field strength, and magnetic susceptibility. A new generation of whole-body MRI units with field strengths of 3.0 T has become increasingly available for routine imaging and currently is undergoing clinical evaluation ( ).
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Materials and methods
Subjects
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MRI Examination
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Table 1
Scan Parameters of the T2* MRI Sequence at 1.5 and 3.0 T
Parameter 1.5 T 3.0 T Matrix scan 256/256 320/512 Slice thickness 6 4 TR 663 573 TE 23 16 Flip angle 18 18 NSA 2 1 Scan duration (min) 3:11 1:35
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Qualitative Analysis
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Quantitative Analysis
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CNR=(I1−I2)/N C
N
R
=
(
I
1
−
I
2
)
/
N
with I 1 = signal intensity of surrounding brain tissue, I 2 = signal intensity of CMB, and N = background noise.
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Statistical Analysis
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Results
Quantitative Results
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Table 2
Results of T2* Imaging at 1.5 and 3.0 T: Quantitative Data
Parameter 1.5 T 3.0 T_P_ Value No. of CMBs detected 41 53 0.001 Signal intensity (mean ± SD) CMB 585.8 ± 473.8 866.7 ± 301.4 Surrounding brain tissue 785.0 ± 633.7 1832.9 ± 304.6 Noise (mean) 11.4 ± 9.8 36.4 ± 6.3 Contrast to noise (mean ± SD) 17.4 ± 8.0 27.8 ± 8.2 0.001
CMBs, cerebral microbleeds; SD, standard deviation.
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Qualitative Results
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Table 3
Results of T2* Imaging at 1.5 and 3.0 T: Qualitative Data
Parameter 1.5 T 3.0 T_P_ Value Visibility of CMBs (mean ± SD) 2.9 ± 1.1 1.3 ± 0.4 <0.001 Overall image quality (mean ± SD) 2.1 ± 1.0 1.3 ± 0.5 <0.001
CMBs, cerebral microbleeds; SD, standard deviation.
Scale from 1 to 5: 1 = complete signal loss to 5 = no detection (only assessable for 1.5).
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Discussion
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Limitations of the Study
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Conclusion
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