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Investigating the Influence of Flip Angle and k -Space Sampling on Dynamic Contrast-Enhanced MRI Breast Examinations

Posted Oct 31, 2014 Updated Apr 10, 2023
By Araminta E.W. Ledger PhD , Marco Borri MPhys , Romney J.E. Pope MRCP FRCR , Erica D. Scurr BSc , Toni Wallace BSc , Cheryl Richardson DCR(R) , Marianne Usher MSc , Steven Allen MRCS FRCR , Robin M. Wilson FRCR FRCPE , Karen Thomas PhD , Nita M. deSouza MD FRCR , Martin O. Leach PhD FMedSci , Maria A. Schmidt PhD
10 min read

Rationale and Objectives

To retrospectively investigate the effect of flip angle (FA) and k -space sampling on the performance of dynamic contrast-enhanced (DCE-) magnetic resonance imaging (MRI) breast sequences.

Materials and Methods

Five DCE-MRI breast sequences were evaluated (10°, 14°, and 18° FAs; radial or linear k -space sampling), with 7–10 patients in each group ( n = 45). All sequences were compliant with current technical breast screening guidelines. Contrast agent (CA) uptake curves were constructed from the right mammary artery for each examination. Maximum relative enhancement, E max , and time-to-peak enhancement, T max , were measured and compared between protocols (analysis of variance and Mann–Whitney). For each sequence, calculated values of maximum relative enhancement, E calc , were derived from the Bloch equations and compared to E max . Fat suppression performance (residual bright fat and chemical shift artifact) was rated for each examination and compared between sequences (Fisher exact tests).

Results

Significant differences were identified between DCE-MRI sequences. E max increased significantly at higher FAs and with linear k -space sampling ( P < .0001; P = .001). Radial protocols exhibited greater T max than linear protocols at FAs of both 14° ( P = .025) and 18° ( P < .0001), suggesting artificially flattened uptake curves. Good correlation was observed between E calc and E max ( r = 0.86). Fat suppression failure was more pronounced at an FA of 18° ( P = .008).

Conclusions

This retrospective approach is validated as a tool to compare and optimize breast DCE-MRI sequences. Alterations in FA and k -space sampling result in significant differences in CA uptake curve shape which could potentially affect diagnostic interpretation. These results emphasize the need for careful parameter selection and greater standardization of breast DCE-MRI sequences.

In the evaluation of breast tumors, the Breast Imaging Reporting and Data System lexicon classifies both lesion morphology and the pattern of contrast agent (CA) uptake with time . Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) is used to increase the specificity of breast MRI examinations, through evaluation of the kinetic behavior of CA uptake . Enhancement curves may demonstrate persistent, plateau, or wash-out behaviors of signal intensity across the dynamic series, where a wash-out of signal intensity is often indicative of malignancy . With smaller lesions in which morphology is indeterminate, this evaluation of CA kinetics becomes increasingly important in diagnosis.

Reliable classification of CA uptake curves requires rapid T 1 -weighted pulse sequences to be designed to provide image intensity directly proportional to CA concentration over the range of expected T 1 values. Effective fat suppression is also required to facilitate the assessment of small lesions. Accurate evaluation and standardization of sequence performance is particularly important because differences across magnetic resonance (MR) systems and sequences can significantly affect sensitivity for detecting lesions . However, the accuracy of sequence assessment with test objects is limited: spatial variations in the B 1 field can hinder verification of image intensity dependence on T 1 and the flip angle (FA) applied to a test object may not accurately represent clinical examinations. In addition, variability in patient size and shape and differences in patient circulation affect T 1 values after intravenous CA administration. An assessment of the impact of sequence parameter changes on the diagnostic capability of an examination would therefore be invaluable. As the internal mammary artery provides a source of arterial enhancement in the examination field of view, we used it to construct signal intensity versus time curves retrospectively and compared these curves between patients examined with breast DCE-MRI sequences of varying FAs and k -space sampling patterns. We simultaneously assessed the effect of these parameters on fat suppression performance. Every sequence evaluated was used in clinical practice and was therefore compliant with current technical breast screening guidelines . Sequences with different k -space sampling coverage pattern and FA were modeled to validate experimental results.

Materials and methods

MRI Protocols

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Table 1

Summary of MRI Protocols

System Philips 1.5 T Intera Philips 1.5 T Achieva Philips 1.5 T Intera Group Notation Rad-10 Rad-14 Rad-18 Lin-14 Lin-18 Sample size 8 10 10 7 10 Flip angle 10° 14° 18° 14° 18°K -space sampling Radial Radial Radial Linear Linear TR/TE, ms 5.10/2.39 5.10/2.35 3.94/1.81 4.58/2.27 4.10/1.97 Number of echoes per shot 100 100 60 100 100 Slice thickness, mm 2.5 2.5 2.0 2.0 2.0 In-plane resolution, mm 0.63 0.63 1.25 1.25 1.25

MRI, magnetic resonance imaging; TE, echo time; TR, repetition time.

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Theoretical Calculation of Relative Enhancement and Fat Suppression Efficiency

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Analysis of Dynamic Contrast-Enhanced Examinations

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Figure 1, Axial T 1 -weighted fat-suppressed dynamic contrast-enhanced magnetic resonance breast image (the first postcontrast dynamic) identifying position of right internal mammary artery together with enlarged image ( inset ). Fat suppression failure in the left breast is also evident.

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Evaluation of Fat Suppression

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Data Analysis

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Results

Calculation of Relative Enhancement and Fat Suppression Efficiency

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Figure 2, Calculated image intensity (arbitrary units) as a function of 1/T 1 for the five dynamic contrast-enhanced magnetic resonance imaging sequences (flip angles 10°, 14°, and 18°, respectively). Radial and Linear sequences with the same flip angle appear coincident at this scale. Hashed lines represent a T 1 of 1200 milliseconds for unenhanced blood and 100 milliseconds at maximum enhancement. Dynamic range is reduced at lower flip angles.

Table 2

Values of E calc with Mean and Standard Deviation of Measured E max

System Philips 1.5 T Intera Philips 1.5 T Achieva Philips 1.5 T Intera Group Rad-10 Rad-14 Rad-18 Lin-14 Lin-18E calc , % 220 348 484 367 486E max , % 113 ± 32 146 ± 35 223 ± 52 196 ± 63 336 ± 112

E calc , calculated maximum relative enhancement; E max , measured maximum relative enhancement.

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Figure 3, Calculated image intensity (arbitrary units) as a function of 1/T 1 for the five dynamic contrast-enhanced magnetic resonance imaging sequences (flip angles 10°, 14°, and 18°, respectively). Radial and Linear sequences with the same flip angle appear coincident at this scale. Unsuppressed signal intensity is represented by the upper black curve , with the lower gray curve corresponding to the suppressed signal intensity. The hashed line denotes a T 1 of 300 milliseconds, chosen to represent the T 1 of fat at 1.5 T. At lower flip angles, the relative difference between unsuppressed and suppressed spins is greatest, resulting in the most efficient fat suppression, while dynamic range is reduced.

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Analysis of Dynamic Contrast-Enhanced Examinations

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Figure 4, Box plots of E max (maximum relative enhancement) split by flip angle and k -space sampling technique. Boxes display median and interquartile range (IQR) and whiskers extend to the extreme values within each group (excluding outliers). Values that fall within IQR 1.5–3 from the outer box limits are defined as outliers and are denoted by an unfilled circle .

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Figure 5, Spread of time to peak enhancement, T max (dynamic frame), across the patient groups. Each dynamic frame was acquired in approximately 1 minute.

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Figure 6, Mean relative enhancement at each dynamic frame in the dynamic series for the dynamic contrast-enhanced magnetic resonance imaging sequences: Lin-18 versus Rad-18 and Lin-14 versus Rad-14. Sequences with the same flip angle but different k -space sampling exhibit a reduction in relative enhancement and enhance later in the dynamic series.

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Fat Suppression Evaluation

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Table 3

Evaluation of Fat Suppression Performance Across the Patient Groups

Group Unsuppressed Fat Chemical Shift Artifact Fat Suppression Successful Fat Suppression Unsuccessful Total Fat Suppression Successful Fat Suppression Unsuccessful Total Rad-10 6 (100.0) 0 (0.0) 6 1 (16.7) 5 (83.3) 6 Rad-14 5 (83.3) 1 (16.7) 6 0 (0.0) 6 (100.0) 6 Rad-18 4 (57.1) 3 (42.9) 7 5 (71.4) 2 (28.6) 7 Lin-14 6 (85.7) 1 (14.3) 7 6 (85.7) 1 (14.3) 7 Lin-18 1 (11.1) 8 (88.9) 9 6 (66.7) 3 (33.3) 9 Total 35 35

Figure 7, (a) Percentage of examinations exhibiting bright regions of unsuppressed fat within each patient group; (b) percentage of examinations affected by chemical shift artifact within each patient group, together with set echo time values (milliseconds) for each dynamic contrast-enhanced magnetic resonance imaging sequence.

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Regions of Unsuppressed Fat

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Chemical Shift Artifact

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Discussion

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Academic Radiology, Volume 21, Issue 11
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