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ORIGINAL ARTICLE
Year : 2011  |  Volume : 59  |  Issue : 6  |  Page : 839-843

Detection of brain lesions at the skull base using diffusion-weighted imaging with readout-segmented echo-planar imaging and generalized autocalibrating partially parallel acquisitions


1 Department of Radiology, The Sixth Affiliated People's Hospital, Shanghai Jiao Tong University, Shanghai, China
2 Siemens Healthcare, MR Collaboration NE Asia, Shanghai, China

Date of Submission07-Nov-2011
Date of Decision08-Nov-2011
Date of Acceptance18-Nov-2011
Date of Web Publication2-Jan-2012

Correspondence Address:
Wen-Bin Li
Department of Radiology, The Sixth Affiliated People's Hospital, Shanghai Jiao Tong University, No. 600, Yi Shan Road, Shanghai 200233
China
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DOI: 10.4103/0028-3886.91361

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 » Abstract 

Objective: To analyze the value of readout-segmented echo-planar imaging (rs-EPI) with parallel imaging and a two-dimensional (2D) navigator-based reacquisition technique in the detection of brain lesions at the skull base. Materials and Methods: A total of 54 patients (male 37, female 17) with suspected skull-base intracranial lesions underwent magnetic resonance imaging (MRI), including pre-T1 weighted imaging, T2-weighted imaging, Fluid Attenuated Inversion Recovery (FLAIR), standard single shot echo-planar imaging diffusion weighted imaging (ss-EPI DWI) and rs-EPI DWI, post-contrast T1-weighted. The total number of lesions and the number of lesions at different sites on all MRI sequences were used as reference measures. Then differences in detecting lesions and image quality between standard ss-EPI DWI and rs-EPI DWI were analyzed. Results: There was a significant difference in the total number of lesions detected by rs-EPI DWI and standard ss-EPI DWI (P = 0.01). But this difference was mainly due to an improved ability of rs-EPI DWI to detect lesions located in the anterior cranial fossa, compared to ss-EPI DWI (P=0.02); the ability of ss-EPI and rs-EPI DWI to detect lesions in the middle cranial fossa and posterior cranial fossa was not significantly different (P = 0.471, P = 0.486, respectively). For image quality, rs-EPI images were significantly better than standard ss-EPI DWI images (P<0.001). Conclusion: The rs-EPI DWI technique is a useful tool for the detection and evaluation of lesions located at the skull base.


Keywords: 2D navigator, parallel imaging, readout-segmented EPI, single-shot ss-EPI, skull base


How to cite this article:
Wei XE, Li WB, Li MH, Li YH, Wang D, Zhang YZ, Jin LX. Detection of brain lesions at the skull base using diffusion-weighted imaging with readout-segmented echo-planar imaging and generalized autocalibrating partially parallel acquisitions. Neurol India 2011;59:839-43

How to cite this URL:
Wei XE, Li WB, Li MH, Li YH, Wang D, Zhang YZ, Jin LX. Detection of brain lesions at the skull base using diffusion-weighted imaging with readout-segmented echo-planar imaging and generalized autocalibrating partially parallel acquisitions. Neurol India [serial online] 2011 [cited 2014 Sep 2];59:839-43. Available from: http://www.neurologyindia.com/text.asp?2011/59/6/839/91361



 » Introduction Top


Preoperative diagnosis and grading are useful to determine the optimal treatment plan in patients with brain tumors. Magnetic resonance imaging (MRI) is undoubtedly the preferred method for the evaluation of brain tumors. However, until now it is not possible to distinguish histological types of different tumors using conventional MRI as brain tumors share many imaging similarities. [1],[2] Recently, diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) measurements have been widely used to detect cellularity and monitor the response of brain tumors to therapy. DWI can be used to differentiate brain cystic lesions. [1],[3] Furthermore, both DWI and ADC can be used to quantitatively measure tumor tissues, peritumoral edema and predict prognosis after treatment, [4],[5] indicating that DWI is a useful tool to distinguish brain lesions and monitor treatment efficacy. Single-shot echo-planar imaging (ss-EPI) is established as the method of choice for standard DWI; however ss-EPI is prone to susceptibility changes at tissue interfaces, [6] such as at the skull base and sinus areas. This leads to image distortion and artifacts, [7],[8] which limits the application of standard ss-EPI DWI for the detection of lesions at tissue interfaces, especially when using ultra-high field strength scanners. Earlier studies have shown that readout-segmented echo-planar imaging (rs-EPI) with parallel imaging and 2D navigator-based reacquisition can generate high-resolution DWI images with a robust correction for motion-induced phase errors which substantially improves the image quality, compared to standard ss-EPI protocols with parallel imaging. [9],[10]

The aim of this study was to use rs-EPI with parallel imaging and a 2D navigator-based reacquisition technique to detect the lesions at the skull base. We hypothesized that rs-EPI DWI can improve image quality and detect more lesions. Therefore, we compared rs-EPI DWI images with standard ss-EPI protocol DWI with parallel imaging in patients with suspected skull-base brain lesions.


 » Materials and Methods Top


Patients

Between March 2010 and March 2011, patients with suspected skull-base intracranial lesions were enrolled in this study. The inclusion criteria were: (1) detection of a lesion or suspected lesion in the skull base on computerized tomography (CT), including anterior, middle and posterior cranial fossa and (2) no history of brain surgery, chemotherapy, radiotherapy, cerebral infarct or hemorrhage. The exclusion criteria were abnormalities in other areas of the brain in addition to the skull base, a history of brain surgery, chemotherapy, radiotherapy, and cerebral infarction or hemorrhage. A total of 54 patients (male 37, female 17) were included in the study. The study was approved by the Institutional Review Board of our hospital and performed in accordance with the Declaration of Helsinki. Written informed consent was obtained from all patients.

MR imaging

MRI was performed using a 3.0T MRI scanner (Magnetom Verio, Siemens Healthcare, Erlangen, Germany) and a 12-channel head coil. The MRI sequences were: pre-contrast T1-weighted imaging (repetition time /echo time ( TR/TE) = 2000/9 ms; TI = 860 ms; slice thickness = 5 mm; Field of View (FOV) = 220 × 220 mm; matrix = 204 × 320); T2-weighted imaging (T2WI: TR/TE = 6000/95 ms; FOV = 220 × 220 mm; matrix, 244 × 384; slice thickness = 5 mm); and FLAIR (TR/TE = 8500/94 ms; TI = 2438.9 ms; slice thickness = 5 mm; FOV = 220 × 220 mm; matrix = 324 × 512). Before injection of contrast agent, standard ss-EPI DWI and rs-EPI DWI with parallel imaging and 2D navigator-based reacquisition were performed. The parameters for standard ss-EPI DWI imaging were: parallel imaging using Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) with an acceleration factor of 2, with FOV = 220 × 220 mm; matrix=162 × 162; phase partial Fourier factor 6/8; slices = 25; slice thickness = 4 mm; echo-spacing 900 s; TR 4,900 ms; TE 94 ms; one scan at b = 0 s/mm 2 and three at b = 1,000 s/mm 2 were applied along the three gradients coinciding with the major physical axes (x, y, and z), four averages, total measurement time = 58 s. The parameters for rs-EPI DWI imaging were: parallel imaging using GRAPPA with an acceleration factor of 2, FOV = 220 × 220 mm; matrix = 192 × 192; slices = 25; slice thickness = 4 mm; number of readout segments per image 11; echo-spacing 320 s; TR 4400 ms; TE 69 ms; one scan at b = 0 s/mm2 and three at b = 1,000 s/mm 2 were applied along the three gradients coinciding with the major physical axes (x, y, and z), one average, total measurement time = 3 min 28 s. After the ss-EPI and rs-EPI DWI scans, post-contrast T1-weighted images were acquired in the axial, coronal and sagittal planes using the same parameters. The total scan time for all sequences was approximately 18 min.

Data processing

The total number of lesions and the number of lesions at different sites on all MRI sequences were used as reference measures. Firstly, all standard ss-EPI DWI images and rs-EPI DWI images were reviewed by two neuroradiologists who were blinded to the DWI image type. They recorded the lesion site and number on every DWI sequence and then image quality at the maximum diameter level of the lesion was evaluated and graded as: I clear; II blurred but could be evaluated; and III blurred and could not be evaluated. Decisions were made by consensus when there was a divergence of opinion between the two neuroradiologists. In patients with multiple lesions, image quality at the maximum diameter level of every lesion was evaluated. Secondly, according to all the MRI sequences, the total number of lesions and number of lesions at each different site were counted as the reference measures by these two neuroradiologists.

Statistical analysis

SPSS for Windows (Version 16.0; Chicago, IL, USA) was used for statistical analysis and P<0.05 was considered significant. Differences in the total lesion number and lesion number at different sites between standard ss-EPI DWI and rs-EPI DWI images were analyzed using the Chi-square test. Differences in the image quality between standard ss-EPI DWI and rs-EPI DWI images were calculated using the Mann-Whitney U test.


 » Results Top


Of the 54 patients, six were excluded due to motion artifacts (n = 4) or negative MRI images (n = 2); thus a total of 48 patients were included in the final analysis. In these 48 patients, a total of 56 lesions were detected on the MRI images with 13 in the anterior cranial fossa, 28 in the middle cranial fossa and 15 in the posterior cranial fossa. A total of 48 lesions were found using standard ss-EPI DWI with seven in the anterior cranial fossa, 26 in the middle cranial fossa and 15 in the posterior cranial fossa. In rs-EPI DWI images, all 56 lesions were detected with 13 in the anterior cranial fossa, 28 in the middle cranial fossa and 15 in the posterior cranial fossa [Table 1]. Overall, the difference in the total number of lesions detected by rs-EPI DWI and standard ss-EPI DWI was significant (P = 0.01). This difference was mainly due to an improved ability of rs-EPI DWI to detect lesions located in the anterior cranial fossa, compared to ss-EPI DWI (P=0.02); the ability of ss-EPI and rs-EPI DWI to detect lesions in the middle cranial fossa and posterior cranial fossa was not significantly different (P = 0.471, P = 0.486, respectively).
Table 1: The total number and sites detected by standard single-shot and high-resolution readout-segmented echo-planar diffusion-weighted images

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For standard ss-EPI DWI images, the image quality was graded as follows: Grade I, 27; Grade II, 16; Grade III, 5. For rs-EPI DWI images, the image quality was graded as follows: Grade I, 49; Grade II, 7; Grade III, 0 [Table 2]. Furthermore, the image quality of rs-EPI images was significantly better than standard ss-EPI DWI images (P<0.001, [Figure 1]).
Figure 1: Comparison of standard single-shot (ss-EPI) and high-resolution readout-segmented echo-planar (re-EPI) diffusion-weighted images (DWI) at the maximum diameter level of a lesion (arrow) located at the right side of the anterior cranial fossa. (a): The rs-EPI DWI image is clear, with Grade I image quality. (b): The standard ss-EPI DWI image is blurred but can be evaluated, with a Grade II image quality. (c): The lesion was detected by homogeneous hyperdensity and calcification on CT. (d): The lesion shows obvious enhancement on the post-T1W image

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Table 2: Comparison of brain lesion image quality in standard single-shot and high-resolution readout-segmented echo-planar diffusion-weighted images

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 » Discussion Top


This study found that the rs-EPI detected an increased number of lesions and had a significantly better image quality, compared to ss-EPI. Although CT is a sensitive tool to display bone destruction, the ability of this technique to describe tumor characteristics in the brain is limited. [11] The role of MRI in the diagnosis of brain tumors occurring at the skull base is widely accepted. [12] DWI can differentiate various tissue components. [13],[14] However, lesions located at tissue interfaces can be easily missed on ss-EPI standard DWI images due to image distortion and blurring. [7],[8] As the structure of the skull base is complex with many tissue interfaces, lesions located at the skull base may be missed on standard DWI images. In this study, eight lesions were missed on standard ss-EPI DWI, while rs-EPI DWI displayed all lesions, indicating that rs-EPI is superior to standard ss-EPI for the detection of lesions occurring at the skull base.

In this study, we used readout-segment echo-planar imaging, parallel imaging and a two-dimensional navigator-based reacquisition to acquire rs-EPI DWI images. This technique was initially described by Robson et al.,[15] and was subsequently further developed to include two-dimensional (2D) navigator correction, parallel imaging, and navigator-based reacquisition. [9] These techniques acquire DWI images with low susceptibility-based image distortion and T2* blurring, and include a robust correction for motion-induced phase artifact, providing a significant improvement in image quality and the ability to find more lesions, compared to ss-EPI DWI. [9],[10] However, the difference in the ability of rs-EPI DWI to detect cranial lesions was only focused on the anterior cranial fossa, and not in the middle and posterior cranial fossa. This could be explained by the fact that there is a greater extent of susceptibility changes due to gas-tissues interfaces in the anterior cranial fossa, compared to in the middle and posterior cranial fossa.

Earlier studies have shown that DWI and ADC are useful for the diagnosis and grading of tumors. [16],[17] Furthermore, DWI and ADC can be used to monitor the effect of chemotherapy and radiotherapy and provide prognostic information. [4],[18],[19] However, ADC values based on standard ss-EPI DWI may be inaccurate, as ss-EPI is prone to image distortion and blurring. In this study, the quality of DWI images was evaluated by two neuroradiologists independently. The quality of rs-EPI images was significantly better than standard ss-EPI images, suggesting that rs-EPI DWI can display lesions located at the skull base more clearly. The improved quality of rs-EPI DWI is explained by the fact that we did not use partial Fourier in the phase-encoding direction, which allows a substantially shorter echo-spacing than ss-EPI, significantly reducing the susceptibility effect and T2* decay. In addition, we used a 2D-navigator based reacquisition scheme, in which the navigator data is used during the scan, and can be combined with rs-EPI and parallel imaging to provide a robust method for high-resolution DWI, which has a low level of image distortion and is insensitive to motion-induced phase error. [6],[8],[9]

In this study, the image quality of seven lesions on rs-EPI imaging was graded as blurred but could be evaluated (Grade II). Of these seven lesions, six were located in the anterior cranial fossa. In particular, one lesion was almost completely located in the ethmoid sinus, and the susceptibility changes observed in this lesion were the largest. These findings indicate that the image quality of lesions located in the anterior cranial fossa may be blurred using rs-EPI [Figure 2], especially when the lesions involve the paranasal sinuses.
Figure 2: Comparison of standard single-shot (ss-EPI) and high resolution readout-segmented echo-planar (re-EPI) diffusion-weighted images (DWI) at the maximum diameter level of a lesion (arrow) located at the right side of the anterior cranial fossa and extending towards the right paranasal sinus. (a): An irregular lesion located in the anterior cranial fossa and extending towards the right paranasal sinus is detected on the T2W image. (b): The lesion shows heterogeneous enhancement on the post-T1W image. (c): The standard ss-EPI DWI image quality is blurred and could not be evaluated accurately, with Grade III image quality. (d): The rs-EPI DWI image quality is blurred but could be evaluated, with a Grade II image quality

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There are some limitations in this study. Firstly, the scan time used in this study for rs-EPI DWI was longer than standard ss-EPI DWI (3 min 28 sec vs. 58 sec, respectively). A long scan time can lead to motion artifacts when used in elderly patients or children. Secondly, although the image quality of rs-EPI DWI images is significantly better than standard ss-EPI DWI images, rs-EPI images of lesions occurring in the anterior cranial fossa were blurred. Finally, there may be some bias due to the fact that the reference values were not histopathological data, but the total number of lesions detected on the basis of all MRI sequences; however, its impact on this study is limited as we just want to evaluate the ability of rs-EPI DWI and standard ss-EPI DWI in detecting lesions located at the skull base.

In conclusion the quality of rs-EPI DWI images is significantly better than standard ss-EPI DWI images, and rs-EPI can detect an increased number of lesions at the skull base. The rs-EPI DWI technique is a useful tool for the detection and evaluation of lesions located at the skull base.

 
 » References Top

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