- Email: [email protected]
Evaluation of focal liver lesions: fast-recovery fast spin echo T2-weighted MR imaging
Clinical Imaging 30 (2006) 322 – 325
Evaluation of focal liver lesions: fast-recovery fast spin echo T2-weighted MR imaging Oguz Akina,4, Lawrence H. Schwartza, Adam Welberb, Cynthia F. Maierc, Douglas R. DeCoratod, David M. Paniceka a
Department of Radiology, Memorial SloanKettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA b Danbury Hospital, Denbury, CT 06810, USA c GE Medical Systems, Waukesha, WI 53201, USA d East River Medical Imaging, New York, NY 10021, USA Received 5 January 2006; accepted 20 February 2006
Abstract Purpose: To compare breath-hold fast-recovery fast spin echo (FR-FSE) and non-breath-hold fast spin echo (FSE) T2-weighted sequences for hepatic lesion conspicuity and image quality at MR imaging. Materials and Methods: Fifty-nine patients with known or suspected liver lesions underwent hepatic MR imaging by using a breath-hold FR-FSE T2-weighted sequence with and without fat suppression and a non-breath-hold FSE T2-weighted sequence with and without fat suppression. Quantitative analysis was made with measurements of the signal intensity of the liver, spleen, background noise, and up to three liver lesions, as well as calculations of the liver signal-to-noise ratio (SNR) and the liver-to-lesion contrast-to-noise ratio (CNR) for each sequence. Qualitative analysis was made for image quality and the number of lesions identified. Statistical analysis was performed by using a single-tailed paired Student’s t test with a 95% confidence interval. Results: SNR and CNR were significantly higher ( P b.05) for FSE with fat suppression than for FR-FSE with fat suppression. No statistically significant difference was seen in terms of SNR and CNR between non-fat-suppressed FSE and FR-FSE sequences. Lesion conspicuity, liver edge sharpness, and clarity of vessels were superior and ghosting was less with the FR-FSE sequences compared with the FSE sequences. Conclusion: Breath-hold FR-FSE technique is a reasonable alternative in T2-weighted imaging of the liver. D 2006 Elsevier Inc. All rights reserved. Keywords: Fast-recovery fast spin echo; Liver lesion; MR imaging
1. Introduction MR imaging of the liver is widely used since MR is more sensitive and specific in the detection and characterization of focal liver lesions than US or CT. T2-weighted sequences have been important elements in hepatic MR imaging. Today, T2-weighted fast spin echo (FSE) sequences are generally considered to be the standard T2-weighted sequence for evaluating the liver. T2-weighted FSE sequences can be obtained with the use of respiratory-triggered or breath-hold techniques to reduce respiratory motion artifacts. Use of the breath-hold technique further improves FSE imaging quality by reducing respiratory artifacts,
4 Corresponding author. Tel.: +1 212 639 3458; fax: +1 212 794 4010. E-mail address: [email protected] (O. Akin). 0899-7071/$ – see front matter D 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.clinimag.2006.02.006
providing shorter acquisition times and thereby increasing patient compliance and throughput. Several authors have investigated the role of breath-hold FSE imaging in the evaluation of liver lesions and have reported varying levels of effectiveness for this method [1–4]. A limitation of the breath-hold T2-weighted FSE sequence is decreased signal intensity (SI) due to proton saturation with short TR. To alleviate this drawback, a modified T2-weighted FSE sequence, fast-recovery FSE (FR-FSE), can be used. In this technique, additional RF pulses are used to accelerate the recovery of longitudinal magnetization to create highly T2-weighted images in a single breath-hold. Initial reports about the usefulness of FR-FSE in liver were promising [5– 8]. The purpose of this study is to compare breath-hold FRFSE with and without fat suppression with non-breath-hold
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
FSE with and without fat suppression in image quality and the detection of liver lesions.
323
does not hold his or her breath at the same location during subsequent breath-holds. 2.3. MR imaging protocol
2. Materials and methods 2.1. Patients This prospective study was conducted on 59 patients (19 men and 40 women; mean age, 60 years) who had focal liver lesions seen on other radiological studies (US or CT) or clinical suspicion due to laboratory abnormalities. The study was approved by the institutional review board for patient safety and privacy, and informed consent was obtained from all of the patients. The study was HIPAA compliant. All of the patients underwent MR imaging, including the four different T2-weighted sequences, which were breathhold FR-FSE with and without fat suppression and nonbreath-hold FSE with and without fat suppression. The diagnoses were made with histopathologic examination of either biopsy or surgical specimens, typical radiological findings, or clinical observation. 2.2. Breath-hold FR-FSE sequence Breath-hold FR-FSE is a modified FSE sequence that utilizes fast-recovery T2 enhancement to increase the signal from long T2 protons. Additional radio-frequency pulses after the final acquisition window are used to drive the recovery of longitudinal magnetization. An additional 1808 refocusing pulse is sent once the last echo in the fast SE echo train has been acquired. A 908 pulse is then used to drive the refocused magnetization back up onto the longitudinal axis instead of allowing it to recover with the T1 processes. After an interval of several repetition times, a steady state of longitudinal magnetization is established with net enhancement of the long T2 components. In addition, an optimized acquisition scheme is used. Conventional FSE acquisition schemes obtain transverse sections that cover the entire liver in each breath-hold and will interleave the sections from subsequent breath-holds. For instance, if 20 sections are required to cover the entire liver, Sections 1, 3, 5, . . ., 19 will be acquired in the first breath-hold and Sections 2, 4, 6, . . ., 20 in the second. Because it is impossible for a patient to hold his or her breath at exactly the same location in each breath-hold, the liver will be at different actual locations during each breathhold, and transverse sections from two breath-holds will not be correctly interleaved. With the use of the breath-hold FRFSE, transverse sections are acquired in groups of contiguous sections. One group of sections is acquired for each breath-hold, and the sections within each group are interleaved during the breath-hold to reduce cross-talk effects. Then, the subsequent group of sections is acquired. Overlapping the groups of sections prevents the possibility of missing tissue between groups of sections if the patient
MR imaging was performed with a 1.5-T magnet (Signa, GE Medical Systems, Milwaukee, WI) with phased-array coils. The T2-weighted imaging included (a) breath-hold FR-FSE (TR = 2500, TE = 200, matrix = 256128–192, NEX = 1, ETL= 18), (b) breath-hold FR-FSE with fat suppression (TR = 2550, TE = 86, matrix = 256128–192, NEX =1, ETL=20), (c) non-breath-hold FSE (TR =6000, TE = 200, matrix =256256, NEX =4, ETL= 18), and (d) non-breath-hold FSE with fat suppression (TR = 4200, TE = 98, matrix = 256256, NEX = 4, ETL=12). Slice thickness was 8 mm with a 2-mm gap for all of the sequences. All of the images were acquired in the axial plane. Precontrast and postcontrast dynamic T1-weighted sequences were also obtained in all of the patients in order to obtain a complete MR imaging examination of the liver. However, these images were not used by the reviewers for the study purposes. 2.4. Quantitative analysis Quantitative analysis was performed on each T2-weighted sequence in each patient by using operator-defined region-ofinterest measurements of mean SI of the normal parenchyma of the liver, up to three largest liver lesions, and the background noise (S.D.). Then, calculations of the liver signal-to-noise ratio (SNR) (SIliver/S.D.noise) and the liver-to-lesion contrastto-noise ratio (CNR) [(SI lesion SIliver)/S.D.noise] were made for each of the four T2-weighted sequences. 2.5. Qualitative analysis Image quality in each of the four T2-weighted sequences was assessed separately by two radiologists blinded to the sequence parameters, patient information, and pathologic findings. Images were compared on a scale of 1–5 (1= poor; 5= excellent) for liver edge sharpness, clarity of vessels, ghosting artifact, motion artifact, lesion conspicuity, overall image quality, uniformity of fat suppression, and the number of lesions identified (up to 10 lesions). 2.6. Statistical analysis Statistical analysis was performed by using a singletailed paired Student’s t test with a 95% confidence interval for the comparison of four different T2-weighted sequences Table 1 Mean liver SNRs and mean liver-to-lesion CNRs for four different pulse sequences FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Liver SNR (S.D.)
Liver-to-lesion CNR (S.D.)
10.48 (7.98) 18.81 (8.56)
41.29 (52.01) 32.12 (40.35)
12.02 (5.38) 28.86 (11.95)
45.27 (74.75) 47.47 (56.18)
324
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
Table 2 Mean number of lesions detected by two blinded reviewers for four different pulse sequences Number of lesions (S.D.) FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Reviewer 1
Reviewer 2
1.76 2.31 1.69 2.10
2.51 2.88 2.46 2.77
(2.01) (2.39) (1.81) (2.20)
(2.36) (2.59) (2.46) (2.62)
obtained by breath-hold FR-FSE with and without fat suppression and non-breath-hold FSE with and without fat-suppression techniques. 3. Results Table 1 summarizes the mean liver SNRs and the mean liver-to-lesion CNRs for four different pulse sequences. According to quantitative analysis by using operator-defined region-of-interest measurements, liver SNRs and liver-tolesion CNRs were 10.48 (S.D. = 7.98) and 41.29 (S.D. = 52.01) for FR-FSE, 18.81 (S.D. =8.56) and 32.12 (S.D. = 40.35) for FR-FSE with fat suppression, 12.02 (S.D. = 5.38) and 45.27 (S.D. = 74.75) for FSE, and 28.86 (S.D. =11.95) and 47.47 (S.D. =56.18) for FSE with fat suppression, respectively. Non-breath-hold FSE with fat-suppression technique provided significantly better ( P b.05) SNR and CNR compared with breath-hold FR-FSE with fat suppression. There was no statistically significant difference in terms of SNR and CNR between non-breath-hold FSE without fat suppression and breath-hold FR-FSE without fat suppression. In the study group, metastases in 25 patients, cysts in 22, hemangiomas in 15, hepatocellular carcinomas in 2, cholangiocarcinomas in 2, sarcomas in 2, focal nodular hyperplasia in 1, and abscess in 1 patient were seen. In 6 patients, no mass was seen on MR imaging. Seventeen patients had more than one type of pathology. Table 2 summarizes the mean number of lesions detected by two blinded reviewers for four different pulse sequences. There was no statistically significant difference between the two reviewers in terms of the number of lesions detected for any of the pulse sequences. More lesions were detected by both reviewers with breath-hold FR-FSE with or without fat-suppression techniques compared with non-breath-hold Table 3 Mean overall image quality qualitatively evaluated by two blinded reviewers on a scale of 1–5 (1 = poor; 5 = excellent) for four different pulse sequences Overall image quality (S.D.) FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Reviewer 1
Reviewer 2
3.62 4.23 3.07 3.59
4.35 4.41 3.74 3.70
(0.86) (0.66) (0.70) (0.70)
(0.78) (0.77) (0.78) (0.90)
Fig. 1. Liver metastasis from colon cancer (arrows on A and B). Lesion conspicuity, liver edge sharpness, and clarity of vessels are superior on breath-hold FR-FSE sequence with fat suppression (TR = 2550, TE = 86) (A) compared with non-breath-hold FSE with fat-saturation sequence (TR = 4200, TE = 98) (B).
FSE with or without fat-suppression techniques. However, these differences also did not reach statistical significance. Table 3 summarizes the mean overall image quality qualitatively evaluated by two blinded reviewers on a scale of 1–5 (1 =poor; 5 = excellent) for four different pulse sequences. The overall image quality was statistically significantly better ( P b.05) on breath-hold FR-FSE sequences compared with non-breath-hold FSE sequences based on the evaluations of both blinded reviewers. The lesion conspicuity, liver edge sharpness, and clarity of vessels were superior and ghosting was less on the breath-hold FR-FSE sequences compared with the non-breath-hold FSE sequences (Fig. 1). 4. Discussion MR imaging is recognized as a very useful method in liver lesion detection and characterization. Therefore, this technique is commonly used in clinical practice today. In many centers, MR imaging examination of the liver routinely includes T1- and T2-weighted sequences, as well as dynamic postcontrast T1-weighted sequences. In some cases, MR angiography and MR cholangiography sequences
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
can also be included in the MR imaging examination of the liver to provide a thorough evaluation. All of the abovementioned sequences are complementary to each other for optimal lesion detection and characterization in the liver. An optimal T2-weighted sequence must provide high soft tissue contrast and must be performed rapidly. Degradation of image quality by respiratory motion artifacts is the most important technical problem to be solved in abdominal MR imaging. Techniques such as respiratory triggering or breath-holding have been used to eliminate respiratory motion artifacts. Respiratory-triggered imaging requires long imaging times and may be ineffective if the patient cannot maintain a regular respiratory pattern. Compared with respiratory-triggering techniques, breathholding techniques are advantageous in terms of shortening the imaging time. Breath-hold T2-weighted FSE imaging techniques generally provide decreased tissue contrast and lesion detection compared with non-breath-hold techniques [2,4]. However, a modified FSE technique with fast recovery (FR-FSE) can provide coverage of the entire liver in one or two breath-holds with better image quality and lesion detection than standard FSE techniques [5–8]. In the FR-FSE technique, residual magnetization in the transverse plane is refocused and flipped back to the longitudinal axis to improve T2 contrast and decrease T1 contrast. Imaging time is reduced due to decreased TR (~2500 ms) while maintaining T2 contrast. The results of our study showed that breath-hold FR-FSE sequences provide lower liver SNRs and liver-to-lesion CNRs than non-breath-hold FSE sequences. However, these differences in liver SNRs and liver-to-lesion CNRs do not cause decreased image quality or lesion conspicuity. As a matter of fact, image quality and lesion conspicuity were better with breath-hold FR-FSE sequences than with nonbreath-hold FSE sequences. Although more lesions were detected with breath-hold FR-FSE sequences, the difference was not statistically significant. In this study, we did not aim to characterize these lesions based on breath-hold FR-FSE or non-breath-hold FSE imaging features. Although T2-weighted images provide important clues about the nature of a hepatic lesion, complete characterization of a hepatic lesion is accomplished by obtaining information from T1- and T2-weighted sequences, as well as from dynamic postcontrast T1-weighted sequences. One limitation of this study was that no gold-standard test was used to determine the true number of liver lesions in the study group. However, our study was designed to compare the breath-hold FR-FSE with and without fat suppression with non-breath-hold FSE with and without fat suppression in terms of detection of liver lesions and image
325
quality. Our purpose was not to investigate the true sensitivity of either technique. Another limitation might be the lack of histopathologic confirmation in some of the cases. However, imaging and clinical follow-up in these cases provided sufficient evidence about the nature of the lesions. Another bias might have originated from the qualitative evaluation of different pulse sequences. Although the reviewers were blinded to the imaging parameters, they might have recognized the type of technique due to differences in images obtained with a particular pulse sequence. In summary, the results of our study showed that FR-FSE sequences, despite their lower SNR and CNR compared with FSE sequences, provide superior image quality with better liver lesion conspicuity than the FSE sequences. The amount of time saved by using breath-hold FR-FSE is undoubtedly another advantage, as it increases patient compliance and throughput. We believe that the breath-hold FR-FSE technique is a reasonable alternative approach for T2-weighted imaging of the liver.
References [1] Rydberg JN, Lomas DJ, Coakley KJ, Hough DM, Ehman RL, Riederer SJ. Comparison of breath-hold fast spin-echo and conventional spinecho pulse sequences for T2-weighted MR imaging of liver lesions. Radiology 1995;194:431 – 7. [2] Soyer P, Normand SL, Givry SC, Gueye C, Somveille E, Scherrer A. T2-weighted spin-echo MR imaging of the liver: breath-hold fast spinecho versus non-breath-hold fast spin-echo images with and without fat suppression. AJR Am J Roentgenol 1996;166:593 – 7. [3] Yamashita Y, Yamamoto H, Nmimoto T, Abe Y, Takahashi M. Phased array breath-hold versus non-breath-hold MR imaging of focal liver lesions: a prospective comparative study. J Magn Reson Imaging 1997; 7:292 – 7. [4] Kanematsu M, Hoshi H, Itoh K, et al. Focal hepatic lesion detection: comparison of four fat-suppressed T2-weighted MR imaging pulse sequences. Radiology 1999;211:363 – 71. [5] Schwartz LH, Welber A, Maier CF, DeCorato DR, Winston CB, Panicek DM. Fast recovery fast spin echo evaluation of focal hepatic lesions (abstr). Radiology 2000;217(P):586 – 7. [6] Katayama M, Masui T, Kobayashi S, et al. Fat-suppressed T2-weighted MRI of the liver: comparison of respiratory-triggered fast spin-echo, breath-hold single-shot fast spin-echo, and breath-hold fast-recovery fast spin-echo sequences. J Magn Reson Imaging 2001;14:439 – 49. [7] Augui J, Vignaux O, Argaud C, Coste J, Gouya H, Legmann P. Liver: T2-weighted MR imaging with breath-hold fast-recovery optimized fast spin-echo compared with breath-hold half-Fourier and non-breath-hold respiratory-triggered fast spin-echo pulse sequences. Radiology 2002; 223:853 – 9. [8] Huang J, Raman SS, Vuong N, Sayre JW, Lu DS. Utility of breath-hold fast-recovery fast spin-echo t2 versus respiratory-triggered fast spinecho T2 in clinical hepatic imaging. AJR Am J Roentgenol 2005; 184:842 – 6.
Evaluation of focal liver lesions: fast-recovery fast spin echo T2-weighted MR imaging Oguz Akina,4, Lawrence H. Schwartza, Adam Welberb, Cynthia F. Maierc, Douglas R. DeCoratod, David M. Paniceka a
Department of Radiology, Memorial SloanKettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA b Danbury Hospital, Denbury, CT 06810, USA c GE Medical Systems, Waukesha, WI 53201, USA d East River Medical Imaging, New York, NY 10021, USA Received 5 January 2006; accepted 20 February 2006
Abstract Purpose: To compare breath-hold fast-recovery fast spin echo (FR-FSE) and non-breath-hold fast spin echo (FSE) T2-weighted sequences for hepatic lesion conspicuity and image quality at MR imaging. Materials and Methods: Fifty-nine patients with known or suspected liver lesions underwent hepatic MR imaging by using a breath-hold FR-FSE T2-weighted sequence with and without fat suppression and a non-breath-hold FSE T2-weighted sequence with and without fat suppression. Quantitative analysis was made with measurements of the signal intensity of the liver, spleen, background noise, and up to three liver lesions, as well as calculations of the liver signal-to-noise ratio (SNR) and the liver-to-lesion contrast-to-noise ratio (CNR) for each sequence. Qualitative analysis was made for image quality and the number of lesions identified. Statistical analysis was performed by using a single-tailed paired Student’s t test with a 95% confidence interval. Results: SNR and CNR were significantly higher ( P b.05) for FSE with fat suppression than for FR-FSE with fat suppression. No statistically significant difference was seen in terms of SNR and CNR between non-fat-suppressed FSE and FR-FSE sequences. Lesion conspicuity, liver edge sharpness, and clarity of vessels were superior and ghosting was less with the FR-FSE sequences compared with the FSE sequences. Conclusion: Breath-hold FR-FSE technique is a reasonable alternative in T2-weighted imaging of the liver. D 2006 Elsevier Inc. All rights reserved. Keywords: Fast-recovery fast spin echo; Liver lesion; MR imaging
1. Introduction MR imaging of the liver is widely used since MR is more sensitive and specific in the detection and characterization of focal liver lesions than US or CT. T2-weighted sequences have been important elements in hepatic MR imaging. Today, T2-weighted fast spin echo (FSE) sequences are generally considered to be the standard T2-weighted sequence for evaluating the liver. T2-weighted FSE sequences can be obtained with the use of respiratory-triggered or breath-hold techniques to reduce respiratory motion artifacts. Use of the breath-hold technique further improves FSE imaging quality by reducing respiratory artifacts,
4 Corresponding author. Tel.: +1 212 639 3458; fax: +1 212 794 4010. E-mail address: [email protected] (O. Akin). 0899-7071/$ – see front matter D 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.clinimag.2006.02.006
providing shorter acquisition times and thereby increasing patient compliance and throughput. Several authors have investigated the role of breath-hold FSE imaging in the evaluation of liver lesions and have reported varying levels of effectiveness for this method [1–4]. A limitation of the breath-hold T2-weighted FSE sequence is decreased signal intensity (SI) due to proton saturation with short TR. To alleviate this drawback, a modified T2-weighted FSE sequence, fast-recovery FSE (FR-FSE), can be used. In this technique, additional RF pulses are used to accelerate the recovery of longitudinal magnetization to create highly T2-weighted images in a single breath-hold. Initial reports about the usefulness of FR-FSE in liver were promising [5– 8]. The purpose of this study is to compare breath-hold FRFSE with and without fat suppression with non-breath-hold
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
FSE with and without fat suppression in image quality and the detection of liver lesions.
323
does not hold his or her breath at the same location during subsequent breath-holds. 2.3. MR imaging protocol
2. Materials and methods 2.1. Patients This prospective study was conducted on 59 patients (19 men and 40 women; mean age, 60 years) who had focal liver lesions seen on other radiological studies (US or CT) or clinical suspicion due to laboratory abnormalities. The study was approved by the institutional review board for patient safety and privacy, and informed consent was obtained from all of the patients. The study was HIPAA compliant. All of the patients underwent MR imaging, including the four different T2-weighted sequences, which were breathhold FR-FSE with and without fat suppression and nonbreath-hold FSE with and without fat suppression. The diagnoses were made with histopathologic examination of either biopsy or surgical specimens, typical radiological findings, or clinical observation. 2.2. Breath-hold FR-FSE sequence Breath-hold FR-FSE is a modified FSE sequence that utilizes fast-recovery T2 enhancement to increase the signal from long T2 protons. Additional radio-frequency pulses after the final acquisition window are used to drive the recovery of longitudinal magnetization. An additional 1808 refocusing pulse is sent once the last echo in the fast SE echo train has been acquired. A 908 pulse is then used to drive the refocused magnetization back up onto the longitudinal axis instead of allowing it to recover with the T1 processes. After an interval of several repetition times, a steady state of longitudinal magnetization is established with net enhancement of the long T2 components. In addition, an optimized acquisition scheme is used. Conventional FSE acquisition schemes obtain transverse sections that cover the entire liver in each breath-hold and will interleave the sections from subsequent breath-holds. For instance, if 20 sections are required to cover the entire liver, Sections 1, 3, 5, . . ., 19 will be acquired in the first breath-hold and Sections 2, 4, 6, . . ., 20 in the second. Because it is impossible for a patient to hold his or her breath at exactly the same location in each breath-hold, the liver will be at different actual locations during each breathhold, and transverse sections from two breath-holds will not be correctly interleaved. With the use of the breath-hold FRFSE, transverse sections are acquired in groups of contiguous sections. One group of sections is acquired for each breath-hold, and the sections within each group are interleaved during the breath-hold to reduce cross-talk effects. Then, the subsequent group of sections is acquired. Overlapping the groups of sections prevents the possibility of missing tissue between groups of sections if the patient
MR imaging was performed with a 1.5-T magnet (Signa, GE Medical Systems, Milwaukee, WI) with phased-array coils. The T2-weighted imaging included (a) breath-hold FR-FSE (TR = 2500, TE = 200, matrix = 256128–192, NEX = 1, ETL= 18), (b) breath-hold FR-FSE with fat suppression (TR = 2550, TE = 86, matrix = 256128–192, NEX =1, ETL=20), (c) non-breath-hold FSE (TR =6000, TE = 200, matrix =256256, NEX =4, ETL= 18), and (d) non-breath-hold FSE with fat suppression (TR = 4200, TE = 98, matrix = 256256, NEX = 4, ETL=12). Slice thickness was 8 mm with a 2-mm gap for all of the sequences. All of the images were acquired in the axial plane. Precontrast and postcontrast dynamic T1-weighted sequences were also obtained in all of the patients in order to obtain a complete MR imaging examination of the liver. However, these images were not used by the reviewers for the study purposes. 2.4. Quantitative analysis Quantitative analysis was performed on each T2-weighted sequence in each patient by using operator-defined region-ofinterest measurements of mean SI of the normal parenchyma of the liver, up to three largest liver lesions, and the background noise (S.D.). Then, calculations of the liver signal-to-noise ratio (SNR) (SIliver/S.D.noise) and the liver-to-lesion contrastto-noise ratio (CNR) [(SI lesion SIliver)/S.D.noise] were made for each of the four T2-weighted sequences. 2.5. Qualitative analysis Image quality in each of the four T2-weighted sequences was assessed separately by two radiologists blinded to the sequence parameters, patient information, and pathologic findings. Images were compared on a scale of 1–5 (1= poor; 5= excellent) for liver edge sharpness, clarity of vessels, ghosting artifact, motion artifact, lesion conspicuity, overall image quality, uniformity of fat suppression, and the number of lesions identified (up to 10 lesions). 2.6. Statistical analysis Statistical analysis was performed by using a singletailed paired Student’s t test with a 95% confidence interval for the comparison of four different T2-weighted sequences Table 1 Mean liver SNRs and mean liver-to-lesion CNRs for four different pulse sequences FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Liver SNR (S.D.)
Liver-to-lesion CNR (S.D.)
10.48 (7.98) 18.81 (8.56)
41.29 (52.01) 32.12 (40.35)
12.02 (5.38) 28.86 (11.95)
45.27 (74.75) 47.47 (56.18)
324
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
Table 2 Mean number of lesions detected by two blinded reviewers for four different pulse sequences Number of lesions (S.D.) FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Reviewer 1
Reviewer 2
1.76 2.31 1.69 2.10
2.51 2.88 2.46 2.77
(2.01) (2.39) (1.81) (2.20)
(2.36) (2.59) (2.46) (2.62)
obtained by breath-hold FR-FSE with and without fat suppression and non-breath-hold FSE with and without fat-suppression techniques. 3. Results Table 1 summarizes the mean liver SNRs and the mean liver-to-lesion CNRs for four different pulse sequences. According to quantitative analysis by using operator-defined region-of-interest measurements, liver SNRs and liver-tolesion CNRs were 10.48 (S.D. = 7.98) and 41.29 (S.D. = 52.01) for FR-FSE, 18.81 (S.D. =8.56) and 32.12 (S.D. = 40.35) for FR-FSE with fat suppression, 12.02 (S.D. = 5.38) and 45.27 (S.D. = 74.75) for FSE, and 28.86 (S.D. =11.95) and 47.47 (S.D. =56.18) for FSE with fat suppression, respectively. Non-breath-hold FSE with fat-suppression technique provided significantly better ( P b.05) SNR and CNR compared with breath-hold FR-FSE with fat suppression. There was no statistically significant difference in terms of SNR and CNR between non-breath-hold FSE without fat suppression and breath-hold FR-FSE without fat suppression. In the study group, metastases in 25 patients, cysts in 22, hemangiomas in 15, hepatocellular carcinomas in 2, cholangiocarcinomas in 2, sarcomas in 2, focal nodular hyperplasia in 1, and abscess in 1 patient were seen. In 6 patients, no mass was seen on MR imaging. Seventeen patients had more than one type of pathology. Table 2 summarizes the mean number of lesions detected by two blinded reviewers for four different pulse sequences. There was no statistically significant difference between the two reviewers in terms of the number of lesions detected for any of the pulse sequences. More lesions were detected by both reviewers with breath-hold FR-FSE with or without fat-suppression techniques compared with non-breath-hold Table 3 Mean overall image quality qualitatively evaluated by two blinded reviewers on a scale of 1–5 (1 = poor; 5 = excellent) for four different pulse sequences Overall image quality (S.D.) FR-FSE FR-FSE with fat suppression FSE FSE with fat suppression
Reviewer 1
Reviewer 2
3.62 4.23 3.07 3.59
4.35 4.41 3.74 3.70
(0.86) (0.66) (0.70) (0.70)
(0.78) (0.77) (0.78) (0.90)
Fig. 1. Liver metastasis from colon cancer (arrows on A and B). Lesion conspicuity, liver edge sharpness, and clarity of vessels are superior on breath-hold FR-FSE sequence with fat suppression (TR = 2550, TE = 86) (A) compared with non-breath-hold FSE with fat-saturation sequence (TR = 4200, TE = 98) (B).
FSE with or without fat-suppression techniques. However, these differences also did not reach statistical significance. Table 3 summarizes the mean overall image quality qualitatively evaluated by two blinded reviewers on a scale of 1–5 (1 =poor; 5 = excellent) for four different pulse sequences. The overall image quality was statistically significantly better ( P b.05) on breath-hold FR-FSE sequences compared with non-breath-hold FSE sequences based on the evaluations of both blinded reviewers. The lesion conspicuity, liver edge sharpness, and clarity of vessels were superior and ghosting was less on the breath-hold FR-FSE sequences compared with the non-breath-hold FSE sequences (Fig. 1). 4. Discussion MR imaging is recognized as a very useful method in liver lesion detection and characterization. Therefore, this technique is commonly used in clinical practice today. In many centers, MR imaging examination of the liver routinely includes T1- and T2-weighted sequences, as well as dynamic postcontrast T1-weighted sequences. In some cases, MR angiography and MR cholangiography sequences
O. Akin et al. / Clinical Imaging 30 (2006) 322–325
can also be included in the MR imaging examination of the liver to provide a thorough evaluation. All of the abovementioned sequences are complementary to each other for optimal lesion detection and characterization in the liver. An optimal T2-weighted sequence must provide high soft tissue contrast and must be performed rapidly. Degradation of image quality by respiratory motion artifacts is the most important technical problem to be solved in abdominal MR imaging. Techniques such as respiratory triggering or breath-holding have been used to eliminate respiratory motion artifacts. Respiratory-triggered imaging requires long imaging times and may be ineffective if the patient cannot maintain a regular respiratory pattern. Compared with respiratory-triggering techniques, breathholding techniques are advantageous in terms of shortening the imaging time. Breath-hold T2-weighted FSE imaging techniques generally provide decreased tissue contrast and lesion detection compared with non-breath-hold techniques [2,4]. However, a modified FSE technique with fast recovery (FR-FSE) can provide coverage of the entire liver in one or two breath-holds with better image quality and lesion detection than standard FSE techniques [5–8]. In the FR-FSE technique, residual magnetization in the transverse plane is refocused and flipped back to the longitudinal axis to improve T2 contrast and decrease T1 contrast. Imaging time is reduced due to decreased TR (~2500 ms) while maintaining T2 contrast. The results of our study showed that breath-hold FR-FSE sequences provide lower liver SNRs and liver-to-lesion CNRs than non-breath-hold FSE sequences. However, these differences in liver SNRs and liver-to-lesion CNRs do not cause decreased image quality or lesion conspicuity. As a matter of fact, image quality and lesion conspicuity were better with breath-hold FR-FSE sequences than with nonbreath-hold FSE sequences. Although more lesions were detected with breath-hold FR-FSE sequences, the difference was not statistically significant. In this study, we did not aim to characterize these lesions based on breath-hold FR-FSE or non-breath-hold FSE imaging features. Although T2-weighted images provide important clues about the nature of a hepatic lesion, complete characterization of a hepatic lesion is accomplished by obtaining information from T1- and T2-weighted sequences, as well as from dynamic postcontrast T1-weighted sequences. One limitation of this study was that no gold-standard test was used to determine the true number of liver lesions in the study group. However, our study was designed to compare the breath-hold FR-FSE with and without fat suppression with non-breath-hold FSE with and without fat suppression in terms of detection of liver lesions and image
325
quality. Our purpose was not to investigate the true sensitivity of either technique. Another limitation might be the lack of histopathologic confirmation in some of the cases. However, imaging and clinical follow-up in these cases provided sufficient evidence about the nature of the lesions. Another bias might have originated from the qualitative evaluation of different pulse sequences. Although the reviewers were blinded to the imaging parameters, they might have recognized the type of technique due to differences in images obtained with a particular pulse sequence. In summary, the results of our study showed that FR-FSE sequences, despite their lower SNR and CNR compared with FSE sequences, provide superior image quality with better liver lesion conspicuity than the FSE sequences. The amount of time saved by using breath-hold FR-FSE is undoubtedly another advantage, as it increases patient compliance and throughput. We believe that the breath-hold FR-FSE technique is a reasonable alternative approach for T2-weighted imaging of the liver.
References [1] Rydberg JN, Lomas DJ, Coakley KJ, Hough DM, Ehman RL, Riederer SJ. Comparison of breath-hold fast spin-echo and conventional spinecho pulse sequences for T2-weighted MR imaging of liver lesions. Radiology 1995;194:431 – 7. [2] Soyer P, Normand SL, Givry SC, Gueye C, Somveille E, Scherrer A. T2-weighted spin-echo MR imaging of the liver: breath-hold fast spinecho versus non-breath-hold fast spin-echo images with and without fat suppression. AJR Am J Roentgenol 1996;166:593 – 7. [3] Yamashita Y, Yamamoto H, Nmimoto T, Abe Y, Takahashi M. Phased array breath-hold versus non-breath-hold MR imaging of focal liver lesions: a prospective comparative study. J Magn Reson Imaging 1997; 7:292 – 7. [4] Kanematsu M, Hoshi H, Itoh K, et al. Focal hepatic lesion detection: comparison of four fat-suppressed T2-weighted MR imaging pulse sequences. Radiology 1999;211:363 – 71. [5] Schwartz LH, Welber A, Maier CF, DeCorato DR, Winston CB, Panicek DM. Fast recovery fast spin echo evaluation of focal hepatic lesions (abstr). Radiology 2000;217(P):586 – 7. [6] Katayama M, Masui T, Kobayashi S, et al. Fat-suppressed T2-weighted MRI of the liver: comparison of respiratory-triggered fast spin-echo, breath-hold single-shot fast spin-echo, and breath-hold fast-recovery fast spin-echo sequences. J Magn Reson Imaging 2001;14:439 – 49. [7] Augui J, Vignaux O, Argaud C, Coste J, Gouya H, Legmann P. Liver: T2-weighted MR imaging with breath-hold fast-recovery optimized fast spin-echo compared with breath-hold half-Fourier and non-breath-hold respiratory-triggered fast spin-echo pulse sequences. Radiology 2002; 223:853 – 9. [8] Huang J, Raman SS, Vuong N, Sayre JW, Lu DS. Utility of breath-hold fast-recovery fast spin-echo t2 versus respiratory-triggered fast spinecho T2 in clinical hepatic imaging. AJR Am J Roentgenol 2005; 184:842 – 6.