BRAIN
Phase-Sensitive T1 Inversion Recovery Imaging: A Time-Efficient Interleaved Technique for Improved Tissue Contrast in Neuroimaging
Ping Houa, Khader M. Hasana, Clark W. Sittona, Jerry S. Wolinskyb and Ponnada A. Narayanaa
+ Author Affiliations
aDepartment of Diagnostic and Interventional Imaging, University of Texas Medical School at Houston, Houston, TX
bDepartment of Neurology, University of Texas Medical School at Houston, Houston, TX
Address correspondence to Ping Hou, PhD, Department of Diagnostic and Interventional Imaging, University of Texas Medical School at Houston, 6431 Fannin Street, MSB 2.100, Houston, TX 77030
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Abstract
BACKGROUND AND PURPOSE: High tissue contrast and short acquisition time are desirable when scanning patients. The purpose of this report is to describe the implementation of a new technique for generating high gray matter (GM) and white matter (WM) contrast in a short scan time, make a quantitative evaluation of the contrast efficiency, and explore its potential applications in neuroimaging.
METHOD: A fully interleaved T1-weighted inversion recovery (T1IR) sequence with phase-sensitive reconstruction (PS-T1IR) is implemented. This sequence is compared with conventional T1-weighted spin-echo imaging (T1SE) and T1-weighted fluid-attenuated inversion recovery (T1FLAIR). The time efficiency and contrast enhancement have been quantitatively analyzed in normal volunteers. The performance of the sequence is evaluated in >30 patients with neurologic disorders. The sensitivity of PS-T1IR relative to T1SE in detecting gadolinium enhancements is also evaluated.
RESULTS: PS-T1IR is more time-efficient than T1SE and generates better GM-WM contrast. It results in the best contrast-to-noise ratio (CNR) efficiency (1.16) compared with T1FLAIR (0.73) and T1SE (0.23). For a typical clinical protocol, PS-T1IR takes only 1:30 minutes versus 2:40 minutes for T1SE imaging for the whole brain coverage. Although gadolinium enhancements are detected with comparable sensitivity on both PS-T1IR and T1SE sequences, in certain instances, the latter sequence appears to be more sensitive in demonstrating gadolinium enhancements within WM.
CONCLUSION: PS-T1IR has the highest CNR efficiency compared with T1FLAIR and T1SE. It is a very practical technique for neuroradiologic applications.
Inversion recovery (IR) sequences are commonly used to suppress the MR signal intensity from CSF (1–2) or fat; the so-called fluid-attenuated inversion recovery (FLAIR) and short tau inversion recovery (STIR) sequences (3), respectively. In addition to suppressing specified tissues, IR pulse sequences can generate T1-weighted images with an intermediate inversion time (TI) of 600–1,200 milliseconds. Several studies have also demonstrated that IR provides superior contrast and greater sensitivity in detecting gadolinium (Gd) contrast enhancement than conventional spin-echo (SE) sequences (4–8). STIR generates high-contrast T1-, T2-, and proton density-weighted images by nulling the fat signal. Like most IR sequences, however, STIR requires long acquisition time, even when combined with the fast spin-echo (FSE) readout. A time-efficient interleaved technique was proposed by Listerud et al (9) for acquiring T2-weighted FLAIR (T2FLAIR) images. In their technique, section excitation and acquisition were both interleaved during the TI and TR periods. This interleaved technique can be adapted for acquiring T1-weighted image. With this truly section and time interleaved technique, the contrast between white matter (WM) and gray matter (GM) is improved by suppressing CSF. Because the T1-weighted FLAIR (T1FLAIR) images are generated by magnitude reconstruction, gain in the image contrast, however, remains limited. Moreover, the images appear blurred compared with conventional T1-weighted SE (T1SE) images. Therefore, despite its speed and robustness, T1FLAIR did not gain wide acceptance in the radiologic community, and T1SE continues to be the sequence of choice for generating T1-weighted images.
Central to all the inversion recovery sequences is the application of an inversion radio-frequency (RF) pulse that flips the longitudinal magnetization from the +z to the −z direction. The magnetization can, therefore, be positive or negative, depending on the TI and tissue T1 values and the time at which the readout sequence is applied. By preserving the sign of the MR signal intensity, the image contrast can be enhanced by selecting appropriate TI value (10, 11). The benefits of the phase-sensitive reconstruction in IR are well known (10–18). Its application, however, has been limited by the artifacts from phase errors and long scan times. The sources of phase errors include non-centering of the echo in the readout window because of errors in the pulse sequence timing and phase-encoding steps, phase shifts from hardware such as bandwidth filters, variation in the patient loading, and coil sensitivity. Different phase-correction strategies, including acquisition of a reference image (16, 17) and estimation of phase from local statistics (10, 11, 15), have been investigated. In this article, we apply the phase-sensitive reconstruction to an interleaved T1-weighted IR pulse sequence (PS-T1IR) for generating images with high tissue contrast in a short scan time and demonstrate its application in neuroimaging.
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Methods
Theory
The signal intensity amplitude in an IR sequence (19) can be written as
Formula
where ρ is the water proton spin density and the other symbols have their usual meaning. The T1 weighting is determined by the expression in the parenthesis and we refer to this as the T1-weighted factor. It can be seen from the above expression that the T1 contrast in an IR sequence is different from that of the SE sequence. The T1-weighted factor is a function of both TR and TI, which are user-selectable and is negative for a short TI and positive for long TI. The advantage of the phase-sensitive reconstructed IR is that the range of the T1-weighted factor is from −1 to 1 instead of 0 to 1 in a T1SE sequence. This increased dynamic range provides greater T1 contrast for different tissues. Because, in practice, TR is not infinite, the actual dynamic range is within −(1 − exp[−TR/T1]) to (1 − exp[−TR/T1]). The traditional magnitude reconstruction in the IR sequence automatically restricts the range of the T1-weighted factor from 0 to (1 − exp[−TR/T1[), and has the potential disadvantage of compromising the contrast between tissues, depending on the value of TI, as demonstrated in Figure 1. It can be observed from Figure 1 that, if the inversion time is set between 400–500 milliseconds, the WM and GM have opposite magnetizations; their contrast in the magnitude-reconstructed images appears minimal. In addition to the potential loss of contrast, magnitude IR images also suffer from the dark line artifact that appears at the tissue borders where the positive and negative signals cancel. The phase-sensitive reconstruction not only suppresses this dark line artifact, but also provides improved GM-WM contrast and hypointense CSF signal intensity, because it has a large negative magnetization.
Fig 1.
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FIG 1.
Behavior of longitudinal magnetization as a function of inversion time before the application of the read-out sequence. The parameters used in these simulations are TR, 2250 milliseconds; T1WM, 600 milliseconds; T1GM, 920 milliseconds; and T1CSF, 4200 milliseconds.
The fully interleaved T1IR sequence is shown in Figure 2. If the minimum sequence play out time is defined as Tmin, which includes the inversion RF pulse, crusher gradients, and FSE data acquisition time, the number of sections covered during inversion time (TI) is TI/Tmin, and the total number of sections in one repetition time (TR) is TR/Tmin. The number of sections covered in TI and TR is truncated to an integer and is further reduced because of the hardware idle time requirements (gradient recovery, delay between the transmitter inactivation and receiver activation, and so forth) and RF safety limitations. Each IR pulse is followed by a readout sequence (FSE, in this case) with a different excitation frequency for specific section location. There is no dead time left in the pulse sequence. This is an optimal approach for time and section interleaving.
Fig 2.
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FIG 2.
Timing diagram of the T1IR sequence. In this diagram, the number of sections packed in one TI is 3, and the maximum number of sections covered in one TR is 7. The upper part shows the interleaved scheme, and the lower part shows how the IR and FSE integrate tightly in timing. Tmin is the time of IR, crusher gradient (in the phase encoding direction) and the FSE acquisition time. If the number of sections packed in the TI is less than the maximum sections allowed, there is a delay time added between crusher gradient and the FSE, and Tmin stays the same.
Subject and Protocols
Five healthy volunteers were scanned with T1SE, T1FLAIR, and PS-T1IR sequences. Both T1SE and PS-T1IR images were acquired on 30 patients with neurologic diseases. Twenty of these patients were administered Gd–diethylene triamine pentaacetic acid as a part of the diagnostic procedure. All volunteers signed the consent form before scanning, in accordance with our institutional regulations.
All scans were performed on a GE 1.5T Signa system (GE Medical Systems, Waukesha, WI) equipped with a gradient system capable of generating maximum gradient amplitude of 40 mT/m per channel with a slew rate of 150 mT/m/msec. A standard quadrature head coil was used for RF transmission and reception. This sequence is based on the GE sequence, T1FLAIR. An adiabatic inversion pulse was used, because it is less sensitive to B1 field inhomogenei
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