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Lesion Enhancement in Radio-Frequency Spoiled Gradient-Echo Imaging: Theory, Experimental Evaluation, and Clinical Implications

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Lesion Enhancement in Radio-Frequency Spoiled Gradient-Echo Imaging: Theory, Experimental Evaluation, and Clinical Implications Lesion Enhancement in Radio-Frequency Spoiled Gradient-Echo Imaging: Theory, Experimental Evaluation, and Clinical Implications 1 1 2 1 Scott Rand, Kenneth R. Maravilla, • and Udo Schmiedl PURPOSE: To investigate the lesser lesion conspicuity after gadolinium contrast infusion with radio-frequency spoiled gradient-echo (SPGR) sequences relative to conventional T1-weighted spin-echo techniques. METHODS: The influences of repetition time, echo time, and flip angle on spin-echo and SPGR signal were studied with mathematical modeling of the image signal amplitude for concentrations of gadopentetate dimeglumine solute from 0 to 10 mM. Predictions of signal strength were verified in vitro by imaging of a doped water phantom. The effects of standard (0.1 mmol/kg) and high-dose (0.3 mmol/kg) gadoteridol on spin-echo and SPGR images were also investigated in three patients. RESULTS: The measured amplitude of undoped water and the rate of increase of doped water signal with increasing gadopentetate concentration (slope) for spin­ echo 600/ 11/1/ 90° (repetition time/ echo time/excitations/flip angle) and SPGR (600/11/190°) were similar and exceeded those of SPGR (35/ 5/ 145°). Greater increases in SPGR doped water signal and its slope were produced by increasing TR than by varying echo-time or flip angle. The subjective lesion conspicuity and measured lesion contrast at 0.3 mmol/kg were greater with spin­ echo (600/11/ 1/90°) than with SPGR (35/5/145°) in all three patients; the measured lesion enhancement was similar for both techniques in two patients and decreased for SPGR in the third patient. CONCLUSIONS: The phantom studies suggest that the short repetition time of 35 msec, typically used in clinical SPGR imaging, is largely responsible for a reduced signal amplitude and a diminished rate of increase of signal with increasing gadopentetate concentration, relative to spin-echo. Phantom and clinical studies suggest that the dose of paramagnetic agent required to achieve SPGR lesion conspicuity with short repetition time comparable with spin-echo would have to be higher than the dose in current clinical use. Index terms: Magnetic resonance, contrast enhancement; Magnetic resonance, experimental; Magnetic resonance, gradient-echo; Magnetic resonance, physics AJNR Am J Neuroradiol 15:27-35, Jan 1994 Radio-frequency spoiled gradient-echo (SPGR) travenous gadopentetate both comparable with sequences are capable of producing images with (1, 2) (Kornmesser W, et al, presented at the tissue contrast and anatomic detail similar to annual meeting of the Society of Magnetic Res­ conventional Tl-weighted spin-echo (SE) tech­ onance in Medicine, New York, August, 1987) niques, but with considerably reduced data ac­ and less than (3) that of spin-echo techniques. quisition times. Previous studies with gradient­ However, reduced postgadolinium lesion en­ echo sequences in which residual transverse co­ hancement has been observed with SPGR relative herences were spoiled with gradient pulses re­ to SE imaging in recent clinical trials, with the ported lesion contrast or enhancement after in- implication of possible undetected or misdi­ agnosed lesions with SPGR (4) (Augenstein H, Sze Received June 4, 1992; accepted pending revision October 26; revision G, presented at the annual meeting of the Radi­ received March 5, 1993. ological Society of North America, Chicago, No­ 1 Department of Radiology, University of Washington Medical Center, vember, 1990). Seattle, WA 98195. Although the dependence of image contrast on 2 Address correspondence to Kenneth R. Maravilla, MD, Department of Radiology, SB-05, University of Washington Medical Center Seattle, operator-dependent pulse-sequence parameters WA 98195. has been investigated in detail for SE and gra­ AJNR 15:27-35, Jan 1994 0195-6108/94/ 1501-0027 dient-echo techniques (5, 6) , the apparent limited © American Society of Neuroradiology effectiveness of gadopentetate in the improve- 27 28 RAND AJNR: 15, January 1994 ment of lesion conspicuity in SPGR imaging re­ Similarly, standard SE images with fixed receiver and mains unexplained (Brant-Zawadzki M, presented transmitter gains, a fixed TR of 4800 msec, and four TE at the Society of Magnetic Resonance in Medicine values of 20, 40, 60, and 80 msec were used to measure Workshop on Contrast-Enhanced Magnetic Res­ transverse relaxation times of the phantom. Measured onance, Napa, CA, May, 1991). The present study signal amplitudes as a function of time (TE) were fit to the was undertaken to determine the reasons for the decaying exponential diminished SPGR lesion contrast relative to SE S = N(T1) * exp[-TE/T2] observed after gadolinium administration and to to solve for pseudo-spin density (N(T1)) and transverse seek means for improved SPGR enhancement. relaxation time (T2). This iterative two-parameter tech­ nique was also provided with General Electric software. Methods Longitudinal (R1) and transverse (R2) relaxivities (1/(mM sec)) were computed with linear least-squares fits to the This study consisted of a three-part investigation com­ Solomon-Bloembergen equations listed in the Appendix posed of mathematical modelling of signal amplitudes with (8). SE and SPGR, in vitro verification of model predictions with a gadolinium-doped water phantom, and a pilot clinical study of three patients who received standard and high Measurement of Gadopentetate-Doped Water Signal doses of a gadolinium contrast agent. Amplitude All imaging studies were conducted on a General Electric Prediction of Gadopentetate-Doped Water Signal 1.5T Signa system (General Electric, Milwaukee, WI). The Amplitude phantom consisted of an array of plastic centrifuge tubes filled with deionized, distilled water doped with graded Theoretical expressions of signal amplitude for the SE concentrations of gadopentetate. Three serial dilution and SPGR pulse sequences are listed in the Appendix. batches were prepared with the following concentrations: Measured water solvent longitudinal (T1) and transverse 0.00100 to 10.0 mM Gd-DTPA in multiplicative increments 3 2 1 (T2) relaxation times and longitudinal (R 1) and transverse of 1.00 X 10- , 1.00 X 10- , 1.00 X 10- , 1.00, and 1.00 (R2) relaxivities of gadopentetate dimeglumine (Gd-DTPA) X 101 mM; 0.100 to 1.00 mM Gd-DTPA in linear (arith­ (Magnevist, Berlex, Cedar Knolls, NJ) were substituted into metic) increments of 0.100 mM; and 1.00 to 10.0 mM Gd­ these expressions to predict (calculate) SE and SPGR signal DTPA, in linear (arithmetic) increments of 1.00 mM. Ref­ amplitudes as a function of gadopentetate concentration erence tubes containing undoped water were also included. (7). The effects of intravoxel incoherent dephasing such as Single section images of the phantom were obtained susceptibility artifacts were neglected in the SPGR predic­ with the SE and SPGR pulse sequences. A 24-cm field of tions, and T2* was set equal to T2. Simulations were view, 5-mm section thickness, 256 X 192 matrix, and a calculated and plotted on a Macintosh II computer (Apple, single data collection (1 excitation) were used for both Cupertino, CA) using the symbolic mathematics program techniques. Averages and standard deviations of region of Mathematica (Wolfram Research, Champaign, lL). interest signal amplitudes from samples within two or three Standard SE images were obtained to measure T1 nominally identical images were obtained for each combi­ relaxation times of the phantom. The receiver and trans­ nation of TR, TE, excitations, and flip angle. Measured mitter amplification gains were fixed, echo time (TE) was signal amplitudes for SE 600/11/1/90° (TR/TE/excita­ fixed at 20 msec, and repetition time (TR) was varied. Eight tions/flip angle), SPGR 600/ 11/1/90°, and SPGR 35/5/ TR values of 150, 300, 600, 900, 1200, 1800, 2400, and 145° were compared directly, without normalization, be­ 4800 msec were used to measure T1 values ranging from cause the receiver amplification gain was identical and the 133 to 2382 msec for gadopentetate concentrations up to transmitter gain was within 0.5 dB for each pulse sequence. 1 mM. Five TR values of 50, 67, 83, 100, and 150 msec Additional sets of SPGR images with fixed receiver and were used to measure T1 values ranging from 20 to 85 transmitter gains were produced in which either TR, TE, msec for gadopentetate concentrations from 2 to 10 mM. or flip angle varied incrementally. For each gadopentetate concentration and TR value, the signal amplitude was defined as the average value within a region of interest cursor placed over the appropriate cen­ Pilot Clinical Studies trifuge tube. Measured signal amplitudes as a function of time (TR) were fit to the rising exponential Multisection clinical SE and SPGR images were obtained in three patients who were participating in a phase Ill clinical S = N(T2) * (1 - (1 + FR) * exp[- TR/T1]) trial investigating the effectiveness of standard versus high to solve for pseudo-spin density (N(T2)), flip angle doses of a new magnetic resonance (MR) contrast agent, 0 0 (a ;FR = -cos(a )), and longitudinal relaxation time (T1). gadoteridol (ProHance, Squibb, Princeton, NJ). Images This iterative three-parameter curve-fitting program was were obtained before and after the intravenous injection of provided with General Electric software available on the graded doses of 0.1 and 0.2 mmol/kg that yielded a operator's console. cumulative dose of 0.3 mmol/kg. SPGR scans were ob- AJNR: 15, January 1994 SPOILED GRADIENT-ECHO ENHANCEMENT 29 Predicted SE & SPGR Signal vs. [Gd-DTPA] s Measured SE & SPGR Signal vs. [Gd-DTPA] 2000 0. 8 ·;:~ = SE 600/ ll / l/90° ~ 0. 6 SPGR 600/ ll /1/90° ] ------------~ ~ --- SE 600/11/1190' 1000 -··-·-• ·--- SPGR 600/1111190' <II 0. 4 SPGR 35/5/1/45° ~ SPGR 35/5/1/45' 0. E 0. 2 < 10 12 10[ Gd l (ml1) 2 6 8 Gd-DTPA Concentration (mM) A 8 Fig. 1. A, Predicted signal amplitudes, S (arbitrary units), versus Gd-DTPA concentration (Gd) (mM), for the SE and SPGR pulse sequences as per equations 1 through 3.
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