Lesion Contrast-to-noise Ratio Research Articles

Magnetic resonance imaging of hyaline cartilage defects at 1.5T and 3.0T: comparison of medium T2‐weighted fast spin echo, T1‐weighted two‐dimensional and three‐dimensional gradient echo pulse sequences

To evaluate and compare the diagnostic accuracy of appropriate magnetic resonance (MR) sequences in the detection of cartilage lesions at 1.5T and 3.0T. Twelve chondral defects of varying depths, widths, and locations were created in the retropatellar hyaline cartilage in six sheep cadaver limbs. Axial images employing three fat-suppressed imaging sequences--(1) a T2-weighted fast spin-echo (FSE) sequence, (2) a two-dimensional (2D) and (3) three-dimensional (3D) gradient-echo (GE) sequence at 1.5T and 3.0T using an extremity quadrature coil--were evaluated by three experienced radiologists. Statistical analysis of the results consisted of receiver operating characteristics (ROC) and significant testing using the bivariate chi-square test. In addition, signal-to-noise ratios (SNR) and contrast-to-noise ratios (CNR) were evaluated with significance testing using the Wilcoxon test. The 3D GE sequence compared favorably with other sequences at 3.0T and 1.5T (Az=0.88 at 3.0T and Az=0.85 at 1.5T) missing only one small grade 2 lesion. 2D GE imaging was inferior to 3D imaging at both field strengths (P<0.05) in general. However, compared to 1.5T, lesion detectability was improved at the higher magnetic field of 3.0T (Az=0.81 and 0.73 at 3.0T and 1.5T, respectively). FSE images showed significantly inferior sensitivity and less anatomical detail compared to the GE sequences at both field strengths (Az=0.64 and 0.72 at 3.0T and 1.5T, respectively; P<0.05). However, compared to 1.5T, lesion detectability SNR and CNR values were superior in all sequences tested at 3.0T. MRI at 3.0T improves SNR and CNR significantly in the most common sequences for cartilage MRI, resulting in an improvement in chondral lesion detection. GE imaging therefore allows resolution to be increased in an acceptable time manner for patient comfort, and the 3D GE fat-suppressed sequence at 3.0T appears to be best suited for cartilage imaging in a clinical setting.

Acute and Subacute Ischemic Stroke at High-Field-Strength (3.0-T) Diffusion-weighted MR Imaging: Intraindividual Comparative Study

To compare signal-to-noise ratios (SNRs), contrast-to-noise ratios (CNRs), image quality, and confidence in diagnosis between 1.5- and 3.0-T diffusion-weighted (DW) magnetic resonance (MR) imaging of ischemic stroke lesions. The study design was approved by the institutional review board, and all patients gave informed consent. In a prospective intraindividual study, 25 patients who had clinical symptoms consistent with ischemic stroke underwent DW MR imaging at both 1.5 T and 3.0 T. The 3.0- or 1.5-T examination was performed immediately one after the other, in random order. Two readers in consensus recorded the presence and number of ischemic lesions and rated image quality and lesion conspicuity. The image SNR and the CNR of the ischemic lesions were quantified. Paired Student t and Wilcoxon matched-pairs signed rank tests were used to test for statistical significance. Image quality at 3.0-T DW MR imaging was consistently lower than that at 1.5-T DW MR imaging owing to greater image distortions (P < .05). Yet, overall SNR and lesion CNR at 3.0 T increased significantly; mean increases were 48.8% (P < .001) and 96.3% (P < .01), respectively. The higher overall SNR and lesion CNR translated into a significantly higher sensitivity in the detection of ischemic lesions at 3.0 T than at 1.5 T. Of the total of 48 lesions that were identified in 19 of the 25 patients, 47 (98%) were diagnosed at 3.0 T and 36 (75%) were diagnosed at 1.5 T. In addition, the conspicuity of the lesions that were visible with both systems was significantly higher at 3.0 T (P < .001). Although 3.0-T DW MR imaging generates greater image distortions, it yields increased SNR and CNR compared with DW MR imaging at 1.5 T. The increased CNR at 3.0 T translates into a significantly improved diagnostic confidence in the detection of focal apparent diffusion coefficient changes in the setting of subacute and acute ischemic stroke.

Is postcontrast trueFISP imaging advantageous?

Contrast of trueFISP images depends mainly on the T2/T1 ratio. Consequently, there is a potential gain in signal intensity after administration of paramagnetic contrast medium despite the strong T2 weighting. The purpose of this study was to analyze signal intensities of abdominal organs after applying contrast medium and to determine whether this yields an improved contrast for pathologies compared with precontrast trueFISP. Fifty patients underwent an abdominal examination, including the trueFISP sequence before and after the administration of contrast medium. All images were obtained with a 1.5 T system. The mean signal-to-noise ratio before and after contrast medium was assessed for abdominal organs, vessels, muscle, and fat. The contrast-to-noise ratio (CNR) of pathologic lesions was calculated. The trueFISP sequence yielded a higher signal-to-noise ratio after application of contrast medium for all organs except for fat and the aorta. CNR of solid lesions (angiomyolipoma, liver adenoma, liver hemangioma, hepatocellular carcinoma) increased whereas contrast of cysts decreased. TrueFISP imaging after application of contrast medium led to better CNR for many solid lesions while cysts showed a diminished contrast. We advise trueFISP imaging sequences before and after contrast medium application.

Dose and image quality. How does the choice of kilovoltage affect patient dose, scattered radiation, and image quality in CT examinations? Submitted by Glen Thomson, Middlemore Hospital, New Zealand.

Published online: 5 December 2003 Springer-Verlag 2003 How does the choice of KV affect patient dose, scattered radiation, and image quality in CT examinations? When the x-ray tube voltage (kV) in CT is reduced while keeping the mAs constant, the patient dose is decreased. Dropping the x-ray tube voltage from 120 to 80 kV at a constant current (mAs) typically reduces the patient dose by about 60% because of the large reduction in x-ray tube output. The choice of kV dose not affect the amount of ‘‘scatter’’—CT has negligible image scatter since the thin detector (up to 2–3 cm) and the data acquisition geometry ensure that most scattered photons miss the detector. The contrast of a lesion relative to the surrounding background generally increases as kV is reduced, but the noise (mottle) level also increases since there are fewer photons at the lower kV (hence the lower patient dose). The lesion contrast-tonoise ratio (CNR) generally gets worse as the kV is reduced at a constant mAs in CT—the increase in contrast is less than the corresponding increase in noise [1]. If one reduces the kV and increases the mAs, however, it may be possible to maintain image quality (CNR) while reducing patient doses. CT optimization with respect to x-ray tube voltage is being investigated by several researchers, with initial findings suggesting that the use of lower kV values is promising for CT performed using iodinated contrast material [2, 3]. To summarize, reducing the CT kV at a constant mAs will always reduce the patient dose, and would probably reduce the lesion CNR. When iodine is administered to a patient, reducing the kV is likely to reduce patient doses with no adverse effect on CNR.

Accelerated dynamic MR imaging with a parallel imaging technique for hypervascular hepatocellular carcinomas: Usefulness of a test bolus in examination and subtraction imaging

To assess the impact of the accelerated dynamic MR imaging (ADMRI) approach using parallel imaging for detecting hypervascular hepatocellular carcinomas (HCCs) and to evaluate the usefulness of a test bolus in examination and subtraction imaging in this setting. Thirty patients with 135 HCCs underwent ADMRI using a two-dimensional gradient-recalled echo sequence with parallel imaging. Seventeen patients were evaluated without a test bolus and 13 patients with a test bolus. The detectability of HCCs was calculated between the groups with and without a test bolus. ADMRI was evaluated regarding the signal-to-noise ratio (SNR) of the lesion and the liver, the contrast-to-noise ratio (CNR) of the lesion vs. the liver, and the feasibility of subtraction images. ADMRI with and without a test bolus had almost equal sensitivity (92.5% and 92.6%). No significant difference was seen in the SNR of lesions and the CNR of lesions vs. livers between both groups. With a test bolus, ADMRI could depict the peak enhancement of nodules on the 2nd or 3rd dynamic phases and optimized the timing of peak lesion enhancement. Subtraction images could be obtained regarding minimal slice misregistration. ADMRI had high detectability of HCCs with and without a test bolus.

Early Detection of Hyperacute Cerebral Infarction in Dogs: Comparison of Unenhanced CT, Diffusion-weighted,Spin-echo T2 - weighted, and Fast FLAIR MR Imaging

Purpose : This study was performed in order to compare unenhanced CT with diffusion-weighted, T2-weight-ed,and fast FLAIR MR imaging in the detection of hyperacute cerebral ischema induced in a dog and to deter-mine whichmodality first detected cerebral ischemia. Materials and Methods : Experimental cerebral infarction was induced bythe occlusion of intracerebral arter-ies using embolic materials (polyvinyl-alcohol, 300 -6 00 μ) introducedthrough a microcatheter into the internal carotid artery of five dogs weighing 12 -20 kg. Serial CT and MR imageswere obtained at one hour intervals from one to five hours after occlusion, and were analyzed independently by tworadiologists. We assessed changes in attenuation, as seen on unenhanced CT and the signal intensity of the lesionon each MR image, and measured the contrast-to-noise ratio (CNR) of the lesions. Results : Ischemic lesions weredetected on unenhanced CT 1 -3 hours after occlusion of cerebral arteries. In all dogs, the lesions were detectedearliest on diffusion-weighted images obtained at 1 hour. They were detect-ed on T2-weighted images at 3 -5 hoursand on fast FLAIR images of 2 -5 hours. The CNR of ischemic lesions increased gradually during the 5-hour period.It was highest on diffusion-weighted images, while on unen-hanced CT, T2-weighted, and fast FLAIR images it wassimilar. Conclusion : Hyperacute ischemic lesions were detected earliest on diffusion-weighted images, and earlieron unenhanced CT than on fast FLAIR or T2-weighted MR image.

MR Imaging of Brain: Comparison between SS-FSE and Fast Spin-Echo T2-Weighted Pulse Sequences

We compared quantitatively and qualitatively a single shot fast spin-echo (SS-FSE) technique with a T2-weighted fast spin-echo (T2-FSE) technique and to determine whether SS-FSE is a valuable alternative to fast SE (FSE) imaging. A total of 41 consecutive patients with brain lesions were retrospectively evaluated. Together with quantitative analysis, contrast-to-noise ratios (CNR), signal-to-noise ratios (SNR), and the percent contrast (%C) were measured for the lesion. Images were evaluated by two radiologists who reached consensus. Visibility (detection), margination, and extent of the lesions were rated separately, according to a five-point scale. The mean lesion CNR was significantly higher on SS-FSE sequences than on fast SE sequences (p=0.0131). The mean lesion %C was also higher on SS-FSE than on T2-FSE sequence (p=0.0222). However, the mean lesion SNR was significantly higher on T2-FSE sequence than on SSFSE sequence (p?0.0222). No significant differences were found in the visibility (detectability) of the lesions between two pulse sequences (p=0.1323). The SS-FSE technique is quantitatively superior to T2-FSE for obtaining T2-weighted images of the brain. In comparison with T2-FSE, SS-FSE provides comparable T2-weighted images with sufficient contrast in the evaluation of brain lesions. Our findings suggest that SSFSE may be useful for ill, uncooperative patients, especially for the children patients requiring sedation.