Bo Geijer

Assessment of regional cerebral blood flow by dynamic susceptibility contrast MRI using different deconvolution techniques

Regional cerebral blood flow (rCBF) was assessed using dynamic susceptibility‐contrast MRI at 1.5 T. A simultaneous dual FLASH pulse sequence and Gd‐DTPA‐BMA (0.3 mmol/kg b.w.) were used for examination of 43 volunteers, measuring rCBF in frontal white matter (WM) and in gray matter in the thalamus (GM). Arterial input functions (AIFs) were registered 1) in the carotid artery and 2) in an artery within the GM/WM slice. The measured concentration‐vs.‐time curve was deconvolved with the AIF using both Fourier Transform (FT) and Singular Value Decomposition (SVD). Relative rCBF was given by the height of the deconvolved response curve. For each volunteer, eight different rCBF maps were calculated, representing different combinations of deconvolution techniques, AIFs, and filters. The average GM–WM rCBF ratios ranged from 2.0–2.2, depending on methodology. Absolute rCBF was 68 ± 28 ml/(min 100 g) in GM and 35 ± 13 ml/(min 100g) in WM (mean ± SD, n = 39). GM–WM rCBF ratios obtained using SVD were 6–10% higher than corresponding ratios obtained using FT. Magn Reson Med 43:691–700, 2000.

A ring-enhancing metastasis with central high signal on diffusion-weighted imaging and low apparent diffusion coefficients

Abstract Diffusion-weighted imaging (DWI) has been reported to be useful in the differential diagnosis between abscesses and cystic or necrotic tumours. However, experience is still limited and the true sensitivity and specificity remain to be determined. Our purpose is to describe a ring-enhancing metastasis of adenocarcinoma with a DWI pattern similar to that reported for abscesses. The tumour had a diameter of 1.5 cm and give signal from its centre similar to that of normal brain on T1-weighted images, whereas it was increased on T2-weighted images, and surrounded by a low signal ring, suggesting a capsule. The signal was high on DWI and the apparent diffusion coefficient (ADC) was low (0.55 × 10–3 mm2/s). The findings were misinterpreted as representing an abscess in the early capsule-formation stage, but the signal pattern probably represented early tumour necrosis with intracellular oedema, but without liquefaction. Findings on DWI during the early capsule formation stage in abscesses and early tumour necrosis are probably similar and must be interpreted with caution.

Diffusion-Weighted MRI Findings in Patients with Capsular Warning Syndrome

Background and Purpose: The ‘capsular warning syndrome’ (CWS) of recurrent stereotyped episodes of motor or sensory dysfunction is clinically well recognized, and is associated with a high risk of imminent lacunar infarction with permanent deficits resembling those of CWS. However, the pathophysiology of CWS has not been well characterized. We report a clinicoanatomic correlation with MR imaging studies in the acute and chronic phases in patients with CWS. Material and Methods: Between April 1997 and March 2001, we prospectively studied 8 patients, mean age 73.3 years, presenting with 4–17 motor or sensorimotor transient ischemic attacks (TIAs; duration 2–90 min) up to 3 days after onset of the first episode. Four patients were free of symptoms between the attacks and had no residua, whereas 4 patients developed a pure motor or sensorimotor stroke within 1–3 days after symptom onset. Diffusion-weighted echoplanar MRI (DWI) and T2-weighted MRI studies were performed within 1 week after symptom onset and were repeated 1–2 months later. Results: Seven of the 8 patients had an appropriate lesion on DWI in the acute phase. DWI abnormalities in the 3 patients with TIAs were 4–10 mm in diameter and confined to the lateral thalamus or medial globus pallidus without involving the internal capsule, whereas 4 patients who developed a stroke had abnormalities localized to the putamen extending to corona radiata (3 patients), or the pontomesencephalic junction (1 patient). All 6 patients who underwent follow-up MRI had an infarct on T2-weighted images corresponding to, but usually smaller than, the acute phase DWI abnormality. Conclusions: Small infarcts in the basal ganglia or the pons, close to central motor pathways, appear to be the primary lesion in CWS. The pathophysiology of CWS is complex, and may involve hemodynamic mechanisms in penetrating arterial territories, as well as molecular mechanisms, such as peri-infarct depolarizations affecting adjacent motor pathways.

Clinical lacunar syndromes as predictors of lacunar infarcts. A comparison of acute clinical lacunar syndromes and findings on diffusion-weighted MRI

Objectives– To evaluate if patients with acute lacunar syndromes have acute lacunar infarcts or other types of cerebral lesions on diffusion‐weighted MRI. Methods– Patients with acute lacunar syndromes underwent echo‐planar diffusion MRI of the brain within 3 days after stroke onset. Localization and size of lesions with hyperintense signal were determined, compared with clinical characteristics and with findings on follow‐up T2‐weighted MRI. Results– Twenty‐three patients participated in the study. Thirteen patients had pure motor stroke, 1 pure sensory stroke, 8 sensorimotor stroke, and 1 ataxic hemiparesis. Twenty‐two patients had at least one lesion with increased signal on diffusion‐weighted MR images. These acute lesions were in the internal capsule/basal ganglia/thalamus in 13 patients, subcortical white matter in 5 patients, brainstem in 2 patients, cortex (multiple small lesions) in 1 patient, and cortex+basal ganglia in 1 patient. The median volume of the lesions was 0.6 ml on the initial examination and on follow‐up, of 17 patients after 1 to 5 months, 0.5 ml. Conclusions– Almost all patients with acute ischemic lacunar syndromes have acute lesions on echo‐planar diffusion‐weighted MRI within 3 days after stroke onset. These lesions are mostly small and subcortical, compatible with lacunar infarcts caused by single penetrating artery occlusion, but in a minor proportion of patients (2 of 23 in our study) a cortical involvement is found.

TRIGGERING IN QUANTITATIVE DIFFUSION IMAGING WITH SINGLE-SHOT EPI

Purpose: the aim of this study was to evaluate any possible effects of brain motion, CSF pulsations and other possible sources of physiological motion in electrocardiographic (ECG) triggered and non-triggered single-shot echo-planar imaging (EPI) measurements of diffusion. Material and Methods: Three different triggering protocols were evaluated in 6 healthy volunteers: 1) ECG triggering with time delay (TD) 100 ms; 2) ECG triggering with TD 400 ms; and 3) no triggering at all. Results: the results obtained showed that white matter mean apparent diffusion coefficient (ADC) values were similar for all triggering protocols and that the reproducibility in ADC measurements using diffusion-weighted (DW) EPI was good. the average standard deviation of the ADC values was, however, higher for ADC maps obtained without ECG triggering. Conclusion: the use of prospective diastolic ECG triggering significantly improves the accuracy of quantitative diffusion measurements but for routine clinical diffusion imaging, where quantitative data is of less importance, the accuracy obtained without ECG triggering can be considered adequate.

Absolute cerebral blood flow measured by dynamic susceptibility contrast MRI: a direct comparison with Xe-133 SPECT

Absolute regional cerebral blood flow (CBF) was measured in ten healthy volunteers, using both dynamic susceptibility-contrast (DSC) magnetic resonance imaging (MRI) and Xe-133 SPECT within-4 h. After i.v. injection of Gd-DTPA-BMA (0.3 mmol/kg b.w.), the bolus was monitored with a Simultaneous Dual FLASH pulse sequence (1.5 s image), providing one slice through brain tissue and a second slice through the carotid artery. ConcentrationC(t)x − (1 TE) ln[S(t)/S(0)] was related to CBF asC(t)=CBF [AIF(t)⊗R(t)], where AIF is the arterial input function andR(t) is the residue function. A singular-value-decomposition-based deconvolution technique was used for retrieval ofR(t). Absolute CBF was given by Zierler’s area-to-height relation and the central volume principle. For elimination of large vessels (ELV), all MRI-based CBF values exceeding 2.5 times the mean CBF value of the slice were excluded. A correction for partial-volume effects (CPVE) in the artery used for AIF monitoring was based on registration of signal in a phantom with tubes of various diameters (1.5–6.5 mm), providing an individual concentration correction factor applied to AIF data registered in vivo. In the Xe-133 SPECT investigation, 3000–4000 MBq of Xe-133 was administered intravenously, and CBF was calculated using the Kanno-Lassen algorithm. When ELV and CPVE were applied. DSC-MRI showed average CBF values from the entire slice of 43±10 ml/(min 100 g) (small-artery AIF) and 48±17 ml (min 100 g) (carotid-artery AIF) (mean±S.D.,n=10). The corresponding Xe-133-SPECT-based CBF was 33±6 ml (min 100 g) (n=10). The relationships of CBF(MRI) versus CBF(SPECT) showed good linear correlation (r=0.74–0.83).

Regional cerebral blood flow distributions in normal volunteers: dynamic susceptibility contrast MRI compared with 99mTc-HMPAO SPECT

Purpose Relative regional cerebral blood flow (rCBF) at rest was measured in 44 volunteers using both dynamic susceptibility contrast (DSC) MRI and 99mTc-HMPAO SPECT on the same day. Method In MRI, a Gd-DTPA-BMA contrast agent bolus (0.3 mmol/kg body wt) was monitored with a simultaneous dual FLASH pulse sequence (time resolution 1.5 s). MRI-based rCBF images were calculated by singular value decomposition-based deconvolution of the measured tissue concentration-time curve with an arterial input function from a small artery within the imaging slice. In the SPECT investigation, 900 MBq of 99mTc-HMPAO was injected intravenously. Relative rCBF in gray matter in the thalamus and in frontal white matter was determined. Results The ratio of relative rCBF in gray matter to relative rCBF in white matter was 2.21 ± 0.57 using MRI and 2.24 ± 0.54 using SPECT (mean ± SD). Conclusion Relative rCBF maps from DSC MRI and 99mTc-HMPAO SPECT showed good agreement, and the MRI-based rCBF ratio correlated with the corresponding SPECT-based ratio (r = 0.79, p < 0.0000006).

The value of b required to avoid T2 shine-through from old lucunar infarcts in diffusion-weighted imaging

Abstract Multiple small infarcts of different ages are common in small-vessel disease. Diffusion-weighted imaging (DWI) is a powerful method for discriminating new from chronic lesions. This can be done on the diffusion-weighted images provided that b is sufficiently high. Our purpose was to determine that critical value of b. We reviewed DWI from a previous study of acute, mainly lacunar strokes, and selected 18 old lacunar infarcts, well defined on uncoded images with b 0 s/m2 (i. e., T2-weighted images) but invisible on DWI with b 1200 × 106 s/m2. We used a 1.5 tesla imager and single-shot echo-planar technique. We had seven separate acquisitions with echo time 123 ms and b in steps between 0 and 1200 × 106 s/m2. Two neuroradiologists blinded to the selection of lesions carried out two different lesion-detection procedures, thereby testing each lesion four times, giving a total of 72 tests of b values. The results were consistent, indicating a level for detection of 800 × 106 s/m2 in two tests, 400–600 × 106 s/m2 in 65 tests and at lower values in the remainder. For imagers up to 1.5 tesla, at long repetition times and an echo time up to 120 ms T2-shine through of old lacunar infarcts can be avoided using b of 1000 × 106 s/m2.

Persistent high signal on diffusion-weighted MRI in the late stages of small cortical and lacunar ischaemic lesions.

Abstract Diffusion-weighted imaging (DWI) is very sensitive to early brain infarcts. However, the late stages have been insufficiently studied. Infarcts in small vessel disease are often multiple and of different ages, and differentiation between new and old lesions might be difficult. We have therefore studied the change with time in DWI of small (< 3 ml) ischaemic lesions. We imaged 21 patients with an acute lacunar syndrome and a lesion visible on early DWI. They all had three MRI examinations 12–58 h (early), 7–16 and 54–144 days after the onset of stroke; 10 patients with high DWI signal on the third examination had a fourth examination 12–28 months after the stroke. MRI was performed at 1.5 T, using echo-planar DWI with 7 b-values from 0 to 1200 × 106 s/m2 and conventional T2-weighted imaging. After 7–16 days 18 of 21 lesions gave high signal on DWI, and 12/16 measurable lesions had a decreased apparent diffusion coefficient (ADC). After 54–144 days ten lesions still gave high DWI signal and two still had an ADC below normal. On the fourth examination there was no remaining high DWI signal and all ADC were higher than normal.

The perfusion fraction in volunteers and in patients with ischaemic stroke

The fractional volume of capillary blood, i.e. the perfusion fraction f, was measured with the aid of an echo-planar imaging protocol originally designed for the measurement of water diffusion. in healthy volunteers, reasonable f values were obtained. in patients with cerebral ischaemic stroke, a marked decrease in the f value was seen in the infarcted region as compared with corresponding values in the contralateral hemisphere. We suggest that perfusion-fraction measurements may add to the diagnostic value of water-mobility examinations in patients with ischaemic disease