A comparison of MR-based attenuation correction in PET versus SPECT.

Attenuation correction (AC) is a critical step in the reconstruction of quantitatively accurate positron emission tomography (PET) and single photon emission computed tomography (SPECT) images. Several groups have proposed magnetic resonance (MR)-based AC algorithms for application in hybrid PET/MR systems. However, none of these approaches have been tested on SPECT data. Since SPECT/MR systems are under active development, it is important to ascertain whether MR-based AC algorithms validated for PET can be applied to SPECT. To investigate this issue, two imaging experiments were performed: one with an anthropomorphic chest phantom and one with two groups of canines. Both groups of canines were imaged from neck to abdomen, one with PET/CT and MR (n = 4) and the other with SPECT/CT and MR (n = 4), while the phantom was imaged with all modalities. The quality of the nuclear medicine reconstructions using MR-based attenuation maps was compared between PET and SPECT on global and local scales. In addition, the sensitivity of these reconstructions to variations in the attenuation map was ascertained. On both scales, it was found that the SPECT reconstructions were of higher fidelity than the PET reconstructions. Further, they were less sensitive to changes to the MR-based attenuation map. Thus, MR-based AC algorithms that have been designed for PET/MR can be expected to demonstrate improved performance when used for SPECT/MR.

Effects of acute hypoxia on postural and kinetic tremor.

Human physiological tremor is a complex phenomenon that is modulated by numerous mechanical, neurophysiological, and environmental conditions. Researchers investigating tremor have suggested that acute hypoxia increases tremor amplitude. Based on the results of prior studies, we hypothesized that human participants exposed to a simulated altitude of 4,500 m would display an increased tremor amplitude within the 6-12 Hz frequency range. Postural and kinetic tremors were recorded with a laser system in 23 healthy male participants before, during, and after 1 h of altitude-induced hypoxia. A large panel of tremor characteristics was used to investigate the effect of hypoxia. Acute hypoxia increased tremor frequency content between 6 and 12 Hz during both postural and kinetic tremor tasks (P < 0.05, F = 6.142, Eta(2) = 0.24 and P < 0.05, F = 3.767 Eta(2) = 0.14, respectively). Although the physiological mechanisms underlying the observed changes in tremor are not completely elucidated yet, this study confirms that acute hypoxia increases tremor frequency in the 6-12 Hz range. Furthermore, this study indicates that changes in physiological tremor can be detected at lower hypoxemic levels than previously reported (blood saturation in oxygen = 80.9%). The effects of hypoxia mainly result from a cascade of events starting with the activation of the hypothalamic-pituitary-adrenal axis causing in turn an increase in catecholamine release, leading to an augmentation of tremor amplitude in the 6- to 12-Hz interval and heart rate increase.

Evaluation of geometric sensitivity for hybrid PET.

UNLABELLED: Hybrid PET systems have spatially varying sensitivity profiles. These profiles are dependent on imaging parameters, namely, number of heads, head configuration, spacing between gantry stops, radius of rotation (RoR), and coincident head acceptance angle. METHODS: Sensitivity profiles were calculated across a 500-mm field of view (FoV) for a representative set of existing and theoretic 2-, 3-, and 4-head hybrid PET systems. The head configuration was defined by alpha(n), which describes the angular separation between head 1 and head n. Simulated configurations were 2 head ([alpha(2)] = [180 degrees ]), 3 head ([alpha(2), alpha(3)] = [120 degrees, 240 degrees ] and [90 degrees, 180 degrees ]), and 4 head ([alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ]). Four transverse acceptance angles, measured from the perpendicular of the crystal to the surface, were simulated: 90 degrees, 45 degrees, 23 degrees, and 11 degrees. Two RoRs were considered: 250 and 300 mm. Each head was rotated through 360 degrees in 128 steps, and no physical collimation was modeled. RESULTS: For a 250-mm RoR and 90 degrees acceptance angle, the sensitivities relative to [alpha(2)] = [180 degrees ] were [alpha(2), alpha(3)] = [120 degrees, 240 degrees ], 183%; [alpha(2), alpha(3)] = [90 degrees, 180 degrees ], 159%; and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ], 317%. Increasing RoR to 300 mm decreased [alpha(2)] = [180 degrees ] sensitivity by approximately 12%; all other configurations were decreased by approximately 75% of their 250-mm RoR sensitivities. Decreasing the acceptance angle to 45 degrees decreased sensitivities to [alpha(2), alpha(3)] = [120 degrees, 240 degrees ], 100%; [alpha(2), alpha(3)] = [90 degrees, 180 degrees ], 105%; and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ], 210%. The 2-head [alpha(2)] = [180 degrees ] system sensitivity was not affected. The configuration was the most important factor affecting the shape of the sensitivity profiles. For a 250-mm RoR and 90 degrees acceptance angle, [alpha(2)] = [180 degrees ] concentrated sensitivity in the FoV center, [alpha(2), alpha(3)] = [120 degrees, 240 degrees ] had a slightly increased peripheral sensitivity, and the profiles for both [alpha(2), alpha(3)] = [90 degrees, 180 degrees ] and [alpha(2), alpha(3), alpha(4)] = [90 degrees, 180 degrees, 270 degrees ] were completely flat. CONCLUSION: Sensitivity profiles are affected strongly by imaging parameters; however, profiles can be shaped to concentrate on an annulus or distribute sensitivity uniformly over the FoV. Also, the 4-head system showed a markedly higher sensitivity than either of the 3-head systems.

Scatter and attenuation correction for brain SPECT using attenuation distributions inferred from a head atlas.

UNLABELLED: Sequential transmission scanning (TS)/SPECT is impractical for neurologically impaired patients who are unable to keep their heads motionless for the extended duration of the combined scans. To provide an alternative to TS, we have developed a method of inferring-attenuation distributions (IADs), from SPECT data, using a head atlas and a registration program. The validity of replacing TS with IAD was tested in 10 patients with mild dementia. METHODS: TS was conducted with each patient using a collimated 99mTc line source and fanbeam collimator; this was followed by hexamethyl propyleneamine oxime-SPECT. IAD was derived by deformably registering the brain component of a digital head atlas to a preliminary SPECT reconstruction and then applying the resulting spatial transformation to the full head atlas. SPECT data were reconstructed with scatter and attenuation correction. Relative regional cerebral blood flow was quantified in 12 threshold-guided anatomic regions of interest, with cerebellar normalization. SPECT reconstructions using IAD were compared with those using TS (which is the "gold standard") in terms of these regions of interest. RESULTS: When we compared all regions of interest across the population, the correlation between IAD-guided and TS-guided SPECT scans was 0.92 (P < 0.0001), whereas the mean absolute difference between the scans was 7.5%. On average, IAD resulted in slight underestimation of relative regional cerebral blood flow; however, this underestimation was statistically significant for only the left frontal and left central sulcus regions (P = 0.001 and 0.002, respectively). Error analysis indicated that approximately 10.0% of the total error was caused by IAD scatter correction, 36.6% was caused by IAD attenuation correction, 27.0% was caused by discrepancies in region-of-interest demarcation from quantitative errors in IAD-guided reconstructions, and 26.5% was caused by patient motion throughout the imaging procedure. CONCLUSION: SPECT reconstructions guided by IAD are sufficiently accurate to identify regional cerebral blood flow deficits of 10%, which are typical in moderate and advanced dementia.

The relative contributions of scatter and attenuation corrections toward improved brain SPECT quantification.

Mounting evidence indicates that scatter and attenuation are major confounds to objective diagnosis of brain disease by quantitative SPECT. There is considerable debate, however, as to the relative importance of scatter correction (SC) and attenuation correction (AC), and how they should be implemented. The efficacy of SC and AC for 99mTc brain SPECT was evaluated using a two-compartment fully tissue-equivalent anthropomorphic head phantom. Four correction schemes were implemented: uniform broad-beam AC, non-uniform broad-beam AC, uniform SC + AC, and non-uniform SC + AC. SC was based on non-stationary deconvolution scatter subtraction, modified to incorporate a priori knowledge of either the head contour (uniform SC) or transmission map (non-uniform SC). The quantitative accuracy of the correction schemes was evaluated in terms of contrast recovery, relative quantification (cortical:cerebellar activity), uniformity ((coefficient of variation of 230 macro-voxels) x 100%), and bias (relative to a calibration scan). Our results were: uniform broad-beam (mu = 0.12 cm(-1)) AC (the most popular correction): 71% contrast recovery, 112% relative quantification, 7.0% uniformity, +23% bias. Non-uniform broad-beam (soft tissue mu = 0.12 cm(-1)) AC: 73%, 114%, 6.0%, +21%, respectively. Uniform SC + AC: 90%, 99%, 4.9%, +12%, respectively. Non-uniform SC + AC: 93%, 101%, 4.0%, +10%, respectively. SC and AC achieved the best quantification; however, non-uniform corrections produce only small improvements over their uniform counterparts. SC + AC was found to be superior to AC; this advantage is distinct and consistent across all four quantification indices.

Importance of bone attenuation in brain SPECT quantification.

UNLABELLED: The purpose of this study was to determine the effects of nonuniform attenuation on relative quantification in brain SPECT and to compare the ability of the Chang and Sorenson uniform attenuation corrections (UACs) to achieve volumetric relative quantification. METHODS: Three head phantoms (dry human skull, Rando and Radiology Support Devices (RSD) phantoms) were compared with a human head using a gamma camera transmission CT (gammaTCT) SPECT system and x-ray CT. Subsequently, the RSD phantom's brain reservoir was filled with a uniform water solution of 99mTc, and SPECT and gammaTCT data were acquired using fanbeam collimation. The attenuating effects of bone, scalp and head-holder in individual projections were determined by an analytical projection technique using the SPECT and gammaTCT reconstructions. The Chang UAC used brain and head contours that were segmented from the gammaTCT reconstruction to demarcate its attenuation map, whereas the Sorenson UAC fit slice-specific ellipses to the SPECT projection data. For each UAC, volumetric relative quantification was measured with varying attenuation coefficients (mus) of the attenuation map. RESULTS: Gamma camera transmission CT and x-ray CT scans showed that the dry skull and Rando phantoms suffered from a dried trabecular bone compartment. The RSD phantom most closely reproduced the attenuation coefficients of the human gammaTCT and x-ray CT scans. The analytical projections showed that the attenuating effects of bone, scalp and head-holder were nonuniform across the projections and accounted for 18%-37% of the total count loss. Volumetric relative quantification was best achieved with the Chang (zero iterations) attenuation correction using the head contour and mu = 0.075 cm(-1); however, cortical activity was found to be 10% higher than cerebellar activity. For all UACs, the optimal choices of mu were experimentally found to be lower than the recommended 0.12 cm(-1) for brain tissue. This result is theoretically supported here. CONCLUSION: The magnitude of errors resulting from uniform attenuation corrections can be greater than the magnitudes of regional cerebral blood flow deficits in patients with dementia, as compared with normal controls. This suggests that nonuniform attenuation correction in brain SPECT imaging must be applied to accurately estimate regional cerebral blood flow.