Thirty years from now: future physics contributions in nuclear medicine

Background

This paper is the first in a series of invited perspectives by pioneers of nuclear medicine imaging and physics. A medical physicist and a nuclear medicine physician each take a backward and a forward look at the contributions of physics to nuclear medicine. Here, we provide a forward look from the medical physicist’s perspective.

Discussion

The author examines a number of developments in nuclear medicine and discusses the ways in which physics has contributed to these. Future developments are postulated in the context of an increasingly personalised approach to medical diagnostics and therapies.

Conclusions

A skill set for the next generation of medical physicists in nuclear medicine is proposed in the context of the increasing complexity of ‘Molecular Imaging’ in the next three decades. The author sees a shift away from ‘traditional’ roles in instrumentation QA to more innovative approaches in understanding radiobiology and human disease.

Summary report of the First International Workshop on PET/MR imaging, March 19-23, 2012, Tübingen, Germany

We report from the First International Workshop on positron emission tomography/magnetic resonance imaging (PET/MRI) that was organized by the University of Tübingen in March 2012. Approximately 100 imaging experts in MRI, PET and PET/computed tomography (CT), among them early adopters of pre-clinical and clinical PET/MRI technology, gathered from March 19 to 24, 2012 in Tübingen, Germany. The objective of the workshop was to provide a forum for sharing first-hand methodological and clinical know-how and to assess the potential of combined PET/MRI in various applications from pre-clinical research to scientific as well as clinical applications in humans. The workshop was comprised of pro-active sessions including tutorials, specific discussion panels and grand rounds. Pre-selected experts moderated the sessions, and feedback from the subsequent discussions is presented here to a greater readership. Naturally, the summaries provided herein are subjective descriptions of the hopes and challenges of PET/MR imaging as seen by the workshop attendees at a very early point in time of adopting PET/MRI technology and, as such, represent only a snapshot of current approaches.

Positron emission tomography (PET) for assessing aerosol deposition of orally inhaled drug products

The topical distribution of inhaled therapies in the lung can be viewed using radionuclides and imaging. Positron emission tomography (PET) is a three-dimensional functional imaging technique providing quantitatively accurate localization of the quantity and distribution of an inhaled or injected PET radiotracer in the lung. A series of transaxial slices through the lungs are obtained, comparable to an X-ray computed tomography (CT) scan. Subsequent reformatting allows coronal and sagittal images of the distribution of radioactivity to be viewed. This article describes procedures for administering [(18)F]-fluorodeoxyglucose aerosol to human subjects for the purpose of determining dose and distribution following inhalation from an aerosol drug delivery device (ADDD). The advantages of using direct-labeled PET drugs in the ADDD are discussed with reference to the literature. The methods for designing the inhalation system, determining proper radiation shielding, calibration, and validation of administered radioactivity, scanner setup, and data handling procedures are described. Obtaining an X-ray CT or radionuclide transmission scan to provide accurate geometry of the lung and also correct for tissue attenuation of the PET radiotracer is discussed. Protocols for producing accurate images, including factors that need to be incorporated into the data calibration, are described, as well as a proposed standard method for partitioning the lung into regions of interest. Alternate methods are described for more detailed assessments. Radiation dosimetry/risk calculations for the procedures are appended, as well as a sample data collection form and spreadsheet for calculations. This article should provide guidance for those interested in using PET to determine quantity and distribution of inhaled therapeutics.

Pulmonary hypertension leads to a loss of gravity dependent redistribution of regional lung perfusion: a SPECT/CT study

OBJECTIVE

Pre-capillary pulmonary hypertension (PHT) is characterised by progressive pulmonary vascular obliteration and loss of vascular reserves. In health, regional lung perfusion redistributes under the influence of gravity due to the presence of recruitable vessels. We investigated a combined single photon emission computed tomography/CT (SPECT/CT) method for assessing the pulmonary circulation by quantifying the gravity dependent redistribution of lung perfusion.

DESIGN

Characterisation of patients versus healthy controls.

PATIENTS

15 patients with pre-capillary PHT and 11 healthy controls.

SETTING

University hospital clinic.

INTERVENTION

Regional lung perfusion was measured using SPECT/CT in two different postures (supine vs upright). A perfusion redistribution index (PRI) was used to quantify the cranial-caudal shift in regional lung perfusion resulting from gravitational (postural) change.

MAIN OUTCOME MEASURE

PRI was compared between cases and controls, and correlated with markers of disease severity in cases.

RESULTS

Patients with pre-capillary PHT had notably reduced PRI compared to controls (0.02±0.06 vs. 0.28±0.15 normalised perfusion/cm, p<0.0001). PRI was significantly associated with prognostic parameters such as 6 min walk distance (r=0.60, p=0.018), functional class (p=0.008), and tricuspid annular plane systolic excursion (r=0.58, p=0.022). The receiver operating characteristic curve showed that PRI differentiated patients with pre-capillary PHT from controls with AUC=0.94 (p<0.001).

CONCLUSIONS

With SPECT/CT, gravity dependent redistribution of lung perfusion can be quantified using the PRI derived from supine and upright perfusion analysis. The potential utility of PRI for the non-invasive detection of PHT and assessment of disease severity warrants further study.

Quantitative SPECT/CT: SPECT joins PET as a quantitative imaging modality

The introduction of combined modality single photon emission computed tomography (SPECT)/CT cameras has revived interest in quantitative SPECT. Schemes to mitigate the deleterious effects of photon attenuation and scattering in SPECT imaging have been developed over the last 30 years but have been held back by lack of ready access to data concerning the density of the body and photon transport, which we see as key to producing quantitative data. With X-ray CT data now routinely available, validations of techniques to produce quantitative SPECT reconstructions have been undertaken. While still suffering from inferior spatial resolution and sensitivity compared to positron emission tomography (PET) imaging, SPECT scans nevertheless can be produced that are as quantitative as PET scans. Routine corrections are applied for photon attenuation and scattering, resolution recovery, instrumental dead time, radioactive decay and cross-calibration to produce SPECT images in units of kBq.ml(-1). Though clinical applications of quantitative SPECT imaging are lacking due to the previous non-availability of accurately calibrated SPECT reconstructions, these are beginning to emerge as the community and industry focus on producing SPECT/CT systems that are intrinsically quantitative.

V/Q scanning using SPECT and SPECT/CT

Planar ventilation-perfusion (V/Q) scanning is often used to investigate pulmonary embolism; however, it has well-recognized limitations. SPECT overcomes many of these through its ability to generate 3-dimensional imaging data. V/Q SPECT has higher sensitivity, specificity, and accuracy than planar imaging and a lower indeterminate rate. SPECT allows for new ways to display and analyze data, such as parametric V/Q ratio images. Compared with CT pulmonary angiography, SPECT has higher sensitivity, a lower radiation dose, fewer technically suboptimal studies, and no contrast-related complications. Any nuclear medicine department equipped with a modern hybrid scanner can now perform combined V/Q SPECT with CT (using low-dose protocols) to further enhance diagnostic accuracy. V/Q SPECT (with or without CT) has application in other pulmonary conditions and in research.

Determination of evidence-based diagnostic thresholds for upper limb lymphedema secondary to treatment for cancer

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Background: Diagnosis of upper limb lymphedema (LE) secondary to treatment for cancer has been complicated by the range of diagnostic thresholds used. Traditional thresholds were arbitrarily chosen but recently normative-based cut-offs have been established. However, the diagnostic power of these thresholds has not been compared to changes identified with lymphoscintigraphy, the gold standard for diagnosis of LE. The aim of this study was to determine thresholds for commonly used clinical diagnostic tools based on lymphatic imaging outcomes. Methods: Women previously diagnosed with LE secondary to treatment for cancer (n=67), and women without LE (n=20) participated. Lymphoscintigraphy was completed and the presence and severity of dermal back flow qualitatively scored as none to mild or moderate to severe by an experienced nuclear medicine physician. On the same day, circumference measurements and segmental bioimpedance spectroscopy (BIS) for 10 cm intervals from the ulnar styloid to 40 cm proximal were recorded. The BIS inter-limb ratio for each segment and the inter-limb circumference differences were compared to diagnostic thresholds based on 2 and 3 standard deviations (SD) above the mean from normative data. Results: The number of BIS segments and circumferences measurements above a 3SD threshold correlated significantly with the dermal backflow score (Rs= 0.710 and 0.824 respectively).The number of abnormal BIS segments detected did not differ significantly between a 2SD and 3SD threshold (χ2=0.23 p=0.63). Of the 20 new abnormal segments detected, 7 were from participants with no or mild evidence of dermal backflow on lymphoscintigraphy including 3 from control participants. In contrast, 46 new abnormal inter-limb circumference differences, all from those with a LE diagnosis, were detected using a 2SD threshold (χ2= 31.785, p<0.001) Conclusions: The need for a standardized, evidenced-based approach for identification of LE is essential. We recommend diagnostic thresholds for segmental BIS be set at 3SD above the mean to minimize false positive diagnoses whereas a lower threshold of 2SD is necessary for the less sensitive inter-limb circumference difference measurements.

An evidence-based review of quantitative SPECT imaging and potential clinical applications

SPECT has traditionally been regarded as nonquantitative. Advances in multimodality γ-cameras (SPECT/CT), algorithms for image reconstruction, and sophisticated compensation techniques to correct for photon attenuation and scattering have, however, now made quantitative SPECT viable in a manner similar to quantitative PET (i.e., kBq cm(-3), standardized uptake value). This review examines the evidence for quantitative SPECT and demonstrates clinical studies in which the accuracy of the reconstructed SPECT data has been assessed in vivo. SPECT reconstructions using CT-based compensation corrections readily achieve accuracy for (99m)Tc to within ± 10% of the known concentration of the radiotracer in vivo. Quantification with other radionuclides is also being introduced. SPECT continues to suffer from poorer photon detection efficiency (sensitivity) and spatial resolution than PET; however, it has the benefit in some situations of longer radionuclide half-lives, which may better suit the biologic process under examination, as well as the ability to perform multitracer studies using pulse height spectroscopy to separate different radiolabels.

Translation of Methodology Used In Human Myocardial Imaging to a Sheep Model of Acute Myocardial Infarction

INTRODUCTION

Pre-clinical investigation of stem cells for repairing damaged myocardium predominantly uses rodents, however large animals have cardiac circulation closely resembling the human heart. The aim of this study was to evaluate whether SPECT/CT myocardial perfusion imaging (MPI) could be used for assessing sheep myocardium following an acute myocardial infarction (MI) and response to intervention.

METHODS

Eighteen sheep were enrolled in a pilot study to evaluate [(99m)Tc]-sestamibi MPI at baseline, post-MI and after therapy. Modifications to the standard MPI protocols were developed. All data was reconstructed with OSEM using CT-derived attenuation and scatter correction. Standard analyses were performed and inter-observer agreement was measured using Kappa (κ). Power determined the sample sizes needed to show statistically significant changes due to intervention.

RESULTS

Ten sheep completed the full protocol. Data processed was performed with pre-existing hardware and software used in human MPI scanning. No improvement in perfusion was seen in the control group, however improvements of 15%-35% were seen after intra-myocardial stem cell administration. Inter-observer agreement was excellent (К=0.89). Using a target power of 0.9, 28 sheep were required to detect a 10-12% change in perfusion.

CONCLUSION

This study demonstrates the suitability of large animal models for imaging with standard MPI protocols and its feasibility with a manageable number of animals. These protocols could be translated into humans to study the efficacy of stem cell therapy in heart regeneration and repair.

Risk factors of hypomineralised second primary molars in a group of Iraqi schoolchildren

AIM

To investigate risk factor/s involved in the development of hypomineralised second primary molars and to relate the location of the affected tooth in the dental arches with the timing of the illness/condition incidence.

STUDY DESIGN

A cluster sample of 1,000, Iraqi 7-9 year-old children were invited to have their second primary molars examined for demarcated hypomineralised lesions.

METHODS

Mothers of 823 children completed a questionnaire-based interview regarding pregnancy and childhood systemic health history. In the clinical examination, the buccal, occlusal and lingual/palatal surfaces of the second primary molar were evaluated for demarcated hypomineralisation lesions by visual examination.

RESULTS

A response rate of 82.3% was obtained. Of the children examined, 53 (6.6%) had hypomineralisation defects in at least one second primary molar and were considered as the hypomineralised second primary molar-affected group. Of the total affected teeth (n=83), maxillary molars were the teeth most frequently affected by hypomineralisation throughout all developmental stages (69.9%). Demarcated opacities were the most prevalent lesion type (71.0%). Ninety-four percent of subjects diagnosed with demarcated defects reported various medical conditions possibly associated with hypomineralisation compared with 44% for their non-affected counterparts. Peri-natal medical conditions (45.3%) were the most frequently reported followed by pre-natal and post-natal conditions (24.5%, 9.4%; respectively).

STATISTICS

Ill health during pregnancy, delivery complications, neonatal complications, acute childhood illness, birth weight and duration of breast feeding were significant potential risk factors (p<0.05). The greater the number of health events reported, the higher was the chance of developing the defect. Children who experienced neonatal complications and whose mothers reported pregnancy and birth problems were approximately six times more likely to have the defect than those whose mothers had delivery complications only (80% vs 14.6%) (p<0.001). Also of those children whose mothers did not report delivery complications, but were breastfed for less than six months, of low birth weight and had history of upper respiratory tract infection, the chance of hypomineralised defects was over four times more likely to happen than in those who did not suffer any of these problems (25.8% vs 6.7%) (p<0.01). No statistically significant association was revealed between the time of the illness/condition occurrence and the location of the tooth in the dental arches.

CONCLUSIONS

Children with hypomineralised second primary molars had experienced more medical conditions than their unaffected peers particularly during the peri-natal period. No single factor was identified as a potential cause, leaving the aetiology of the defect unclear.

Airway closure on imaging relates to airway hyperresponsiveness and peripheral airway disease in asthma

The regional pattern and extent of airway closure measured by three-dimensional ventilation imaging may relate to airway hyperresponsiveness (AHR) and peripheral airways disease in asthmatic subjects. We hypothesized that asthmatic airways are predisposed to closure during bronchoconstriction in the presence of ventilation heterogeneity and AHR. Fourteen asthmatic subjects (6 women) underwent combined ventilation single photon emission computed tomography/computed tomography scans before and after methacholine challenge. Regional airway closure was determined by complete loss of ventilation following methacholine challenge. Peripheral airway disease was measured by multiple-breath nitrogen washout from which S(cond) (index of peripheral conductive airway abnormality) was derived. Relationships between airway closure and lung function were examined by multiple-linear regression. Forced expiratory volume in 1 s was 87.5 ± 15.8% predicted, and seven subjects had AHR. Methacholine challenge decreased forced expiratory volume in 1 s by 23 ± 5% and increased nonventilated volume from 16 ± 4 to 29 ± 13% of computed tomography lung volume. The increase in airway closure measured by nonventilated volume correlated independently with both S(cond) (partial R(2) = 0.22) and with AHR (partial R(2) = 0.38). The extent of airway closure induced by methacholine inhalation in asthmatic subjects is greater with increasing peripheral airways disease, as measured by ventilation heterogeneity, and with worse AHR.

Quantifying lung shunting during planning for radio-embolization

A method is proposed for accurate quantification of lung uptake during shunt studies for liver cancer patients undergoing radio-embolization. The current standard for analysis of [(99m)Tc]-MAA shunt studies is subjective and highly variable. The technique proposed in this work involves a small additional peripheral intravenous injection of macroaggregated albumin (MAA) and two additional static acquisitions (before and after injection) to quantify the absolute activity in the lungs as a result of arterio-venous shunting. Such quantification also allows for estimates of absorbed dose to lung tissue at the time of treatment based on MIRD formalism. The method was used on six radio-embolization patients attending the department for lung shunt analysis. Quantitative values for each were compared to a previously validated technique using fully quantitative SPECT/CT imaging, treated as the gold standard. The average difference between absolute activity shunted to the lungs calculated by the proposed technique compared to the previously validated technique was found to be 2%, with a range of (1-8)%. The proposed method is simple and fast, allowing for accurate quantification of lung shunting and estimates of absorbed dose to lung tissue at treatment, and may one day be used in a one-stop procedure for planning and therapy in a single interventional procedure.

Controversies in Diagnosis of Pulmonary Embolism

The approach to the diagnosis of acute pulmonary embolism (PE) is under constant revision with advances in technology, noninvasive approaches, and increasing awareness of the risks of ionizing radiation. Optimal approaches in some categories of patients are controversial. Data are insufficient for evidence-based recommendations. Therefore, this survey of investigators in the field was undertaken. Even among experts there were marked differences of opinion regarding the approach to the diagnosis of acute PE. Although CT pulmonary angiography was usually the imaging test of choice, the respondents were keenly aware of the dangers of ionizing radiation. In view of advances in scintigraphic diagnosis since the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) trial, ventilation/perfusion (V/Q) lung scans or perfusion scans alone and single photon emission computed tomography (SPECT) V/Q lung scans are often recommended. The choice depends on the patient’s age, gender, and complexity of the findings on the plain chest radiograph.

Using an external gating signal to estimate noise in PET with an emphasis on tracer avid tumors

The purpose of this study is to establish and validate a methodology for estimating the standard deviation of voxels with large activity concentrations within a PET image using replicate imaging that is immediately available for use in the clinic. To do this, ensembles of voxels in the averaged replicate images were compared to the corresponding ensembles in images derived from summed sinograms. In addition, the replicate imaging noise estimate was compared to a noise estimate based on an ensemble of voxels within a region. To make this comparison two phantoms were used. The first phantom was a seven-chamber phantom constructed of 1 liter plastic bottles. Each chamber of this phantom was filled with a different activity concentration relative to the lowest activity concentration with ratios of 1:1, 1:1, 2:1, 2:1, 4:1, 8:1 and 16:1. The second phantom was a GE Well-Counter phantom. These phantoms were imaged and reconstructed on a GE DSTE PET/CT scanner with 2D and 3D reprojection filtered backprojection (FBP), and with 2D- and 3D-ordered subset expectation maximization (OSEM). A series of tests were applied to the resulting images that showed that the region and replicate imaging methods for estimating standard deviation were equivalent for backprojection reconstructions. Furthermore, the noise properties of the FBP algorithms allowed scaling the replicate estimates of the standard deviation by a factor of 1/square root N, where N is the number of replicate images, to obtain the standard deviation of the full data image. This was not the case for OSEM image reconstruction. Due to nonlinearity of the OSEM algorithm, the noise is shown to be both position and activity concentration dependent in such a way that no simple scaling factor can be used to extrapolate noise as a function of counts. The use of the Well-Counter phantom contributed to the development of a heuristic extrapolation of the noise as a function of radius in FBP. In addition, the signal-to-noise ratio for high uptake objects was confirmed to be higher with backprojection image reconstruction methods. These techniques were applied to several patient data sets acquired in either 2D or 3D mode, with (18)F (FLT and FDG). Images of the standard deviation and signal-to-noise ratios were constructed and the standard deviations of the tumors' uptake were determined. Finally, a radial noise extrapolation relationship deduced in this paper was applied to patient data.

The impact of changing dental needs on cost savings from fluoridation

BACKGROUND

Although community water fluoridation has been one of the cornerstone strategies for the prevention and control of dental caries, questions are still raised regarding its cost-effectiveness. This study assessed the impact of changing dental needs on the cost savings from community water fluoridation in Australia.

METHODS

Net costs were estimated as Costs((programme)) minus Costs((averted caries).) Averted costs were estimated as the product of caries increment in non-fluoridated community, effectiveness of fluoridation and the cost of a carious surface. Modelling considered four age-cohorts: 6-20, 21-45, 46-65 and 66+ years and three time points 1970s, 1980s, and 1990s. Cost of a carious surface was estimated by conventional and complex methods. Real discount rates (4, 7 (base) and 10%) were utilized.

RESULTS

With base-case assumptions, the average annual cost savings/person, using Australian dollars at the 2005 level, ranged from $56.41 (1970s) to $17.75 (1990s) (conventional method) and from $249.45 (1970s) to $69.86 (1990s) (complex method). Under worst-case assumptions fluoridation remained cost-effective with cost savings ranging from $24.15 (1970s) to $3.87 (1990s) (conventional method) and $107.85 (1970s) and $24.53 (1990s) (complex method). For 66+ years cohort (1990s) fluoridation did not show a cost saving, but costs/person were marginal.

CONCLUSIONS

Community water fluoridation remains a cost-effective preventive measure in Australia.