Pulmonary: scintigraphy – ventilation and perfusion
Synonym(s): V/Q scintigraphy
IntroductionA. Pulmonary perfusion scintigraphy
- This is a sensitive, non-invasive method of demonstrating pulmonary perfusion.
- Qualitative and quantitative information can be obtained.
- Performing a perfusion and ventilation study of the lungs increases the specificity.
- The radiopharmaceutical used is 99mtechnetium (Tc) human macroaggregated albumin (MAA).
- The radionuclide 99mTc emits gamma rays only of the energy 140 KeV and has a half-life of 6 h.
- Radiopharmaceutical dose of 99mTc MAA, 1 MBq/kg.
- On intravenous injection the 99mTc MAA particles are stopped on first pass through the pulmonary circulation by occluding small branches of the pulmonary artery and capillaries. The resultant distribution of the 99mTc MAA reflects the regional perfusion of the lungs.
- The occlusion of the vessels lasts approximately 4-8 h, with the 99mTc MAA particles being phagocytosed by the cells of the reticulo-endothelial system.
- This is a sensitive, non-invasive method of demonstrating pulmonary ventilation.
- Qualitative and quantitative information can be obtained.
- Performing a ventilation and perfusion study of the lungs increases the specificity of the study.
- Gases, pseudogases and aerosols can be used to study lung ventilation. However, radioactive gases provide physiologically ideal agents for representing the distribution of air within the lungs.
- 81mkrypton (Kr) gas is the agent of choice. It is the daughter product of 81rubidium (Rb) (half-life 4.6 h) and is extracted from the parent by passing humidified air through the 81Rb generator. 81mKr has the following advantages over other radiopharmaceuticals:
- It is a physiological gas.
- A short half-life of 13 s, therefore allowing disposal into the atmosphere.
- Gamma ray energy of 190 keV making it ideal for imaging with conventional gamma camera systems.
- Low radiation dose.
- Simultaneous acquisition of perfusion possible due to its different gamma energy to 99mTc.
- Little crosstalk with 99mTc.
- Easy to use.
- Multiple views possible.
- Good quality images.
- However, 81mKr has the following disadvantages:
- It is expensive.
- ?Short half-life of generator.
- Limited availability.
- 99mTc-diethylenetriaminepentaacetic acid (DTPA) aerosol:
- 99mTc DTPA particles can be used to image lung ventilation.
- Particles being significantly larger, behave differently to gases and are therefore distributed differently.
- Nevertheless, they can be used to give a reasonable representation of the distribution of ventilation if the mean particle size is small enough to ensure alveolar deposition, ie <5 um.
- 99mTc DTPA is cheap, easy to use and readily available. However, a high quality nebuliser in a shielded lead unit and a suitable scavenging system are required.
- In addition, simultaneous acquisition of perfusion and ventilation images is not possible, particles are deposited at sites of turbulence and adhere to mucus (which may produce hot spots) and the radiation dose is high.
- 99mTc Technegas:
- A pseudogas, which consists of microscopic particles of carbon.
- Technegas has the advantage in that it is technetium labelled and its distribution within the lung is closer to that of air than aerosols.
- Its main disadvantage is that the dispensing system and its associated equipment is extremely expensive.
- 133 Xenon (Xe) gas and 127 xenon (Xe) gas:
- Both have the disadvantages that image quality is poor when used when used conventional gamma camera systems this is due to the high energy gamma rays emitted by 127Xe (173 and 202 keV) and the low energy gamma emissions from 133Xe.
- Other disadvantages are the exhaled gas must be trapped and dealt with safely due to their long half-lifes (127Xe - 30 days and 133Xe 5.3 days) and only a single view is possible.
- As an aid to diagnosis, prognosis and monitoring response to treatment in pulmonary conditions such as requine asthma Equine asthma and exercise induced pulmonary hemorrhage (EIPH) Lung: EIPH (exercise-induced pulmonary hemorrhage).
- Quantifying pulmonary damage following infectious, eg pneumonia, or non-infectious insults.
- Important roles in future research into pulmonary function, pathological processes and in the development of treatment for respiratory disease.
- Pulmonary scintigraphy is a sensitive non-invasive method of demonstrating the functional status of the lungs, but has a low specificity. If an abnormality in ventilation and or perfusion is demonstrated with pulmonary scintigraphy, further diagnostic modalities must be employed to identify the disease process. However, radiographing the horse's thorax at the time of the study in order to categorize any changes seen in the lung scintigram can increase the specificity of the study.
- Provides information about regional lung perfusion that is not available from conventional investigations.
- Limited availability - specialist centers only.
- Low specificity - results should be interpreted with clinical examination and other diagnostic techniques.
- Equipment and radiopharmaceuticals are expensive.
- Radiographer must have a good understanding of the principles of nuclear medicine is essential in order to achieve high quality diagnostic images.
- Radiographer must have a good understanding of the necessary legislation pertaining to nuclear medicine and radiation protection.
- Other aids in investigating pulmonary function are:
- Auscultation Thorax: auscultation , percussion Thorax: percussion .
- Radiography Thorax: radiography .
- Ultrasonography Ultrasonography: thorax .
- Endoscopy Respiratory: endoscopy and sampling of the respiratory tract.
- Resting pulmonary function tests, eg esophageal manometry, dynamic compliance, airway resistance, bronchial reactivity, forced oscillatory mechanics, arterial blood gas analysis Blood: gas analysis .
- Exercising pulmonary function tests Lung: pulmonary function test , eg ventilation, gas exchange.
- Quality control checks on the gamma camera performed daily prior to use, takes approximately 30 min.
- Pulmonary perfusion scintigraphy only: 30 min.
- Pulmonary ventilation only: 30 min.
- Pulmonary ventilation and perfusion scintigraphy: 60 min.
Criteria for choosing test
- History and details of previous clinical examinations are required. The horse should be assessed and a preliminary diagnosis made.
- Image interpretation. Uniformity and symmetry are the most important criteria in the normal uptake pattern within the lungs.
- Dedicated imaging room with controlled environment (air conditioning).
- Large field of view gamma camera.
- Low energy general-purpose collimator.
- Imaging computer and associated nuclear medicine software.
- Image recording device.
- Assay calibrator.
- Syringe shield.
- Lead lined storage container for collecting waste.
- Small compressor pump.
- Ventilation delivery system.
- Radioactive marker source.
- Disposable gloves.
- Radioactive decontamination kit.
- Jugular catheter inserted.
- Horse weighed in order to calculate radiopharmaceutical dose.
- Sedation required Anesthesia: standing chemical restraint , eg romifidine Romifidine 0.4 ml/kg IV.
- Blinkers and cotton wool in the ears used to avoid distractions from movement of camera and other noise.
Step 1 - Preparation
- The horse is administered 99mtechnetium-macroaggregated albumin (MAA) IV at a single dose of 1 MBq/kg, injected via a catheter into the left or right jugular vein.
- An anatomical marker containing approximately 2 MBq 99mTc is placed midway across the lung fields at the level of the dorsal spinous processes (ideally out of the lung fields) to aid post-acquisition processing of the lateral images. This is removed from the field of view when acquiring the dorsal images.
- The horse is fitted with a large perspex mask with a central adapter holding a two-way non-rebreathing valve. The mask is sealed to the horse's head by a tight fitting, shaped rubber gasket stretched over the open end of the mask.
- The outlet from the 81Rb generator is connected to the inlet of the inspiratory port by a 200 cm extension line with an internal diameter of 1mm and a volume of 1.8 ml.
- The gamma camera is positioned in order to acquire the first image (left lateral caudal lung). Once positioned 81mKr gas is eluted from the 81Rb generator at a flow rate of 500 ml/min. The airflow is generated from a small compressor. The air is humidified by passing it through water prior to passing it through the 81Rb generator.
Step 1 - Imaging
- A dual energy acquisition is commenced when the count rate from the 81mkrypton pulse height analyzer reaches 5000 counts/s (cps).
- Acquisition parameters acquired for both left lateral and dorsal views of the lungs are a simultaneous, dynamic acquisition, acquiring sixty, two-second frames, using a 128 x 128 matrix. All images are evaluated using a commercial nuclear medicine software package.
- High quality radiographs of the thorax are obtained from each horse immediately post-pulmonary scintigraphy whilst still sedated (provided the dose rate from the horse is below 2.5 microseveirts/h) in order to categorize any changes in the lung scintigram.
Step 2 - Quantitative image analysis
- Initially all images are corrected for motion and the dynamic sequence summed to produce a static image.
- The data is then organized to produce separate static files: left lateral cranial lung ventilation (V) and perfusion (Q), left lateral caudal lung V and Q, left dorsal lung V and Q, and right dorsal lung V and Q.
- Prior to processing the left cranial and caudal lateral lung images the following pre-processing is performed. The left lateral cranial lung V and Q static images are displayed side by side. No registration of the images is required as both images V and Q images are acquired simultaneously and therefore perfectly matched in the acquisition matrix.
- Using the radiolabeled anatomical marker a region of interest (ROI) is drawn vertically down from the centre of the marker and expanded so as to include the entire lung caudal to this marker. The area within the ROI on both V and Q images is masked out and the resultant images saved. This is repeated on the left lateral caudal lung V and Q images but the ROI is expanded to include the entire lung cranial to the anatomical marker.
- The borders of the V and Q images are defined using a 10% threshold ROI, which draws a line between pixels with a value equal to 10% of the maximum pixel value in the image. The ventilation images are normalized to the perfusion images (NV). The perfusion image data is multiplied by ten and one was added to each pixel value (10*Q image).
- The offset of one is required to avoid a zero divide condition. The NV image is divided by the 10*Q image resulting in a V/Q ratio image multiplied by a factor of 100. This V/Q ratio imaged is saved as a new image file. From the V/Q ratio image a histogram of the distribution of pixel counts is produced.
- See diagram of distribution of V/Q ratios in healthy and horses affected with equine asthma .
Step 3 - Qualitative image analysis
- The resulting image files are concatenated and displayed:
- Left lateral cranial lung Q, V and V/Q ratio.
- Left lateral caudal lung Q, V and V/Q ratio.
- Left dorsal lung Q, V and V/Q ratio and right dorsal lung Q, V and V/Q ratio.
- The images are visually inspected, together with the thorax radiographs, and any comments regarding the distribution and pattern of uptake are recorded.
- Horses that have had an injection of a radiopharmaceutical do constitute a radiation hazard. The horse must be stabled for the following 24 h and a temporary controlled area must be demarcated. The necessary radiation protection legislation must be complied with.
- The horse may be discharged 24 h post-procedure provided the dose rate from the horse has fallen to below 2.5 microseveirts/h.
- Recent references from PubMed and VetMedResource.
- Votion D M, Coghe J D & Lekeux P M (2001) Comparison of deposition images obtained by use of an ultrafine 99m-technetium-labeled carbon dry aerosol with ventilation images obtained by use of 81m-krypton gas for evaluation of pulmonary dysfunction in calves. Am J Vet Res 62 (12), 1881-1886 PubMed.
- Coghe J, Votion D & Lekeux P (2000) Comparison between radioactive aerosol, technegas and krypton for ventilation imaging in healthy calves. Vet J 160 (1), 25-32 PubMed.
- Votion D M, Roberts C A, Marlin D J & Lekeux P M (1999) Feasibility of scintigraphy in exercise-induced pulmonary haemorrhage detection and quantification - preliminary studies. Equine Vet J Suppl 30, 137-142 PubMed.
- Votion D, Vandenput S, Duvivier H, Art T & Lekeux P (1997) Analysis of equine scintigraphical lung images. Vet J 153 (1), 49-61 PubMed.
- Votion D, Ghafir Y, Munsters K, Duvivier D H, Art T & Lekeux P (1997) Aerosol deposition in equine lungs following ultrasonic nebulisation versus jet aerosol delivery system. Equine Vet J 29 (5), 388-393 PubMed.
Other sources of information
- Sharp P F, Gemmell H G & Smith F W (1989) Practical Nuclear Medicine. Oxford University Press.