Ernest M. Scalzetti

How Do Radiographic Techniques Affect Image Quality and Patient Doses in CT

The radiation dose received by patients who undergo CT examinations has become a subject of considerable interest. Adult effective doses for head CT examinations are of the order of 1 to 2 mSv, and for single body examinations, patient doses are typically between 4 and 6 mSv. These doses are high in comparison to most other types of radiological examinations that use ionizing radiation. Patient CT doses may also be compared with natural background (3 mSv/year), dose limits to members of the public (1 mSv/year), and the highest level of occupational exposure, which is about 5 mSv/year. The advent of multi-slice technology will serve to increase CT utilization, as well as individual doses for any given examination. Radiologists are responsible for medical radiation doses to their patients, and it is imperative that they understand the relationship between radiation dose and image quality. In this review, we address the impact that variations in radiographic techniques (ie, selected values of X-ray kVp and mAs) have on patient doses as well as the quality of the resultant CT images.

Patient size and x-ray transmission in body CT.

Abstract— Physical characteristics were obtained for 196 patients undergoing chest and abdomen computed tomography (CT) examinations. Computed tomography sections for these patients having no evident pathology were analyzed to determine patient dimensions (AP and lateral), together with the average attenuation coefficient. Patient weights ranged from approximately 3 kg to about 120 kg. For chest CT, the mean Hounsfield unit (HU) fell from about −120 HU for newborns to about −300 HU for adults. For abdominal CT, the mean HU for children and normal-sized adults was about 20 HU, but decreased to below −50 HU for adults weighing more than 100 kg. The effective photon energy and percent energy fluence transmitted through a given patient size and composition was calculated for representative x-ray spectra at 80, 100, 120, and 140 kV tube potentials. A 70-kg adult scanned at 120 kVp transmits 2.6% of the energy fluence for chest and 0.7% for abdomen CT examinations. Reducing the patient size to 10 kg increases transmission by an order of magnitude. For 70 kg patients, effective energies in body CT range from ∼50 keV at 80 kVp to ∼67 keV at 140 kVp; increasing patient size from 10 to 120 kg resulted in an increase in effective photon energy of ∼4 keV. The x-ray transmission data and effective photon energy data can be used to determine CT image noise and image contrast, respectively, and information on patient size and composition can be used to determine patient doses.

Unilateral pulmonary edema after talc pleurodesis.

Talc is commonly given after drainage of the pleural space to create pleural symphysis. Recognized complications of pleural drainage followed by talc pleurodesis include reexpansion pulmonary edema, pneumonia, and adult respiratory distress syndrome. This report describes a complication of talc pleurodesis that appears not to have been appreciated previously. Chest radiographs obtained before and after talc pleurodesis were evaluated in a total of 108 patients in three groups; 89 of these patients were receiving palliative therapy for malignant pleural effusion. Approximately 16% of the 108 patients developed a transient interstitial process in the lung ipsilateral to the treated pleural space. The recognized complications are inadequate to account for these radiographic findings. Other interstitial diseases such as hydrostatic pulmonary edema and lymphangitic carcinomatosis also are not adequate explanations. The observed complication is most likely the result of endothelial damage leading to a capillary leak type of pulmonary edema.

Use of the VIP-Man model to calculate energy imparted and effective dose for x-ray examinations.

Abstract— A male human tomographic model was used to calculate values of energy imparted (&egr;) and effective dose (E) for monoenergetic photons (30–150 keV) in radiographic examinations. Energy deposition in the organs and tissues of the human phantom were obtained using Monte Carlo simulations. Values of E/&egr; were obtained for three common projections [anterior-posterior (AP), posterior-anterior (PA), and lateral (LAT)] of the head, cervical spine, chest, and abdomen, respectively. For head radiographs, all three projections yielded similar E/&egr; values. At 30 keV, the value of E/&egr; was ∼1.6 mSv J−1, which is increased to ∼7 mSv J−1 for 150 keV photons. The AP cervical spine was the only projection investigated where the value of E/&egr; decreased with increasing photon energy. Above 70 keV, cervical spine E/&egr; values showed little energy dependence and ranged between ∼8.5 mSv J−1 for PA projections and ∼17 mSv J−1 for AP projections. The values of E/&egr; for AP chest examinations showed very little variation with photon energy, and had values of ∼23 mSv J−1. Values of E/&egr; for PA and LAT chest projections were substantially lower than the AP projections and increased with increasing photon energy. For abdominal radiographs, differences between the PA and LAT projections were very small. All abdomen projections showed an increase in the E/&egr; ratio with increasing photon energy, and reached a maximum value of ∼13.5 mSv J−1 for AP projections, and ∼9.5 mSv J−1 for PA/lateral projections. These monoenergetic E/&egr; values can generate values of E/&egr; for any x-ray spectrum, and can be used to convert values of energy imparted into effective dose for patients undergoing common head and body radiological examinations.

A METHOD TO OBTAIN MEAN ORGAN DOSES IN A RANDO PHANTOM

A quantitative method of obtaining average organ dose from point measurements made in the male RANDO phantom is described for 24 compact organs of interest in patient dosimetry. A three-dimensional Cartesian coordinate system was created by considering each of the 36 RANDO phantom sections as the z coordinate, and using a rectangular frame to assign x and y coordinates relative to the center of each section. Anatomical atlases and clinical experience were used to identify the location and extent of each organ and tissue in the RANDO phantom. This proposed scheme is comparable to one used in a commercial phantom and offers investigators a comprehensive protocol for obtaining mean organ doses in the RANDO phantom.

PLEUROPULMONARY BLASTOMA SIMULATING AN EMPYEMA IN A YOUNG CHILD

Pleuropulmonary blastoma is a rare childhood malignancy that may simulate an empyema both clinically and radiographically. A 3-year-old boy with fever, cough, and abdominal pain developed complete opacification of the left hemithorax with contralateral mediastinal shift over the course of several weeks. At thoracotomy, a pleuropulmonary blastoma was discovered. The radiology, pathology, and clinical course of this rare neoplasm are discussed.

Protective pneumothorax for needle biopsy of mediastinum and pulmonary hilum.

The risk of performing CT-guided transthoracic needle biopsy of some mediastinal and pulmonary hilar masses is increased by the presence of intervening lung. A series of patients is presented in whom a protective pneumothorax provided access for biopsy of masses in the mediastinum and pulmonary hilum. Review of Interventional Radiology records revealed 24 patients who had biopsies of mediastinum or pulmonary hilum, in whom protective pneumothorax was used, or attempted, to provide percutaneous access for biopsy. Characteristics of these patients and their procedures were reviewed. Percutaneous access to the pleural space was gained in 21/24 (88%) of patients. A protective pneumothorax was established in 19 (79%); 2 patients had pleural adhesions that prevented the lung from being displaced. The process of creating the protective pneumothorax added a mean time of 17 minutes to the procedure (range 6-30 minutes). All patients had biopsy using coaxial technique, with either a 20-gauge or 18-gauge core biopsy instrument, in addition to needle aspiration. Air leak requiring tube drainage occurred in 1/19 (5%) of patients who had a protective pneumothorax, and in 2/5 (40%) of patients in whom protective pneumothorax was not established. Percutaneous creation of a protective pneumothorax is a safe method that provides access for needle biopsy of deep lesions in the chest without traversing aerated lung.