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Perspective |
1
Department of Radiology, Duke University Medical Center, Erwin Rd., Hospital
North, Rm, 1508D, Durham, NC 27710.
2
Department of Radiology, Children's Hospital Medical Center, 3333 Burnet Ave.,
Cincinnati, OH 45229.
Received November 3, 1999;
accepted after revision February 3, 2000.
Address correspondence to D. P. Frush
Introduction
 Top Introduction Chest Radiography: Conventional...CTSonographyFunctional Lung Imaging: PET...ConclusionReferences |
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Although there is a large amount of information available about chest imaging technology in adults, comprehensive information is limited in the pediatric population [2]. Because of the unique applications and challenges of contemporary pediatric thoracic imaging, this article discusses state-of-the-art chest imaging techniques and applications in neonates, infants, and children. These techniques include conventional screen-film and digital radiography, helical (including CT fluoroscopy) and ultrafast CT, sonography, PET, and MR imaging.
Chest Radiography: Conventional and Digital Radiography
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Conventional Radiography
For nearly a century, chest radiographs have been acquired using
screen-film receptors, with film serving as both the image detector and the
medium for display. Film-based imaging offers many practical advantages,
including relatively low system cost, high spatial resolution, operational
simplicity, and high reliability. Unfortunately, there are also inherent
limitations of film-based radiography. These limitations include a relatively
narrow radiographic sensitivity range, which makes an optimum exposure in
children critically dependent on radiographic technique, and a nonlinear
response to X rays as depicted by the Hurter and Driffield curves
(Fig. 1). Therefore, contrast
enhancement varies depending on the background optical density, with lower
contrast appearing in regions of lower optical density. As a practical matter,
high-quality film-based images may be routinely acquired using fixed equipment
for which automatic exposure control (photo timing) is used; however, these
images are more difficult to obtain in environments in which portable imaging
is used such as pediatric intensive care units, which constitute most chest
examinations performed in hospitals.
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Traditional screen-film recorders typically provide an excellent depiction of fine structures in the radiolucent lung region in which the X-ray transmission level (image signal) is high. Unfortunately, in denser chest areas in which X-ray transmission is lower (e.g., the mediastinal and subdiaphragm regions), this same characteristic can result in objectionable levels of image noise. Recent innovations such as the zero-crossover, asymmetric screen-film system have been developed to address this problem. In this type of detector, one screen-emulsion combination is a low-speed, high-contrast receptor that is optimized for imaging the fine detail of the well-exposed radiolucent lung. The other screen-emulsion pair is faster with lower resolution and is primarily responsible for imaging the low-exposure regions of the chest. The result is a conventional film image possessing excellent lung detail and reduced noise appearance. Several film manufacturers now offer asymmetric screen-film systems for chest radiography in older pediatric patients. Additionally, we use an asymmetric screen-film system (InSight Pediatric Imaging System; Eastman Kodak, Rochester, NY) with a low-attenuation cassette that is specifically designed for imaging young pediatric patients (birth to 3 years).
Digital Radiography
In the past two decades, digital radiography of the pediatric chest has
emerged as a viable alternative to conventional techniques
[4,5,6]
and has been recognized as such by the American College of Radiology
[7]. Unlike film-based
radiography, in which film serves as both the receptor and display medium, in
digital systems, image detection is completely independent of image display.
This setup offers many opportunities for improved image capture and
presentation.
Although several types of digital radiography systems are now available, they all share some common fundamental characteristics. First, digital detectors possess a wide sensitivity latitude (dynamic range) and a linear response to exposure over the entire sensitivity range. As a result, useful images can be acquired at both very low and very high exposure settings, virtually eliminating the need for repeated exposures caused by improper technique, which makes cassette-based digital radiography systems well suited for bedside radiography in intensive care units [8,9,10]. Additionally, a wide range of digital image processing options make it possible to adjust image contrast or detail enhancement to aid in specialized diagnostic tasks or to satisfy differing radiologists' preferences. Finally, digital images can be rapidly transmitted to one or more remote locations for soft-copy review at computer workstations, printed on film (hard copy) with duplicate original images or modified images, and archived for later reference. Problems caused by lost images are eliminated in this environment.
Although radiographic contrast on digital images is largely independent of X-ray exposure factors, image quality is directly affected by exposure intensity. In general, the signal-to-noise ratio increases as the X-ray exposure increases. Consequently, there are practical limits to how much patient exposures can be reduced without adversely affecting the diagnostic value of the image. Likewise, precautions must be taken to guard against unnecessarily high exposures, which produce excellent images but at the cost of higher patient dose.
Digital radiography systems based on photostimulable phosphor detectors were first introduced in 1983 [11]. Sometimes called computed radiography, phosphor radiography systems are now available from many manufacturers. Although each type of system has unique properties, the fundamental principles of operation are similar.
Phosphor radiography systems are based on a reusable phosphor plate that is used instead of the traditional screen-film receptor. The imaging plate is coated on one side with a layer of photostimulable phosphor material (barium fluorohalide) and may be used with conventional imaging equipment at customary exposure levels. When the imaging plate is exposed to X rays (Fig. 2), energy proportional to the local X-ray beam intensity is absorbed and stored in metastable energy traps in the phosphor material, forming a latent image. For image readout, the phosphor plate is scanned by a laser beam that releases the stored energy as visible light. Emitted light levels are detected and amplified by a photomultiplier tube, converted to electrical charge, and digitized to form the digital image. Finally, the imaging plate is erased and readied for reuse through exposure to high-intensity light in the image readout system.
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Image quality in phosphor radiography has been investigated in an effort to ascertain whether dose reductions can be achieved relative to screen-film examinations without sacrificing image quality. Although early reports indicated a potential for patient dose reduction [12, 13], subsequent investigations revealed that exposure levels used with phosphor radiography systems should be at least equal to those used for conventional screen-film imaging to ensure comparable image quality [14,15,16].
A new digital chest radiography system based on a selenium detector was introduced in 1993 (Thoravision; Philips Medical Systems, Hamburg, Germany). Amorphous selenium is an X-ray photoconductor that has long been recognized for its high intrinsic spatial resolution and excellent detection efficiency [17,18,19]. Laboratory experiments have revealed detective quantum efficiency approximately twice that of conventional and phosphor-based systems [20].
The new chest radiography system, previously described in detail [20, 21], is a dedicated chest unit that is suitable for pediatric chest imaging in children who are old enough to stand. The system incorporates an aluminum drum coated with a thin layer of amorphous selenium, which serves as the X-ray beam detector (Fig. 3A,3B). Before X-ray exposure, the drum is slowly rotated beneath a charging subsystem that deposits a uniform positive charge density on the selenium surface. When the patient is properly positioned and the X-ray exposure is initiated, drum rotation is stopped and the exposure is made. Where X-ray beams strike the selenium, electron-hole pairs are created and the freed electrons travel to the surface, where they neutralize the positive charge on the drum face, resulting in a charge image. The charge pattern is read by an array of scanning electrometer probes and then digitized, producing a digital image.
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Unlike phosphor detectors that use a two-stage, laser-stimulated readout method for image acquisition, selenium-based detectors directly convert X-ray photons to electric charge with no intervening stage. This eliminates several sources of image noise, resulting in improved image quality. In clinical observer preference studies in adults, radiologists have consistently rated the quality of the selenium-based digital images superior to conventional radiographs for the depiction of normal anatomy in nearly all test categories and equivalent to radiographs for the depiction of the remaining chest features [22,23,24] (Figs. 4,5A,5B,5C,5D,6A,6B).
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At the present time, the only commercially available digital radiography systems suitable for pediatric chest imaging are phosphor systems and selenium drum digital chest radiography systems. In our department, phosphor radiography is used for all portable examinations. We have found this reduces the need for repeated examinations, and the images are quickly and easily transmitted to monitors in the clinical services. Most other chest radiography is performed using asymmetric screen-film systems because this technology is more readily available than the selenium-based digital images. However, we found selenium technology helpful in larger patients, especially in those in whom examination of a wide contrast range is important (i.e., orthopedic or cardiac devices).
In the future, a new class of large-area, flat-panel X-ray beam detectors will become available, offering high-quality digital images [25, 26] and rapid, direct readout of the digital image data. The emergence of this new class of detectors will likely create new opportunities for pediatric chest radiography.
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Current helical CT is possible to perform because of slip-ring technology and improvements in detectors and computers [30]. As a result of these achievements in technology, the predominant benefits of scanning are faster scanning and volume acquisition of data, providing improved reformations and three-dimensional reconstruction capabilities [29, 31].
Data are acquired faster on helical CT scanners than on slice-by-slice scanners. This has resulted in an important decreased reliance on sedation in pediatric practice [32, 33]. With the advent of multislice technology, in which an increased number of detectors provides simultaneous acquisition of data sets, we are finding that sedation is not routinely necessary in infants and young children. With the recent acquisition of a multislice scanner (LightSpeed QXi; General Electric Medical Systems, Milwaukee, WI), more than 97% of children younger than 6 years old were successfully imaged without the use of sedatives (Frush DP, unpublished data).
Faster scanning decreases motion artifacts and may improve pediatric chest CT [34]. Faster scanning can be particularly beneficial in infants and young children because scanning is routinely performed during quiet breathing [29]. Even with helical CT acquired during quiet breathing, image quality is not substantially degraded (and we found no evidence of such degradation in the literature) (Fig. 7A,7B,7C). In fact, it has been shown in a canine model comparing controlled with suspended respiration, that suspended respiration did not significantly increase the detection of pulmonary metastases [35].
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Faster CT scanning allows examinations to be performed during optimal contrast enhancement of the cardiovascular system. This is important in infants and young children who routinely receive a limited total volume of contrast material. Because helical CT examinations are completed faster than older conventional axial scanning, excellent enhancement can be achieved in the neck, chest, abdomen, and pelvis of children [36]. Faster scanning obviates the previous and suboptimal practice of splitting the contrast bolus [27] between the abdomen and chest, with reduced contrast enhancement of both regions.
General protocols for scanning the chest in infants and children are available [27]. Because of the wide variation in sizes encountered in the pediatric population, radiologists must be familiar with adult protocols and be able to adjust CT parameters for smaller children. These parameters include collimation and tube current. Our experience is that the latter parameter is often neglected in pediatric chest CT. In fact, in a review of 55 referral CT examinations, the mean tube current used in children 8 years old or younger was more than 200 mA (Frush DP, unpublished data), an amount more appropriate for adult chest scanning [27]. For children, recommended tube currents range from 25 to 170 mA [27, 29, 37]. The lower tube currents in this range are more appropriate for examinations focused on lung parenchyma [37, 38].
Applications of helical CT in children include those in which conventional slice-by-slice technology is indicated. Congenital lung, airway, and mediastinal abnormalities; infectious disorders and sequelae (e.g., abscess, empyema, bronchiectasis); interstitial lung disease; chronic lung disease; parenchymal and mediastinal malignancy and metastasis surveillance; and trauma are all familiar uses of CT [27, 28]. Although some of these applications are similar to those in adults, there is a greater call for staging of lung cancer, cancer surveillance, and pulmonary embolism in the adult chest CT [27]. To date, reported applications of helical CT in children include examination of pulmonary sequestration, cystic fibrosis, congenital and acquired airways disorders, bronchiectasis, lung transplantation, and the cardiovascular system [27,28,29, 39,40,41,42,43].
Anatomic examination of the airway includes three-dimensional reconstructions and virtual bronchoscopy. One article describes experience with this technique in young children [44], and little information is available comparing the technology with bronchoscopy. Smaller airways and motion-related artifacts from breathing can limit image quality in young children. Three-dimensional depiction of subtle bronchiectasis is provided by volume reconstruction [40], although this has not been shown to affect clinical treatment in children.
Helical CT of lung parenchyma has not changed the way we think about parenchymal disorders, even though overlapping reconstructions have been shown to increase the detection of fine anatomic detail, such as that of lung nodules [45].
A specific vascular application of helical CT is the examination of pulmonary sequestration. Helical CT is excellent for depicting the systemic arterial supply to the lung and is more useful than MR imaging or sonography in depicting disorders of the lung that may mimic sequestration [39]. However, in general, vascular (Fig. 8A,8B,8C) and cardiac applications of helical CT (i.e., CT angiography) in the pediatric chest are limited [39, 42, 43] and await further testing and technical refinements. Similar to body CT angiography in children, the small size of structures and respiratory (and cardiac) motion can substantially limit applications of imaging techniques. Despite these limitations, we have found that the fast imaging times using multislice helical CT have provided the opportunity to perform CT angiography of processes involving the aorta (e.g., coarctation, vascular rings) and proximal pulmonary arteries (e.g., stenosis after surgery) in small children and infants previously available only on MR imaging [42]. Advantages of helical CT angiography include easier scheduling, faster imaging times, and decreased reliance on sedation.
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An area in which helical CT is of value over conventional chest CT is in the functional examination of the lung. One recent investigation [46] used a temporally segmented reconstruction of a slice scanned during the respiratory cycle to distinguish air-trapping from nonobstructive hyperinflation in children with an area of hyperlucent lung. With this technique, investigators were able to determine lung attenuation and to reconstruct the respiratory cycle. When the attenuation values were plotted against time, cyclic variation was present when there was no air-trapping, and unchanged lung attenuation indicated air-trapping.
Improved depiction of mediastinal, pleural, or chest wall abnormalities is a compelling but unproven diagnostic benefit of multiplanar or three-dimensional reconstructions. Whereas most, if not all, information can be extracted by the radiologist from the axial data, the benefits for clinical subspecialties in understanding the appearance or treatment (i.e., surgery) of these abnormalities should not be underestimated.
Whereas helical CT uses a continuously rotating X-ray source and detector set, ultrafast CT technology is based on a fixed set of detectors and movement of an electronically focused beam. Ultrafast CT images are obtained in a fraction of a second. Examinations can be performed with cine mode (50 msec) or volume mode (50-100 msec) image acquisition times. These very short acquisition times make this CT technology attractive for imaging the chest because of the elimination of cardiac-related or respiratory-related motion artifacts; for example, artifacts inherent in imaging children with tachypnea.
Together with an appropriately timed IV contrast material bolus, cardiovascular and airway anatomy can be revealed [47]. However, there is no evidence that this technology has provided information unavailable by more frequently performed imaging such as echocardiography and cardiac catheterization and angiography. In adults, cardiovascular applications are primarily used for the examination of coronary artery disease [48] and pulmonary embolism [49]. Despite the advantages of ultrafast scanning, fewer centers have ultrafast scanning equipment than helical CT scanning equipment. There are few studies in the literature on pediatric airways disorders [50]. It is likely that with even faster helical CT systems and the development of cardiac gating capabilities, the more widely available helical technology will provide similar information to that of current ultrafast CT.
Several manufacturers offer CT with rapid reconstruction algorithms that provide segmented (e.g., every 60°, or one sixth of a second for a 1-sec complete gantry rotation) data display, and a fluoroscopic display of CT sections. As with helical CT, CT fluoroscopy was possible only with the development of slip-ring technology. In conjunction with the tableside control of table movement and image acquisition, the technique is primarily used for interventional procedures [51].
Advantages of CT fluoroscopy include a superior depiction of anatomy compared with routine radiographic fluoroscopy and a more real-time display of interventions than with routine helical CT (for which imaging is longer and the display is at the control room console). As with any CT-guided procedure, CT fluoroscopy provides superior depiction of regions, such as the chest and bone, compared with sonography. This is an important advantage in the pediatric population, in which most imaging guidance has been performed with sonography. CT-guided procedures have been technically difficult in sedated children because of the potential for movement during the procedure. With fluoroscopic display, movements are not a problem because adjustments can be made at the tableside. Although not proven, our experience is that fluoroscopic procedures are completed more quickly than procedures using CT-guidance without fluoroscopy; however, we still perform most abdominal, pleural, and peripheral lung and chest wall procedures with sonographic guidance.
To perform CT fluoroscopy, radiologists and technicians need time to learn to use the equipment effectively and efficiently. Other disadvantages include more difficult scheduling than with sonography, slightly more cumbersome access within the gantry using handheld or biopsy-assistance devices, and the use of ionizing radiation. This last point is an important one because there is, as with routine fluoroscopy, an accumulation of dose during the procedure. With judicious selection of tube current and directed and limited use of CT fluoroscopy, exposure can be minimized (Fig. 9).
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There are no current applications or technical recommendations for CT fluoroscopy in children. A potential noninterventional use of CT fluoroscopy in children is the dynamic examination of the airway in the setting of tracheobronchomalacia.
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Functional Lung Imaging: PET and MR Imaging
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TopIntroductionChest Radiography: Conventional...CTSonographyFunctional Lung Imaging: PET...ConclusionReferences |
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Positron Emission Tomography
FDG PET has clinical use in neurologic, cardiac, and oncologic
applications, particularly in adults
[54]. FDG is a glucose analog
that is disproportionately concentrated in tissues with high rates of
metabolism. Malignant tissues have a high accumulation of FDG because of
higher glucose use and increased active transport sites for glucose on cell
membranes. The fluorine incorporated into FDG undergoes positron-emitting
decay and then an annihilation reaction that produces two 511-keV photons
emitted at 180° to each other
[55]. Images are generated
through the detection of these emitted photons.
The most recent advances in FDG PET of the thorax have been in adults. FDG PET has been shown to be accurate in the examination and staging of lung cancer and in differentiating benign from malignant solitary pulmonary nodules [55]. The establishment of definitive roles for FDG PET in pediatric chest imaging has been slow. The high cost of the equipment required to perform PET has limited mass marketing [56], especially for the pediatric population. As a result, little has been published on FDG PET of the pediatric chest [57, 58]. However, several business and technical developments may contribute to PET becoming more ubiquitous, increasing access for pediatric imaging. The recent development of multiple distribution sites that commercially produce FDG has eliminated the need for owning a cyclotron and radiochemistry laboratory, expanding PET capacities to multiple new centers [54]. Additionally, the development of gamma cameras, which use coincidence detection of annihilation radiation, has created an alternative to dedicated PET scanners, making PET more financially feasible for many centers [56].
Currently, there are no pediatric body malignancies that are routinely imaged with FDG PET for diagnosis, staging, or follow-up after therapy. Concerning pediatric chest imaging, PET may play an increasing role in the examination of childhood lymphoma. FDG PET has been shown to be sensitive for disease detection and to be accurate in staging the extent of disease [59, 60] (Fig. 11). FDG PET has been advocated as an accurate and more cost-effective way to stage lymphoma as compared with the combination of conventional techniques that are currently used [60]. With respect to monitoring the response of lymphoma to therapy, an interval decrease in the uptake of FDG has been associated with a high response to therapy and superior longterm outcome in patients with lymphoma [59]. Another pediatric malignancy that can involve the chest by primary tumor or metastatic lung disease is neuroblastoma. In one recent review [62], it was noted that most neuroblastomas and their metastases are readily detected on FDG PET even when they are not metaiodobenzylguanidine avid, suggesting that FDG PET is helpful in defining the extent of disease in children with neuroblastoma that is non—metaiodobenzylguanidine avid.
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Currently, FDG PET is used as a problem solving technique for difficult oncologic cases. We have found FDG PET helpful when anatomic staging is in question or when CT or bone scan reveals imaging findings that are not definitely attributable to malignancy. Similarly, FDG PET has been helpful when two imaging techniques reveal contradictory findings. PET is also useful in differentiating changes that occur after therapy and viable residual tumor that develops after surgery and radiation and chemotherapy.
MR Imaging
MR imaging of the thorax in children has traditionally been performed for
anatomic examination. With the advent of newer MR imaging applications,
functional lung imaging is now possible. One technique of functional MR
imaging is fast MR imaging of the airway. Although experience is preliminary,
dynamic airways abnormalities such as tracheobronchomalacia can be revealed
noninvasively (Hite SH, Cunningham T, Faust R, Stillman AE, Rimall FL,
presented at the Radiological Society of North America meeting, November
1998). Another functional application of MR imaging is with the use of the
investigational technique hyperpolarized 3He. Whereas MR imaging
based on hydrogen protons has become the optimal imaging technique for many
organ systems and disease states, the role of MR imaging in the examination of
lung abnormality has been extremely limited because of the low concentration
of hydrogen protons in the aerated lung
[62]. Inadequate signal
intensity has previously hindered the generation of meaningful MR images of
diseased lungs. However, recent developments have shown that the lungs can be
revealed on MR imaging by using a hyperpolarized ventilation contrast agent
[62,
63]. Helium-3 is
hyperpolarized through a process that involves a collision exchange between
mixed rubidium and helium, heating to 180°C, and high-intensity laser
illumination for several hours
[63]. The hyperpolarized
3He product can have a concentration of polarized molecules up to
30% [62,
63]. In comparison, when a
magnetic field is applied to human tissues, as is the case with conventional
hydrogen-based MR imaging, only one proton in 10,000,000 is polarized
[62]. Therefore,
3He-enhanced MR imaging offers a potential increase in
signal-to-noise ratio of 100,000 times that of conventional hydrogen proton MR
imaging.
Helium-3—enhanced MR imaging can be performed on a conventional MR imaging system after it is equipped with a broad-band receiver that can adjust the radiofrequency for the gyromagnetic ratio of both 3He and hydrogen-1 and a torso surface coil designed for 3He-enhanced imaging [62, 63]. Immediately after the patient inhales 3He, gradient-echo imaging is performed in the coronal plane during a single breath-hold. Because signal intensity is generated by the hyperpolarized substance and is not related to T-1 TR, very short TRs can be used, resulting in image acquisition times as short as 10 sec [62]. The resulting images reflect lung ventilation, much like a nuclear ventilation scan. However, anatomic detail is revealed at high spatial resolution. To better evaluate morphologic abnormalities and to help define the anatomic extent of the nonventilated lung, T2-weighted fast spin-echo images with respiratory gating are also obtained.
In healthy subjects, 3He-enhanced MR images reveal high signal intensity in the large airways, such as the trachea and main bronchi, and homogeneous high signal intensity is also seen throughout the pulmonary parenchyma [62] (Fig. 12). Lack of lung ventilation is revealed by absent or decreased signal [63] (Fig. 13). Early investigations have suggested that 3He-enhanced MR imaging may be useful in the examination of obstructive airways disease [64], including cystic fibrosis.
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Concerning cystic fibrosis, to facilitate the objective examination of existing and newly developed therapeutic regimens such as gene therapy and mucolytic agents, image scoring systems have been used to objectively evaluate the progression of lung disease [65,66,67,68,69,70]. Most scoring systems have been based on high-resolution CT and chest radiography, which evaluate morphologic changes of cystic fibrosis, such as bronchiectasis, peribronchial thickening, mucus plugging, and emphysema [65,66,67,68,69,70]. However, these morphologic changes, as seen on high-resolution CT and chest radiography, may not be evident in early pulmonary disease. Functional imaging may offer a more sensitive method to detect early changes of cystic fibrosis in patients with minimal disease [67, 71, 72]. This type of imaging is of particular importance in evaluating progression in young patients with minimal disease and minimal or absent findings on high-resolution CT. The detection of early lung changes in cystic fibrosis is critical because many protocols evaluating new therapies involve young children with minimal or no lung disease.
Preliminary studies at our institution have shown that abnormalities of both morphology and function can be depicted on MR imaging with 3He enhancement [54]. In all the patients with cystic fibrosis studied, the functional 3He-enhanced images revealed more severe ventilation abnormalities than would be expected on the basis of the degree of morphologic changes present [64] (Fig. 13). Because 3He-enhanced MR imaging can provide morphologic and functional information and radiation is not used, it may represent an optimal technique on which a grading scale can be based to evaluate the degree and progression of pulmonary changes of cystic fibrosis on a serial basis. The same type of grading system based on 3He-enhanced MR imaging may be useful in the examination of other types of obstructive airways diseases such as bronchiolitis obliterans and reactive airways disease.
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TopIntroductionChest Radiography: Conventional...CTSonographyFunctional Lung Imaging: PET...ConclusionReferences |
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TopIntroductionChest Radiography: Conventional...CTSonographyFunctional Lung Imaging: PET...ConclusionReferences |
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