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Original Research |
1 Department of Radiology, Division of Body Imaging, Boston University Medical
Center, 88 E Newton St., Atrium 2, Boston, MA 02118.
2 Present address: Department of Radiology, Boston Veterans Administration
Healthcare System, West Roxbury, MA.
3 Department of Radiology, Division of Abdominal Imaging, Massachusetts General
Hospital, Boston, MA.
Received March 15, 2006;
accepted after revision October 11, 2006.
Address correspondence to B. C. Lucey.
Abstract
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MATERIALS AND METHODS. We performed 291 CT-guided interventional procedures using a low dose of radiation. The subjects were 165 men and 126 women 22-89 years old with a mean age of 65 years. CT fluoroscopy was not used. The procedures were 201 percutaneous biopsies and 90 percutaneous aspiration or drainage procedures. Before the procedure, images were obtained with standard mAs of 175-250 mAs. All subsequent CT was performed at a reduced mAs. Technical success of catheter placement and biopsy was calculated, and the results were compared with those of procedures performed over the previous 12 months with standard radiation doses. Patient weight, lesion size, and number of CT acquisitions needed to complete the procedure were recorded.
RESULTS. All but three aspiration or drainage procedures performed at 30 mAs were successful, for a success rate of 96.7%. The technical success rate of biopsy performed at 30 mAs was 93.5%. In the cases of 13 patients undergoing biopsy, the masses were not identified with low-dose technique, and these procedures were completed at a higher dose. Results were independent of patient weight and lesion size. The technical success rate was 98% for percutaneous drainage performed at a standard radiation dose in the 12 months before introduction of the low-dose technique. The technical success rate was 87.5% for biopsy performed at a standard radiation dose in the 12 months before introduction of the low-dose technique. The complication rate of the low-dose technique was comparable to that of the standard-dose technique.
CONCLUSION. Low-dose radiation technique using 30 mAs results in technical success for both catheter placement and percutaneous biopsy comparable to standard radiation dose.
Keywords: biopsy • CT technique • interventional radiology • radiation dose
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In our experience, most CT-guided interventional procedures are performed with the imaging parameters used for diagnostic CT. The radiation exposure to the patient is directly proportional to the effective tube current used. The aims of our study were, first, to evaluate the feasibility of using the low tube current of 30 mAs to perform a range of interventional procedures, including percutaneous biopsy, aspiration, and catheter drainage, and, second, to assess the technical success of use a low dose compared with a standard dose in similar interventional procedures.
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The procedures performed were percutaneous biopsy (n = 201), needle aspiration (n = 8), and percutaneous catheter placement (n = 82). The biopsies were performed on the following structures: lung (n =90), liver (n = 20), adrenal gland (n = 9), kidney (n = 8), lymph node (n =29), omentum (n = 5), bone (n =6), mediastinal mass (n = 9), and soft-tissue mass (n = 25). Eight patients underwent needle aspiration alone for diagnostic or therapeutic purposes. In four of these cases, the procedure was performed to identify infected hematoma before surgical evacuation. In the other four cases, the procedure was performed for complete aspiration of small collections deemed too small for catheter placement. Catheters were placed percutaneously for drainage of abscess (n = 77), infected hematoma (n =3), biloma (n = 1), and suprapubic catheter placement in the urinary bladder (n = 1). The weight of each patient was recorded to identify whether patient weight affected the success rate of the procedure.
To assess how biopsy and drainage performed with low-dose radiation compared with standard-dose radiation, we reviewed the results of all CT-guided interventional procedures undertaken in the 12 months (September 2001-September 2002) before this study. During the comparison period, 85 men and 74 women (age range, 25-87 years; mean age, 63 years) underwent 159 CT-guided procedures at the standard radiation dose (175-250 mAs). The procedures were 104 biopsies and 55 percutaneous catheter drainage procedures. Biopsy was performed on the following structures: lung (n =70), lymph node (n = 12), soft tissue (n = 9), liver (n = 8), adrenal gland (n = 3), and pancreas (n =2).
Imaging Technique
All available previous imaging studies were evaluated for planning of
patient positioning and the optimal route for intervention. Once the patent
was placed in position on the CT gantry, a radiopaque grid was applied to the
body surface to overlie the lesion. Preliminary CT was performed through the
target area at the standard dose, determined by the patient's body habitus, to
identify the lesion for biopsy and to delineate the surrounding anatomic
features. The mAs used for this initial limited CT ranged from 175 to 250 mAs.
All subsequent CT to guide needle and catheter placement, to check the final
position before intervention, to confirm the adequacy of intervention, and to
detect complications after procedures was performed at the lower tube current
of 30 mAs. Procedural imaging was performed with an incremental technique. An
image was obtained at the level of the needle, and one or two images were
obtained both cranially and caudally in relation to this position until an
image showed the needle within the target.
The low threshold of 30 mAs was chosen because it was the lowest effective tube current available on our CT machine (PQ 5000, Philips Medical Systems). If lesion conspicuity was poor at this low tube current, the radiologist performing the procedure had the option at any time of increasing the mAs sufficiently to complete the procedure safely and successfully. The peak kilovoltage was kept constant for each procedure and varied between 120 and 140 kVp in all cases. The slice thickness of CT scans depended on the individual case and varied between 3 and 5 mm, 3 mm being used for lesions less than 2 cm in diameter and 5 mm being used for lesions larger than 2 cm.
Interventional Technique
All interventional procedures were performed by one of four attending
radiologists or by a resident or fellow under the direct supervision of one of
these four attending radiologists. Standardized techniques were used to
perform all percutaneous biopsy and drainage procedures. A coaxial needle
technique was used for all biopsies. Initially a 17- or 19-gauge outer coaxial
needle (Easy Core Biopsy System, Boston Scientific) was incrementally advanced
to the lesion while intermittent CT checks were made. Once optimal positioning
was obtained and confirmed, multiple aspirations were performed with a 20- or
22-gauge fine needle (Chiba, Remington Medical). These specimens were reviewed
by a cytopathologist on site to ensure adequate sampling of the lesion. A
definitive diagnosis was not rendered at this time.
After adequacy of sampling was confirmed, multiple core biopsy specimens were obtained with a standard biopsy gun (Easy Core Biopsy System, Boston Scientific). The combination of a 17-gauge outer coaxial needle and 18-gauge biopsy gun was used for all abdominal biopsies. Lung biopsies were performed with a 19-gauge outer coaxial needle and a 20-gauge biopsy needle. Multiple CT acquisitions were required during biopsy to redirect the coaxial needle for better positioning, to check that needle position was maintained during the biopsy, and to ensure no developing complications such as pneumothorax and bleeding necessitated immediate corrective action.
All percutaneous catheter drainage procedures were performed with the tandem trocar technique [10]. This technique involved initial placement of a 10-, 15-, or 20-cm-long 20-gauge needle (Chiba, Ballard Medical Products) into the collection under CT guidance. Aspiration was performed to obtain a diagnostic sample. Drainage catheters ranging in size from 8 to 12 French were advanced into the collection to parallel the Chiba needle. Once the catheter was successfully delivered and coiled in the collection, complete aspiration and saline irrigation were performed. All eight CT-guided diagnostic aspirations were performed with one placement of a 20-gauge needle (Chiba, Ballard Medical Products). Postprocedural CT at 30 mAs was performed after each procedure to assess the result of the procedure and to detect complications.
Technical Success
Technical success of biopsy was defined as either a diagnosis at pathologic
examination or a true-negative result as proved by clinical or imaging
follow-up findings. Technical success of catheter placement was defined as
successful placement of the catheter into the target collection to be drained.
Success was not based on long-term outcome such as successful resolution of
the collection or patient recovery. Any failure to obtain adequate samples at
biopsy or inability to place a catheter into a collection was considered a
technical failure. The patients underwent clinical follow-up (time from
procedure to last visit and examination) for a range of 1-14 months and a mean
of 3 months.
A number of secondary factors that can influence technical success of intervention were measured and recorded at the time of the procedure. These included patient weight, size of the target lesion, and number of separate CT acquisitions from start to finish of the procedure. All these factors were evaluated to determine whether they influenced the technical success of intervention.
For the comparison group, demographic features, clinical details, and interventional details were retrospectively obtained from electronic patient records and review of CT scans obtained at the time of the procedure. Patient weight, lesion size, and number of CT acquisitions during the interventions were recorded. Information on significant complications arising from the interventional procedure was obtained from the clinical records. The biopsy results were obtained from review of pathologic reports. This information was used to calculate the technical success of procedures for the comparison group. Finally, the low-dose and standard-dose groups were compared to determine the existence of a significant difference between these two groups with respect to patient demographics and technical success.
Statistical Analysis
Statistical analysis was performed with SPSS statistical software (version
10.0.0, SPSS). Quantitative variables such as age, patient weight, lesion
size, number of CT acquisitions, and technical success were compared by use of
the Student's t test. Qualitative variables, such as sex and other
demographic variables, were compared by use of the chisquare test. In all
statistical analyses in this study, p < 0.05 was considered
significant.
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Biopsy
Biopsy performed at the low-dose technique of 30 mAs was technically
successful in 188 (93.5%) of the 201 cases and failed in 13 (6.5%) of the
cases. At 30 mAs, all lung, bone, adrenal, renal, mediastinal, and omental
masses were clearly identified, and biopsy was successful. In the successful
biopsy group, malignancy (either primary or metastatic) was diagnosed in 175
cases (Fig. 1A,
1B), and a nonmalignant
diagnosis made in 13 cases. The nonmalignant diagnoses were reactive
inflammatory cells (n = 9), chondroid hamartoma (n = 1),
tuberculous infection (n = 1), inflammatory pseudotumor (n =
1), and rheumatoid nodule (n =1). In each of the nine patients in
whom inflammatory cells were identified, follow-up chest CT 1 and 5 months
after diagnosis showed total or partial resolution of the mass, indicating the
results were truly negative for malignancy. Patients who underwent successful
biopsy weighed 90-290 pounds (41-132 kg) and had a mean weight of 170 pounds
(77 kg).
Biopsy at 30 mAs failed in 13 (6.5%) of the cases because of inadequate sampling in one case and inability to visualize the lesion in the other 12 cases. Inadequate sampling occurred in a patient with a retroperitoneal lymph node. Despite clear visualization of the needle within the nodal mass at biopsy, the pathology report indicated the findings were nondiagnostic. In the other 12 patients, the biopsy target was not clearly identified at 30 mAs. These targets were six liver masses, four lymph nodes, and two soft-tissue masses. In these cases, biopsy was performed successfully at 65 mAs for eight patients, 125 mAs for two patients (one lymph node and one soft-tissue mass), 100 mAs for one patient with a liver mass, and 220 mAs for another patient with a liver mass (Fig. 2A, 2B, 2C, Table 1). Malignancy was diagnosed in 11 of these 12 patients. In the other patient, the diagnosis was cavernous hemangioma. The patients in whom procedures failed weighed 140-245 pounds (64-111 kg) with a mean weight of 162 pounds (73 kg). At statistical analysis, no significant difference (p >0.05) was found between the weight of the patients in the successful group and that of the patients in the failed group.
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The diameter of the lesions biopsied ranged from 8 to 11 cm with a mean of 2.8 cm. At 30 mAs, the mean size of masses successfully biopsied was 2.9 cm, and the mean size of masses unsuccessfully biopsied was 2.7 cm. At statistical analysis, there was no significant difference (p > 0.05) between the two groups with respect to size of the lesions biopsied. The number of CT acquisitions needed for biopsy at the low dose ranged from two to 10 with a mean of 3.2. The number of CT acquisitions needed to perform biopsy at standard doses ranged from two to eight with a mean of 3.6. At statistical analysis, there was no significant difference (p > 0.05) between the low-dose and standard-dose groups with respect to age, sex, weight, lesion size, number of CT acquisitions, and technical success of procedure.
Contrast material (100 mL of 300 mg I/mL iohexol) was administered IV to seven patients with liver masses to help identify the mass for biopsy (Fig. 3A, 3B). The IV contrast material was administered because the liver mass was not identified on the preliminary unenhanced CT scans obtained at biopsy with the standard radiation dose. In five of these seven patients, liver biopsy was completed successfully at 30 mAs. In the other two patients, tube currents of 65 and 220 mAs had to be used for lesion visualization and successful biopsy.
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CT-guided percutaneous catheter drainage at 30 mAs was successful in 80 (97.6%) of 82 patients (Fig. 4A, 4B, 4C, Table 1). The size of the collections ranged from 3 to 18 cm with a mean size of 8 cm. In the two patients with unsuccessful drainage, a catheter was placed successfully at 65 mAs. In these two patients, the contrast between the collection and the surrounding structures was insufficient at 30 mAs for safe placement of a catheter, and the radiologist decided to perform the procedure at 65 mAs. The patients in the successful drainage group weighed 120-320 pounds (54-145 kg) with a mean weight of 190 (86 kg) pounds. The patients who needed the tube current increase to 65 mAs weighed 160 and 195 pounds (73 and 88 kg).
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Comparison Group
The rate of technical success of biopsy performed at standard-dose
radiation in the 12 months preceding the low-dose study was 87.5% (91 of 104
cases). The results were malignant in 74 cases and definitively benign in 17
cases. In the other 12.5% of the patients (13 of 104), the biopsy results were
either insufficient for evaluation or were inconclusive. Eleven of these
patients underwent biopsy of a lung mass, and the other two underwent lymph
node biopsy. Also in the year preceding the study, percutaneous catheter
drainage under standard-dose CT guidance was successful in 54 (98%) of 55
patients. The one failure was inability to drain an interloop collection; the
catheter coiled between the collection and a loop of bowel.
In summary, percutaneous biopsy performed at 30 mAs had a technical success rate of 93.5% and a failure rate of 6.5%. Percutaneous catheter drainage and needle aspiration performed at 30 mAs had a technical success rate of 96.7% and a failure rate of 3.3%. For these types of procedures, there was no significant difference between the technical success group and the technical failure group with respect to patient weight, lesion size, and number of CT acquisitions. The technical success of percutaneous procedures performed with low-dose CT compared with standard-dose CT was 99% versus 87.5% for biopsy and 100% versus 98% for catheter drainage. For both types of procedure, there was no significant difference between the low-dose and the standard-dose groups with respect to age, sex, patient weight, lesion size, number of CT acquisitions, and technical success. The main cause of technical failure at 30 mAs was the lack of contrast resolution of lesions, particularly lesions of the liver.
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The need for radiation dose reduction in the patient population undergoing interventional procedures sometimes is questioned. Many of the patients are elderly and have life-threatening conditions such as malignant tumors and abscesses that necessitate treatment. Radiologists, however, must adhere to the principles of ALARA in all cases and not become patient- or procedure-specific, provided patient safety and outcome are not compromised. In addition, not all patients are elderly, nor do all patients have malignant disease. For example, young patients with multiple abscesses of Crohn's disease need repeated CT-guided percutaneous catheter drainage, and substantial dose accumulation can occur over the course of the disease. The easiest parameter to change to achieve the greatest radiation reduction is tube current. When the effective tube current is decreased to 30 mAs from tube currents of 180-240 mAs, radiation exposure to the patient decreases sixfold to eightfold. The aim of this study was to see whether a tube current of 30 mAs was as effective as a standard tube current for CT guidance in most interventional radiologic procedures.
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Lesion size did not appear to have an undue influence on technical success. Because of the relative lack of procedure failures in this study and the varied locations of the lesions, it is difficult to estimate the influence of lesion size. It seems that smaller lesions in one body part would be less visible with low-dose techniques. Lesion size was not a major consideration in our study and is unlikely to reflect a major problem in the practice of interventional radiology because many biopsies are performed in areas that have high intrinsic contrast enhancement, such as lungs and lymph nodes surrounded by fat (Fig. 6A, 6B).
A greater problem than patient weight or lesion size was the location of the target lesion. If the target lesion was located in an area of little intrinsic contrast enhancement, such as the liver, low-dose techniques were possible in most cases, but the mAs had to be increased from 30 to approximately 65 mAs in some cases. In addition, the number of CT acquisitions needed to complete an intervention at the low dose was not significantly different from that with standard dose. This finding suggests that the degree of difficulty of interventional procedures is the same at lower and higher tube currents.
Our results support the use of a low radiation dose for CT-guided procedures. In most cases, the mAs can be reduced to 30 mAs. In the few cases in which the target lesion was not clearly identified at 30 mAs, increasing the mAs to 65 mAs resulted in a technical success rate of 98% for drainage, aspiration, and biopsy combined. Only one procedure required the standard mAs of 220 mAs. An intuitive finding is that the conspicuity of lung and bone masses was least affected by the reduction in radiation dose given the intrinsic contrast between lung and bone and the soft-tissue target mass. Conversely, liver biopsy provided the greatest challenge to the low-dose technique because of the lack of inherent contrast between normal liver parenchyma and the target mass, particularly in the absence of IV contrast enhancement. Despite this factor, 18 of the 20 liver masses were biopsied successfully at 65 mAs or lower; only one procedure required 220 mAs.
Small masses, such as lymph nodes and masses in solid parenchymal organs, are challenging. The use of 30 mAs decreased the conspicuity of small lymph nodes. In 27 (93%) of 29 patients undergoing lymph node biopsy, however, biopsy was successful at 65 mAs or less. Therefore, low-dose techniques for CT-guided procedures are routinely successful. In almost all cases, the radiation exposure of the patient can be substantially decreased. The most difficult lesions with which to achieve technical success with low-dose radiation are small lesions in obese patients when the target lesion is located in an area of low intrinsic contrast enhancement. The option of increasing the mAs for any procedure or at any time during a procedure is always available to the interventional radiologist performing the procedure.
CT fluoroscopy has been promoted as an alternative to conventional CT techniques for CT-guided procedures. The use of CT fluoroscopy has prompted much discussion concerning both the time-saving nature of CT fluoroscopy and the radiation exposure of both patient and radiologist. Several studies have shown that use of CT fluoroscopy decreases the length of CT-guided procedures [11, 12], but this finding is not universally accepted [13]. Time saving for either technique can be extremely difficult to prove or disprove because interventional procedures vary greatly from patient to patient, as does operator experience. Claims concerning the radiation dose from CT fluoroscopy are ambiguous. Several authors [14, 15] have reported that the use of CT fluoroscopy exposes patients to less radiation than conventional CT-guided procedures. Others [12, 13, 16, 17] have reported that CT fluoroscopy exposes patients to increased radiation doses. CT fluoroscopy without doubt exposes radiologists to more radiation than do conventional CT techniques. The extent of this exposure is a subject of debate [12-17]. Given the ongoing debate concerning radiation exposure from CT fluoroscopy, we believe that using low-dose techniques with conventional CT guidance may be the optimal way to irrefutably decrease patient radiation exposure during CT-guided procedures.
There were two major limitations to this study. The first was that we did not perform a randomized controlled study to compare low-dose with standard-dose CT-guided procedures. Use of a control group who underwent procedures at standard doses in the year before our study introduced its own limitations owing to the retrospective nature of the data collection. Second, we did not measure contrast-to-noise ratio of the target lesion at the low dose in an objective evaluation of this factor with respect to technical outcome. Empirically, however, we found poor contrast the most important factor related to technical failure.
In conclusion, we found that most common CT-guided interventional procedures, including percutaneous biopsy and catheter drainage, can be performed safely and successfully at 30 mAs. Patient weight, lesion size, and lesion position did not influence technical success. Poor contrast resolution, particularly for lesions in the liver, was the main cause of technical failure in a few patients. However, with incremental increases in effective tube current, all lesions were clearly identified, and intervention was successfully completed.
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