Abstract
This chapter deals with PET/CT neoplasm imaging. It includes extensive information on the appropriate uses and interpretation of flourine-18 fluorodeoxyglucose scans. Other PET radiopharmacueitcals are also covered such as fluorine-18 sodium fluoride and gallium-68 agents, as are recent developments for prostate cancer imaging.
Keywords
PET/CT neoplasm imaging, FDG, fluorine-18 sodium fluoride, DOTATATE, carbon-11 choline, carbon-11 acetate, fluorine-18 fluciclovine, PMSA agents
18 F-FDG PET Imaging
Positron emission tomography (PET) in clinical practice is performed using the hybrid instrumentations of PET/computed tomography (CT) and, to a lesser extent, PET/magnetic resonance imaging (MRI). The most commonly used radiopharmaceutical is fluorine-18 fluorodeoxyglucose ( 18 F-FDG). PET physics and the radiopharmacologic properties of 18 F-FDG have been discussed in Chapter 1 , and PET/CT instrumentation and quality control (QC) have been presented in Chapter 2 . Use of 18 F-FDG for brain and thyroid neoplastic applications are also included in Chapters 3 and 4 , respectively. 18 F-FDG imaging in patients with suspected infections and inflammatory conditions is covered in Chapter 12 . PET/CT skeletal scintigraphy using 18 F sodium fluoride ( 18 F-NaF) is presented in Chapter 8 .
This chapter is primarily devoted to PET/CT imaging with 18 F-FDG and includes image interpretation and quantitation in the setting of neoplasm imaging. Although 18 F-FDG PET/CT is a powerful tool for neoplasm imaging, the sensitivity and specificity vary widely among tumor types (as does its clinical usefulness) for staging and/or treatment assessment. The second part of this chapter includes some clinically promising (non-FDG) PET tumor imaging agents.
Indications
PET/CT and PET/MRI in the setting of neoplasm are useful for a number of indications, especially when they replace other conventional imaging procedures, guide or obviate the need for invasive diagnostic or therapeutic procedures, or result in a change in patient management and/or treatment outcome. Although PET/CT is much more commonly performed, both PET/CT and PET/MRI are broadly comparable for use in whole-body oncologic imaging. PET/CT detects more lesions and is superior for lung and mediastinal imaging, whereas PET/MRI appears helpful for brain, liver, breast, bone, head and neck, and some pelvic tumors. PET/MRI remains limited by motion artifacts, particularly in the chest, diaphragm, and bowel. It also takes 50 to 80 minutes to perform, which is much longer than for PET/CT. Table 11.1 compares the relative values of common imaging procedures (including PET/CT) when diagnosing or assessing a specific neoplastic disease.
| Tumor | 18 F-FDG PET/CT | 99m Tc- MDP or 18 F-NaF † | Other NM | Radiographs | Ultrasound | CT or MRI |
|---|---|---|---|---|---|---|
| Lymphoma Lymphoma (low grade [small cell lymphocytic and MALT]) | +++ Poor | Rare | Gallium + | Rare | +++ | |
| Prostate | Poor | +++ | + | ++ | ++ | |
| Melanoma | +++ For distant metastases Poor for local disease | + | Sentinel node ++ | + | ++ CT for small lung nodules | |
| Lung (non-small cell) (bronchoalveolar cell) | +++ Poor | ++ | + | +++ | ||
| Myeloma | ++ | + | ++ | +++ MRI | ||
| Ovary | ++ 70–90% | +++ Also for screening | +++ 95% MRI, 50–90% CT | |||
| Uterus | ++ 90% | + | +++ 90% | |||
| Cervix | ++ 75–90% | ++ | ||||
| Colorectal | +++ Mostly for regional and distant metastases | Rare | Barium enema poor | Endoscopic +++ | +++ MRI detects smaller hepatic metastases than PET/CT | |
| Bladder | Poor | + | ++ | |||
| Kidney | + | + | ++ | +++ | ||
| Thyroid | ++ | Rare | 123 I-NaI | ++ | ++ | + |
| Thyroid medullary cancer | +++ 96% | Octreotide 41% | ++ | |||
| Head and neck | +++ | Rare | + | +++ | ||
| Esophagus | ++ Not for local spread; use for regional and distant disease | For T staging endoscopic +++ | +++ | |||
| Stomach | ++ 50% Not for local spread; use for regional and distant disease | Rare | UGI poor | For T staging endoscopic +++ | +++ | |
| Osteosarcoma | ++ | +++ | ++ | +++ | ||
| Sarcoma (low grade) | Poor | + | +++ | |||
| Breast | ++ Not for primary or axillary disease; use for distant disease and chemotherapy response | +++ | Sentinel node ++PEM (adjunctive) ++ | +++ Mammography | + | + |
| Hepatoma | + 30% have uptake at staging | Rare | Gallium + | +++ | ||
| Pancreas | ++ | Rare | Endoscopic ++ | +++ | ||
| Carcinoid | Poor | 90% for bone metastases | 111 In-Octreotide 80–95% 123 I-MIBG 35–85% DOTA compounds ‡ | + | ||
| Gastrinoma | Octreotide 75–93% DOTA compounds ‡ | + | ||||
| VIPoma | 111 In-Octreotide 88% DOTA compounds ‡ | + | ||||
| Poorly differentiated neuroendocrine | ++ | No | ||||
| Benign pheochromocytoma | + 60% | 123 I-MIBG 85–100% 111 In-Octreotide 65–75% | +++ | |||
| Malignant pheochromocytoma | ++ 75–85% | 123 I-MIBG 85–100% 111 In-Octreotide 87% DOTA compounds ‡ | +++ | |||
| Paraganglioma | ++ 74–88% | 111 In-Octreotide 94% 123 I-MIBG 57–78% DOTA compounds ‡ | 95–100% localized; 45% for metastases | |||
| Neuroblastoma | Stage 1 and 2 only | Stage 4 123 I-MIBG for bone and bone marrow disease DOTA compounds ‡ | ++ |
* Value (+ [low] to +++ [high]) is the opinion of the authors, based on clinical practice and the peer-reviewed literature.
‡ DOTA tracers have not been in clinical use long enough to assess accuracy in subtypes of NETs.
Although PET/CT is very helpful for staging certain tumors, the value of intertreatment scans and post-treatment surveillance scans can be variable. Intertreatment scans can be useful to alter therapy by detecting progression or stability of disease, indicating ineffectiveness of treatment and the need for other approaches, or noting a favorable response that allows moderation or even cessation of therapy. Follow-up scans after treatment are indicated if there are other equivocal imaging tests, clinical suspicion of recurrence, or changing tumor markers. Routine 18 F-FDG PET/CT surveillance follow-up for cancer in asymptomatic patients should generally be avoided. Such monitoring does not significantly improve outcome and is associated with false positives, leading to unnecessary invasive tests, increased cost, increased radiation, and anxiety.
Patient Preparation for 18 F-FDG Imaging
The biodistribution of 18 F-FDG is affected by blood glucose levels. Although there can be competitive inhibition of the 18 F-FDG uptake by high levels of blood glucose, the primary adverse effect of elevated serum glucose is the resultant elevation of insulin levels. Patients should fast for 4 to 6 hours before a scan (preferably overnight) so that basal insulin levels will allow for optimal images. Elevated insulin levels may degrade scans by increasing cardiac and skeletal muscle uptake of FDG. In general, serum glucose levels should be less than 150 mg/dL, but glucose levels up to 200 mg/dL are usually acceptable for satisfactory image quality. Imaging patients with diabetes can be especially challenging. In those taking long-acting insulin the evening before the study, imaging should be performed in the early morning after an overnight fast. If possible, regular insulin should not be used 2 to 4 hours before the examination. Patients taking regular insulin in the morning should do so along with breakfast by 6 am and undergo imaging in the late morning or early afternoon. Although short-acting insulin may be useful in some patients, in general the use of insulin is best avoided altogether. In type 2 diabetics, metformin may be discontinued, if possible, 48 hours before the study to prevent any interfering bowel activity, especially if gastrointestinal pathology is a consideration.
Because there is significant excretion of 18 F-FDG via the kidneys (and 18 F-FDG is not reabsorbed as glucose is), good hydration is recommended. Accumulation in the bladder requires that the patient void as completely as possible before beginning the scan, especially if pelvic pathology is suspected. Diuretics may be helpful in some patients when clearing activity from the kidneys and ureters is crucial. Extreme exercise should be avoided for 24 hours prior to the examination to decrease muscle uptake.
The typical administered activity in adults for 18 F-FDG is in the range of 10 to 25 mCi (370 to 925 MBq), or about 0.14 mCi (5.5 MBq)/kg. The injection is intravenous, and the line should be flushed with 20 to 30 mL of saline. Portacaths or indwelling catheters should not be used unless absolutely necessary because retention in the reservoirs and catheter tips can cause errors in evaluation of the chest. If axillary or supraclavicular lymph node involvement is a consideration (e.g., breast cancer or upper extremity melanomas), the injection should be made in the upper extremity on the opposite side of the lesion. Imaging is typically performed 45 to 60 minutes after injection of 18 F-FDG. The patient should not talk or chew gum for 30 minutes after injection of the 18 F-FDG because this causes increased uptake in the muscles of the larynx and pharynx and the mastication musculature. For brain imaging, the patient should be injected in a dimly lit, quiet room to avoid excess brain stimulation, which will alter FDG distribution.
The patient is usually positioned with the arms up except when the suspected pathology is in the head or neck. In this case, the arms should be down. In some cases when pathology is suspected in both the neck and the chest, it may be necessary to perform scans with the arms in both positions to detect small lesions. In patients with neck muscle pain, muscle relaxants (e.g., Valium) may be useful to decrease interfering activity in neck musculature.
Normal 18 F-FDG Distribution and Variants
General
The percentage of administered 18 F-FDG activity distributed in major tissues is approximately as follows: urine (second hour) 20% to 40%, brain 7%, liver 4.5%, heart 3.3%, red marrow 1.7%, kidneys 1.3%, and lungs 0.9%. It is important to know the normal distribution and normal variants in 18 F-FDG accumulation ( Fig. 11.1 ) so as not to confuse them with actual pathology ( Tables 11.2, 11.3, and 11.4 ) and to reduce false-positive results. Approximately 40% of referring clinicians report overinterpretation as a major concern as many interpreting physicians are more concerned about missing a neoplastic focus than having a false-positive result. However, in some instances, PET scans may actually be positive before lesions become clinically or histopathologically apparent.
| Tissue Level | Comment |
|---|---|
| High Tissue Level | |
| Brain (cerebellum) 8.2 | High in gray matter, low in white matter |
| Tonsils 3.4–4.1 | |
| Kidneys, ureters, bladder | Due to excretion |
| Ascending colon (contents) | Usually tubular in distribution |
| Myocardium 4.3 | If fasting; if not fasting, SUV can exceed 20 |
| Brown or USA fat | Frequently females and in cold environment |
| Uterus | During menses |
| Moderate Tissue Level | |
| Submandibular glands 2.2 | Decreases with age |
| Liver 2.1 | |
| Blood 2.3 | |
| Testes 2.7 | |
| Mild Tissue Level | |
| Blood pool 1.0 (standard) | |
| Aorta | Measured in descending aorta at level of hilum |
| Spleen 1.8 | |
| Nasopharynx 1.7 | |
| Esophagus 1.9 | |
| Low Tissue Level | |
| Thyroid 1.4 | Can be high with Graves disease or thyroiditis |
| Stomach 1.6–2.3 | Can be focally intense |
| Pancreas 1.5 | |
| Rectum and colon 1.0–1.6 | Contents may be much higher |
| Adrenal gland 1.4 | |
| Vertebrae 1.6 | |
| Red marrow 1.0 | Can be high after stimulating agents |
| Penis | |
| Vagina | |
| Ovaries | Depends on menstrual cycle; lowest after menstruation |
| Minimal or No Activity Usually Seen | |
| Skeletal muscle 0.9 | Much higher if active or tense |
| Lung 0.4–0.7 | Higher value in lower vs. upper lung; values decline with age |
| Breast 0.6 | More in young women and with lactation |
| Lymph nodes | |
| Ovaries | Postmenopausal |
| Tissue/Organ | Activity Level | Comments |
|---|---|---|
| Brain | ||
| Ictal seizure focus | High | Very rarely done due to need to remain still and poor temporal resolution of PET |
| Interictal seizure focus | Low | Review temporal lobes |
| Radiation necrosis | Low | |
| Recurrent tumor | Variable | If increased activity, suspect recurrence |
| Dementia (Alzheimer) | Low posterior temporoparietal cortical activity | Often identical pattern to Parkinson dementia |
| Dementia (FTLD-Pick) | Low frontal lobes | |
| Dementia (Multi-infarct) | Scattered small areas of decreased activity | |
| Cerebellar diaschisis (“crossed”) | Low area in one hemisphere | Low activity in cerebellum contralateral to supratentorial stroke, tumor, trauma, etc. |
| Huntington disease | Low activity in caudate nucleus and putamen | |
| Heart | ||
| Infarct | Low | Minimal myocardial activity if patient has been fasting; very intense if meal with large amounts of glucose recently ingested |
| Hibernating | Normal or increased | |
| Stunned | Normal or increased | |
| Neoplasm | ||
| Head and neck | Variable: with cell type | |
| Brain metastases | Low | Poor sensitivity against normal high activity of gray matter |
| Thyroid | Moderate | Especially helpful in poorly differentiated lesions with low or absent iodine-131 uptake |
| Lung | Moderate | Low in bronchoalveolar cell cancer, (adenocarcinoma in situ), carcinoid, well-differentiated cancer, and mucinous metastases |
| Breast | Moderate | |
| Hepatoma | Variable: often low | |
| Esophagus | High | Usually focal, not linear or diffuse |
| Stomach | Variable | Interfering normal activity |
| Colorectal | High | Usually focal, not tubular |
| Melanoma | High | |
| Lymphoma | Variable | Higher in aggressive forms |
| Renal and bladder | Variable: often low | Interfering normal activity |
| Skeletal metastases | Variable | |
| Uterine | Variable | |
| Cervical | Variable | |
| Testicular | Variable | |
| Prostate | Low or absent | |
| Infection | ||
| Pneumonia | Moderate to high | |
| Cellulitis | Moderate | |
| Osteomyelitis | High | |
| Granulomas | ||
| Sarcoidosis | High | If active disease |
| Tuberculosis | High | If active disease |
| Trauma | ||
| Recent surgery | Variable | |
| Fracture | Variable with age of onset | |
| Aortic graft | Moderate | |
| Radiation therapy | Moderate | May be increased for up to several months |
| Anatomic Region | Normal Distribution | Normal Variants | Uptake in Benign Lesions, Potential False (+) | Low-Uptake Lesions, Potential False (−) |
|---|---|---|---|---|
| Head/neck | Brain: gray matter, cortex, basal ganglia Neck: soft palate, tongue, vocal cords, palatine tonsils, adenoids, parotids, salivary glands, thyroid Neck muscles: laryngeal, masticators, genioglossus | Head: extraocular muscles Neck: brown fat | Head: sinusitis Neck: thyroiditis, Graves disease, goiter, benign thyroid nodules, Warthin tumor, reactive lymph nodes | Brain metastases Low-grade gliomas |
| Chest | Mediastinum: heart muscle and atria, thymus in children and young adults, esophagus | Mediastinum: thymic rebound, base of ventricles, brown fat Axilla: lymph nodes (dose infiltration) Breast: premenopausal, hormone therapy | Aortic atherosclerosis Esophagitis, Barrett esophagus, after esophageal dilation procedure Hiatal hernia Empyema Pleurodesis Pneumonia Radiation pneumonitis Granulomatous diseases: tuberculosis, sarcoidosis, histoplasmosis, aspergillosis, coccidioidomycosis Mycobacterium avium-intracellulare, atypical mycobacteria Reactive lymph nodes Breast: inflammation, biopsy site, mastitis, fibroadenomas (low), gynecomastia | Lung: bronchioloalveolar cancer (adenocarcinoma in situ), carcinoid, solitary pulmonary nodule < 1 cm Breast: lobular carcinoma, carcinoma in situ, tubular carcinoma |
| Abdomen | Kidneys Ureters Bladder Stomach Small bowel Colon Liver Spleen (low) | Brown fat: perinephric | Vascular bypass grafts Colonic Adenoma/polyps Adrenal hypertrophy Pancreatitis | MALT Liver: hepatocellular cancer (40–50%), small lesions Kidney: renal cell cancer |
| Pelvis | Ureters Bladder Bowel Uterus (menses) Testes Penis (low) | Uterine fibroids Endometriosis Corpus luteum cysts Vascular bypass grafts | ||
| Skeleton/marrow | Skeletal muscle Brown fat: supraclavicular, paravertebral, intercostal | Marrow: G-CSF Tx (intense) Rad Tx (decreased) | Inflammation/infection Arthritis Phlebitis Spondylodiscitis Pigmented villonodular synovitis Benign bone lesions: fibrous dysplasia, Paget disease, nonossifying fibroma, giant cell tumor, eosinophilic granuloma, aneurysmal bone cyst, enchondroma, osteomyelitis | Chondrosarcoma Plasmacytoma Low-grade sarcomas (osseous and soft tissue) Sclerotic metastases |
| Skin | Inflammation/infection Wound healing | |||
| Any location | Percutaneous lines/tubes | Inflammation/infection Granulomatous diseases Postradiation change Wound healing Reactive lymph nodes Tense musculature Resolving hematoma Ostomies Vascular grafts Atheromatous disease Healing fractures Arthritis | Small lesions, nodal micrometastases Low-grade tumors, especially lymphomas (small lymphocytic cell) Metastases adjacent to fluorodeoxyglucose–avid primary cancer or high-activity organs |
Brain
The gray matter of the brain (cortex, basal ganglia, and thalami) is always high in 18 F-FDG uptake because the cells of gray matter have a high metabolic rate and use glucose as their primary substrate. When acquiring scans of the face and neck, it is important to exclude the brain from the acquisition to be able to see abnormalities that do not have as much activity as gray matter.
Vocal Cords
In the neck, activity near the vocal cords is seen if the patient was talking at the time of injection ( Fig. 11.2 ). The activity can be minimized by having the patient remain silent during injection and during the early uptake of 18 F-FDG. Speaking during the uptake period may also increase 18 F-FDG activity in the tongue. After lingual or laryngeal surgery, asymmetric activity can often be seen in the residual intact musculature and be difficult to differentiate from residual or recurrent tumor. An intense 18 F-FDG focus in the lower neck just lateral to the midline may be caused by compensatory activation of an intact laryngeal muscle when the contralateral vocal cord is paralyzed due to any cause, including mediastinal tumor involvement of the recurrent laryngeal nerve.
Tonsils, Salivary Glands, and Thyroid
The tonsils (especially the palatine tonsils), lymphoid tissue at Waldeyer ring, and the parotid and submandibular glands can normally accumulate 18 F-FDG. This activity usually decreases with patient age and should be symmetric. Thyroid activity is normally not seen. However, mild diffuse activity can occasionally be seen in normal glands. Significantly increased activity can occur with thyroiditis or Graves disease ( Fig. 11.3 ). An enlarged inhomogeneous gland should raise the possibility of a nodular goiter. An unexpected intense focal area raises the possibility of thyroid malignancy (20% to 45%), and further evaluation should be performed.
Thymus
Thymic activity can be seen normally in children but also occasionally in young adults up to the age of about 30 years. It is very uncommon in adults older than 30 years. Increased activity can be seen as a result of thymic rebound after chemotherapy in children and young adults but also occasionally in older adults ( Fig. 11.4 ).
Muscle and Brown Fat
In the resting skeletal muscle, 18 F-FDG uptake is usually low. Increased activity is often seen in the shoulder girdle (especially teres minor) and upper back if the patient was tense at the time of injection or within 30 minutes after injection. Muscle relaxants or antianxiety medications may be helpful in some patients, especially when imaging the neck, where the strap muscles often demonstrate increased activity. Heavy exercise within the 24 hours before the examination can increase muscle uptake, as can elevated insulin levels ( Fig. 11.5 ).
Activity can also be seen normally in the diaphragmatic crura, intercostal muscles, psoas muscles, thoracic paravertebral muscles, forearms, and muscles of mastication. In patients with severe dyspnea, there is often increased activity in intercostal muscles as a result of the increased work of breathing (see Fig. 11.2 ). At any location, symmetry and a diffuse or typical linear configuration are often helpful to distinguish normal muscle activity from pathology.
Activity in the supraclavicular region is typically a result of one of three causes: muscle activity, activity in lymph nodes resulting from pathology, or accumulation in brown fat. Muscle activity is seen in about 5% of patients and is usually linear and bilateral. Activity in brown fat occurs in about 2% to 5% of patients (mostly female) and in about 15% of children. Uptake in brown fat is typically bilateral, symmetric, and more often focal than linear wherever it occurs ( Fig. 11.6 ). Standardized uptake values (SUVs) can range from 2 to 20, with the mean often about 7 or more (well above the threshold value worrisome for malignancy) and thus, if not recognized as benign, may be the cause of false-positive interpretations. Uptake appears to be related in part to sympathetic stimulation during periods of anxiety or shivering in response to ambient temperature and is seen more often with an acute lowering of temperature in the winter months or even in a cold, air-conditioned room. In patients with activity in neck fat, about one-third will also show focal activity in mediastinal, paravertebral, axillary, supraclavicular, posterior intercostal, and abdominal (especially perinephric, perihepatic, and para-aortic) locations ( Fig. 11.7 ). Several interventions have been suggested to prevent uptake of FDG in brown fat, including a warmer study environment or premedication with benzodiazepine or propranolol. However, with coregistered PET/CT images localizing this activity to fat in typical symmetric patterns, these interventions are usually unnecessary.
Heart
Left ventricular 18 F-FDG activity is usually faint, but can be intense under certain conditions. Although the myocardium uses fatty acids as its primary substrate, it will switch to glucose if high levels are available or if the patient has been exercising. A large amount of cardiac activity often means that the patient did not fast in preparation for the examination, and the heart is actively using readily available glucose (see Fig. 11.1 ). Intense cardiac activity is usually not a problem but can interfere with a diagnosis of pathology located adjacent to the heart.
After 12 hours of fasting, the heart switches from glucose to fatty acid metabolism, and the activity becomes the same as the background blood pool. The switch from intense activity to faint activity is often not uniform, and the base of the heart tends to be the last section to convert from glucose metabolism. It is important not to interpret lack of activity near the left ventricular apex as an infarct without additional information. It is also important not to mistake isolated activity at the base of the heart for pathology such as abnormal mediastinal lymph nodes. The right ventricle usually has faint activity when compared with that of the left ventricle, unless there is right ventricular hypertrophy. Atrial activity is not infrequently noted, especially in the right atrium. This can be spotty and, again, should not be mistaken for abnormal lymph nodes.
Aorta
18 F-FDG activity is seen in the aortic wall of about 60% to 70% of older adults. The uptake can be focal or bandlike and is usually more intense in the lower descending thoracic aorta than in the ascending portion or arch. Thoracic aortic uptake appears to be more common in women and patients with hyperlipidemia. The degree of activity appears to be unrelated to calcium deposition but has been shown to correlate with macrophage content in the atherosclerotic plaques, possibly reflecting the metabolic activity of atherosclerotic change. Infected vascular grafts often demonstrate increased activity, conforming to the shape of the grafts. However, aortofemoral bypass grafts routinely have some mild diffuse increased activity for years even if not infected.
Lungs
The lungs have relatively low uptake, appearing photopenic on attenuation-corrected images. However, on images that are not attenuation corrected, they will appear to have increased activity. Pleural effusions from congestive failure will not accumulate activity, but empyemas and malignant effusions often do.
Breast
Breast activity slightly above that in the blood pool is normal and is more common in young women and in postmenopausal women on hormone replacement therapy. The 18 F-FDG activity in the breast is related to the glandular volume and density. As expected, fatty breasts have less activity than do dense breasts. However, even in dense breasts, the SUV is almost always less than 1.5 (much below the 2.5 value that may suggest malignant tissue). Breast activity is notably greater in lactating females ( Fig. 11.8 ), but the activity is in the breast tissue and not in the milk. It is not necessary to discontinue breastfeeding in these patients. However, there may be significant dose concerns to the baby from direct radiation emission from the mother, and as a result, some authors recommend that breastfeeding be suspended for 8 to 12 hours after the injection. Thus an infant feeding just before patient injection may be advised.
Gastrointestinal Tract
Activity in the bowel is exceedingly variable in terms of both intensity and location. The 18 F-FDG activity in the bowel is located in the mucosa and not in the luminal contents. The FDG is not excreted by the liver and biliary system into the bowel. Low-level activity is sometimes seen in the normal esophagus and should be uniform throughout its length. Fusiform or focal esophageal activity should raise the possibility of pathology, including neoplasm, although esophagitis or inflammation related to gastroesophageal reflux can produce a similar appearance.
Activity in the stomach is often greater than that in the liver, and a contracted normal stomach or hiatal hernia can appear very intense, causing difficulties in evaluation of esophagogastric junction and gastric neoplasms. Focal activity in the stomach should prompt a search for a gastric mass on the CT. Small-bowel activity is usually less than that seen in the colon and is commonly located in the lower abdomen or pelvis. Activity in the small bowel tends to be faint, but activity in the colon can normally be quite intense. It is often helpful to view the rotating three-dimensional display (maximum intensity projection [MIP] image) to be able to distinguish normal colonic activity from pathology. The pattern of uptake in the colon may be diffuse, segmental, or nodular. It is usually highest in the cecum and ascending colon ( Fig. 11.9 ) presumably because of the higher concentration of lymphocytes in this region. Diffuse activity is most often nonpathologic, whereas segmental uptake may imply inflammation. Nodular focal intense uptake is seen in about 1% of patients having studies for nongastrointestinal reasons. This has a positive predictive value of 70% to 85% because of lesions such as polyps, villous adenomas, and carcinoma. Approximately 30% of such findings will prove to be malignant, and the majority of the remainder will be premalignant. Metformin is known to be a cause of increased small and large bowel activity, which may interfere with evaluation of known or suspected lesions in these regions.
