Electromyographic activity in lower limb muscles is temporally associated with the slow phase of oxygen uptake during cycling

Although the “slow” phase of pulmonary oxygen uptake () appears to represent energetic processes in contracting muscle, electromyographic evidence tends not to support this. The present study assessed normalized integrated electromyographic (NIEMG) activity in eight muscles that act about the hip, knee and ankle during 8 min of moderate (<ventilatory threshold) and very heavy (>ventilatory threshold) cycling in six male cyclists. was measured breath by breath during four repeated trials at each of the two intensities. Moderate and very heavy exercise followed a 4‐min period of light exercise (50 W). During moderate exercise the slow phase was absent and NIEMG in all muscles did not increase after the first minute of exercise. During very heavy exercise, the slow phase emerged (time delay=58 ± 16 s) and increased progressively (time constant=120 ± 35 s) to an amplitude (0.83 ± 0.16 L/min) that was approximately 21% of the total response. This slow phase coincided with a significant increase in NIEMG in most muscles, and differences in NIEMG activities between the two intensities revealed “slow” muscle activation profiles that differed between muscles in terms of the onset, amplitude and shape of these profiles. This supports the hypothesis that the slow phase is a function of these different slow muscle activation profiles.     

Development of aerobic and anaerobic power in adolescent rowers: a 5‐year follow‐up study

We aimed to determine whether the physical and physiological superiority of early‐maturing rowing athletes, observed at ages 12–13 years, over that of their late‐maturing counterparts observed at the same ages, still persists at 17–18 years of age, when all adolescent athletes are expected to have completed pubertal development. We hypothesized that this superiority of early maturers would not be observed at reassessment, as late maturers would have likely “caught up” with their early‐maturing peers. Twenty‐one male rowers were assessed at age 12.8 ± 0.5 years and again at 17.5 ± 0.5 years (mean ± SD). They were divided into groups of early‐maturing and late‐maturing rowing athletes based on Tanner's sexual maturity ratings. A two‐way repeated‐measures MANOVA followed by a series of ANOVAs with one within‐subject factor (time) and one between‐subject factor (group) indicated significant (P≤0.003) within‐subject and between‐subjects main effects for lean body mass (LBM), maximal oxygen uptake (), and mean power (MP). The group × time interaction effects were significant for LBM (P=0.003), (P=0.004), but not for MP (P=0.171). Over 5 years, early‐maturers' advantage dwindled in terms of LBM (+38% to +9%), (+47% to +9%), and MP (+76% to +15%); however, these differences may still be considered practically relevant. The proposed hypothesis was not supported.     

Skyscraper running: physiological and biomechanical profile of a novel sport activity

Skyscraper running is here analyzed in terms of mechanical and metabolic requirements, both at the general and at the individual level. Skyscraper runners' metabolic profile has been inferred from the total mechanical power estimated in 36 world records (48–421 m tall buildings), ranked by gender and age range. Individual athlete's performance (n=13) has been experimentally investigated during the Pirelli Vertical Sprint, with data loggers for altitude and heart rate (HR). At a general level, a non‐linear regression of Wilkie's model relating maximal mechanical power to event duration revealed the gender and age differences in terms of maximum aerobic power and anaerobic energy resources particularly needed at the beginning of the race. The total mechanical power was found to be partitioned among: the fraction devolved to raise the body center of mass , the need to accelerate the limbs with respect to the body , and running in turns between flights of stairs . At the individual level, experiments revealed that these athletes show a metabolic profile similar to middle‐distance runners. Furthermore, best skyscraper runners maintain a constant vertical speed and HR throughout the race, while others suddenly decelerate, negatively affecting the race performance.     

Relationship between aerobic and anaerobic parameters and functional classification in wheelchair basketball players

Participation in sports for individuals with disabilities continues to gain popularity. In order to provide fair and equitable competition among persons with different disabilities and functional capacity, a separate functional classification system has been devised for each sport. The aims of the present study were to evaluate aerobic and anaerobic performance of wheelchair basketball athletes and verify a correlation with the International Wheelchair Basketball Federation functional classification system. For this, 17 highly trained male Brazilian basketball wheelchair athletes (25.4±4.4 years) from the national team who had taken part in the Athens 2004 Paralympic Games were assessed. These athletes were submitted to cardiopulmonary exercise testing and Wingate‐like 30‐s sprint test using upper limbs. The present study demonstrated that the functional classification score correlated with relative (r=0.90; P<0.0001) and absolute peak power (r=0.50; P=0.0353) and absolute mean power (r=0.93; P<0.0001) obtained from the Wingate‐like 30‐s sprint test and also correlated with absolute O₂ peak (r=0.68; P=0.0026) and O₂ at ventilatory threshold (r=0.71; P=0.0014), measured on cardiopulmonary exercise testing. Therefore, our findings support the functional classification created to classify athletes' functional capability on the court, which also correlated with aerobic and anaerobic performance parameters of the elite wheelchair basketball players.     

Effects of nitrate supplementation in trained and untrained muscle are modest with initial high plasma nitrite levels

Nitrate () supplementation resulting in higher plasma nitrite () is reported to lower resting mean arterial blood pressure (MAP) and oxygen uptake (VO₂) during submaximal exercise in non‐athletic populations, whereas effects in general are absent in endurance‐trained individuals. To test whether physiologic effects of supplementation depend on local muscular training status or cardiovascular fitness, male endurance‐trained cyclists (CYC, n=9, VO₂‐max: 64±3 mL/min/kg; mean±SD) and recreational active subjects serving as a control group (CON, n=8, 46±3 mL/min/kg), acutely consumed nitrate‐rich beetroot juice ([] ~9 mmol) (NIT) or placebo (PLA) with assessment of resting MAP and energy expenditure during moderate intensity (~50% VO₂‐max) and incremental leg cycling (LEG‐ex) and arm‐cranking exercise (ARM‐ex). NIT increased (P<.001) resting plasma by ~1200% relative to PLA. Plasma increased ~25% (P<.01) with a significant change only in CYC. LEG‐ex VO₂ (~2.60 L/min), ARM‐ex VO₂ (~1.14 L/min), and resting MAP (~87 mm Hg) remained unchanged for CYC, and similarly for CON, no changes were observed for LEG‐ex VO₂ (~2.03 L/min), ARM‐ex VO₂ (~1.06 L/min), or resting MAP (~85 mm Hg). VO₂‐max was not affected by supplementation, but incremental test peak power was higher (P<.05) in LEG‐ex for CYC in NIT relative to PLA (418±47 vs 407±46 W). In both CYC and CON, high initial baseline values and small increases in plasma after NIT may have lowered the effect of the intervention implying that muscular and cardiovascular training status is likely not the only factors that influence the physiologic effects of supplementation.     

Effects of short‐term training and detraining on VO2 kinetics: Faster VO2 kinetics response after one training session

This study examined the time course of short‐term training and detraining‐induced changes in oxygen uptake () kinetics. Twelve men (24 ± 3 years) were assigned to either a 50% or a 70% of training intensity (n = 6 per group). was measured breath‐by‐breath. Changes in deoxygenated‐hemoglobin concentration (Δ[HHb]) were measured by near‐infrared spectroscopy. Moderate‐intensity exercise on‐transient and Δ[HHb] were modeled with a mono‐exponential and normalized (0–100% of response) and the ratio was calculated. Similar changes in time constant of () were observed in both groups. The combined group mean for decreased ～14% (32.3 to 27.9 s, P < 0.05) after one training session with a further ～11% decrease (27.9 to 24.8 s, P < 0.05) following two training sessions. The remained unchanged throughout the remaining of training and detraining. A significant “overshoot” in the ratio was decreased (albeit not significant) after one training session, and abolished (P < 0.05) after the second one, with no overshoot observed thereafter. Speeding of kinetics was remarkably quick with no further changes being observed with continuous training or during detraining. Improve matching of local O₂ delivery to O₂ utilization is a mechanism proposed to influence this response.     

Peak oxygen uptake and regional oxygenation in response to a 10‐day confinement to normobaric hypoxia

We investigated the effect of hypoxic acclimatization per se, without any concomitant influence of strenuous physical activity on muscle and cerebral oxygenation. Eight healthy male subjects participated in a crossover‐designed study. In random order, they conducted a 10‐day normoxic (CON) and a 10‐day hypoxic (EXP) confinement. Pre and post both CON and EXP confinements, subjects conducted two incremental‐load cycling exercises to exhaustion; one under normoxic, and the other under hypoxic (F_IO₂ = 0.154) conditions. Oxygen uptake (), ventilation (), and relative changes in regional hemoglobin oxygenation (Δ([HbO₂]) in the cerebral cortex and in the serratus anterior (SA) and vastus lateralis (VL) muscles were measured. No changes were observed in the CON confinement. Peak work rate and were similar pre and post in the EXP confinement, whereas increased in the EXP post normoxic and hypoxic trials (P < 0.05). The exercise‐induced drop in VL Δ[HbO₂] was less in the post‐ than pre‐EXP trial by 4.0 ± 0.4 and 4.2 ± 0.6 μM during normoxic and hypoxic exercise, respectively. No major changes were observed in cerebral or SA oxygenation. These results demonstrate that a 10‐day hypoxic exposure without any concomitant physical activity had no effect on normoxic or hypoxic , despite the enhanced VL oxygenation.     

Relationship between changes in pulmonary kinetics and autonomic regulation of blood flow

Various regulatory mechanisms of pulmonary oxygen uptake () kinetics have been postulated. The purpose of this study was to investigate the relationship between vagal withdrawal, measured using RMSSDRR, the root mean square of successive differences in cardiac interval (RR) kinetics, a mediator of oxygen delivery, and kinetics. Forty‐nine healthy adults (23 ± 3 years; 72 ± 13 kg; 1.80 ± 0.08 m) performed multiple repeat transitions to moderate‐ and heavy‐intensity exercise. Electrocardiography, impedance cardiography, and pulmonary gas exchange parameters were measured throughout; time domain measures of heart rate variability were subsequently derived. The parameters describing the dynamic response of , cardiac output () and RMSSDRR were determined using a mono‐exponential model. During heavy‐intensity exercise, the phase II τ of was significantly correlated with the τ of RR (r = 0.36, P < 0.05), Q (r = 0.67, P < 0.05), and RMSSDRR (r = 0.38, P < 0.05). The τ describing the rise in Q explained 47% of the variation in τ, with 30% of the rate of this rise in Q explained by the τ of RR and RMSSDRR. No relationship was evident between kinetics and those of Q, RR, or RMSSDRR during moderate exercise. Vagal withdrawal kinetics support the concept of a centrally mediated oxygen delivery limitation partly regulating kinetics during heavy‐, but not moderate‐, intensity exercise.     

A 4‐week instructed minimalist running transition and gait‐retraining changes plantar pressure and force

The purpose of this study is to compare changes in plantar pressure and force using conventional running shoes (CRS) and minimalist footwear (MFW) pre and post a 4‐week MFW familiarization period. Ten female runners (age: 21 ± 2 years; stature: 165.8 ± 4.5 cm; mass: 55.9 ± 3.2 kg) completed two 11 km/h treadmill runs, 24 hours apart, in both CRS and MFW (pretest). Plantar data were measured using sensory insoles for foot strike patterns, stride frequency, mean maximum force (), mean maximum pressure () and eight mean maximum regional pressures. Subjects then completed a 4‐week familiarization period consisting of running in MFW and simple gait‐retraining, before repeating the tests (posttest). During the pretests, 30% of subjects adopted a forefoot strike in MFW, following familiarization this increased to 80%; no change occurred in CRS. A significant decrease in in both MFW and CRS (P = 0.024) was observed from pre‐post, and a significant decrease in heel pressures in MFW. was higher in MFW throughout testing (P < 0.001).A 4‐week familiarization to MFW resulted in a significant reduction in in both the CRS and MFW conditions, as well as a reduction in heel pressures. Higher was observed throughout testing in the MFW condition.     

Regional variations of T2* in healthy and pathologic achilles tendon in vivo at 7 Tesla: Preliminary results

The aim of this study was to investigate T in the Achilles tendon (AT), in vivo, using a three‐dimensional ultrashort time echo (3D‐UTE) sequence, to compare field strength differences (3 and 7 T) and to evaluate a regional variation of T in healthy and pathologic tendon. Ten volunteers with no history of pain in the AT and five patients with chronic Achilles tendinopathy were recruited. 3D‐UTE images were measured with the following echo times, at echo time = [0.07, 0.2, 0.33, 0.46, 0.59, 0.74, 1.0, 1.5, 2.0, 4.0, 6.0, and 9.0 ms]. T values in the AT were calculated by fitting the signal decay to biexponential function. Comparing volunteers between 3 and 7 T, short component T was 0.71 ± 0.17 ms and 0.34 ± 0.09 ms (P < 0.05); bulk long component T was 12.85 ± 1.87 ms and 10.28 ± 2.28 ms (P < 0.05). In patients at 7 T, bulk T was 0.53 ± 0.17 ms (P = 0.045, compared to volunteers), T was 11.49 ± 4.28 ms (P = 0.99, compared to volunteers). The results of this study suggest that the regional variability of AT can be quantified by T in in vivo conditions. Advanced quantitative imaging of the human AT using a 3D‐UTE sequence may provide additional information to standard clinical imaging. Finally, as the preliminary patient data suggest, T may be a promising marker for the diagnosis of pathological changes in the AT. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Aerobic capacity in speed‐power athletes aged 20–90 years vs endurance runners and untrained participants

We studied relationships between age and aerobic capacity in three groups of subjects adhering to different exercise modalities. A total of 203 men aged 20–90 years were examined: 52 speed‐power track and field athletes (SP), 89 endurance runners (ER) and 62 untrained individuals (UT). Maximal exercise characteristics were obtained during a graded treadmill test until exhaustion: oxygen uptake (), heart rate (HR_max), oxygen pulse (O₂ Pulse_max) and maximal distance (Dist_max). Information about training history and weekly training amount was collected. A linear model of regression was adopted. in SP was lower than in ER, but significantly higher than in UT. The cross‐sectional rates of decline in body mass‐adjusted and Dist_max were significantly smaller in SP than in ER and UT. About 80 years of age, the levels of and Dist_max reached similar values in SP and ER. The decline in HR_max, but not in O₂ Pulse_max was suggested as a cardiac adaptation accounting for between‐group differences in loss. Weekly training volume was a significant positive predictor of age‐related changes in aerobic capacity. In conclusion, not only endurance, but also speed‐power exercise appears adequate to ensure an elevated aerobic capacity at old age.     

Influence of 4 weeks of bovine colostrum supplementation on neutrophil and mucosal immune responses to prolonged cycling

Bovine colostrum (COL) has been advocated as a nutritional countermeasure to exercise‐induced immune dysfunction. The aims of this study were to identify the effects of 4 weeks of COL supplementation on neutrophil responses and mucosal immunity following prolonged exercise. In a randomized double‐blind, parallel group design, participants [age 28 ± 8 years; body mass 79 ± 7 kg; height 182 ± 6 cm; maximal oxygen uptake () 55 ± 9 mL/kg/min] were assigned to 20 g per day of COL (n = 10) or an isoenergetic/isomacronutrient placebo (PLA; n = 10) for 4 weeks. Venous blood and unstimulated saliva samples were obtained before and after 2.5 h of cycling at 15% Δ (～55–60% ). A significantly greater formyl‐methionyl‐leucyl phenylalanine‐stimulated oxidative burst was observed in the COL group compared with PLA group (P < 0.05) and a trend toward a time × group interaction (P = 0.06). However, there was no effect of COL on leukocyte trafficking, phorbol‐12‐myristate‐13‐acetate‐stimulated oxidative burst, bacterial‐stimulated neutrophil degranulation, salivary secretory IgA, lactoferrin or lysozyme (P > 0.05). These findings provide further evidence of the beneficial effects of COL on receptor‐mediated stimulation of neutrophil oxidative burst in a model of exercise‐induced immune dysfunction.     

Reduced oxygen cost of running is related to alignment of the resultant GRF and leg axis vector: A pilot study

This pilot study investigated whether a 10‐week running program (10wkRP), which reduced the oxygen cost of running, affected resultant ground reaction force (GRF), leg axis alignment, joint moment characteristics, and gear ratios. Ten novice, female runners completed a 10wkRP. Running kinematics and kinetics, in addition to oxygen consumption () during steady‐state running, were recorded pre‐ and post‐10wkRP. decreased (8%) from pre‐10wkRP to post‐10wkRP. There was a better alignment of the resultant GRF and leg axis at peak propulsion post‐10wkRP compared with pre‐10wkRP (10.8 ± 4.9 vs 1.6 ± 1.2°), as the resultant GRF vector was applied 7 ± 0.6° (P = 0.008) more horizontally. There were shorter external ankle moment arms (24%) and smaller knee extensor moments (23%) at peak braking post‐10wkRP. The change in was associated with the change in alignment of the resultant GRF and leg axis (r_s = 0.88, P = 0.003). As runners became more economical, they exhibited a more aligned resultant GRF vector and leg axis at peak propulsion. This appears to be a self‐optimization strategy that may improve performance. Additionally, changes to external ankle moment arms indicated beneficial low gear ratios were achieved at the time of peak braking force.     

Quantitative MRI measurement of lung density must account for the change in T with lung inflation

Purpose

To evaluate lung water density at three different levels of lung inflation in normal lungs using a fast gradient echo sequence developed for rapid imaging.

Materials and Methods

Ten healthy volunteers were imaged with a fast gradient echo sequence that collects 12 images alternating between two closely spaced echoes in a single 9‐s breathhold. Data were fit to a single exponential to determine lung water density and T. Data were evaluated in a single imaging slice at total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). Analysis of variance for repeated measures was used to statistically evaluate changes in T and lung water density across lung volumes, imaging plane, and spatial locations in the lung.

Results

In normal subjects (n = 10), T (and [lung density/water density]) was 1.2 ± 0.1 msec (0.10 ± 0.02), 1.8 ± 0.2 ms (0.25 ± 0.04), and 2.0 ± 0.2 msec (0.27 ± 0.03) at TLC, FRC, and RV, respectively. Results also show that there is a considerable intersubject variability in the values of T.

Conclusion

Data show that T in the lung is very short, and varies considerably with lung volume. Thus, if quantitative assessment of lung density within a breathhold is to be measured accurately, then it is necessary to also determine T. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.     

A single‐scan T mapping method based on two gradient‐echo images with compensation for macroscopic field inhomogeneity

T‐weighted imaging (TWI) and quantitative T mapping with conventional gradient‐echo acquisition are often hindered by severe signal loss induced by macroscopic field inhomogeneity. Various z‐shimming approaches have been developed for TWI/T mapping in which the effects of macroscopic field inhomogeneity are suppressed while the sensitivity of T‐related signal intensity to alterations in the microscopic susceptibility is maintained. However, this is often done at the cost of significantly increased imaging time. In this work, a fast T mapping method with compensation for macroscopic field inhomogeneity was developed. A proton density‐weighted image and a composite T‐weighted image, both of which were essentially free from macroscopic field inhomogeneity‐induced signal loss, were used for the T calculation. The composite T‐weighted image was reconstructed from a number of gradient‐echo images acquired with successively incremented z‐shimming compensation. Because acquisition of the two images and z‐shimming compensation were realized in a single scan, the total acquisition time for obtaining a T map with the proposed method is the same as the time taken for a conventional multiecho gradient‐echo imaging sequence without compensation. The performance and efficiency of the proposed method were demonstrated and evaluated at 4.7 T. Magn Reson Med 60:1388–1395, 2008. © 2008 Wiley‐Liss, Inc.     

Validation of MRI biomarkers of hepatic steatosis in the presence of iron overload in the ob/ob mouse

Purpose:

To validate the utility and performance of a T correction method for hepatic fat quantification in an animal model of both steatosis and iron overload.

Materials and Methods:

Mice with low (n = 6), medium (n = 6), and high (n = 8) levels of steatosis were sedated and imaged using a chemical shift‐based fat‐water separation method to obtain magnetic resonance imaging (MRI) fat‐fraction measurements. Imaging was performed before and after each of two superparamagnetic iron oxide (SPIO) injections to create hepatic iron overload. Fat‐fraction maps were reconstructed with and without T correction. Fat‐fraction with and without T correction and T measurements were compared after each injection. Liver tissue was harvested and imaging results were compared to triglyceride extraction and histology grading.

Results:

Excellent correlation was seen between MRI fat‐fraction and tissue‐based fat quantification. Injections of SPIOs led to increases in R (=1/T). Measured fat‐fraction was unaffected by the presence of iron when T correction was used, whereas measured fat‐fraction dramatically increased without T correction.

Conclusion:

Hepatic fat‐fraction measured using a T‐corrected chemical shift‐based fat‐water separation method was validated in an animal model of steatosis and iron overload. T correction enables robust fat‐fraction estimation in both the presence and absence of iron, and is necessary for accurate hepatic fat quantification. J. Magn. Reson. Imaging 2012;35:844–851. © 2011 Wiley Periodicals, Inc.     

B(1) mapping with selective pulses

Knowledge of B distribution is crucial for many applications, such as quantitative MRI. A novel method has been developed to improve the accuracy of the conventionally applied double‐angle method for B mapping. It solves the remaining issues raised by the use of selective pulses for slice selection to accelerate the acquisition process. A general approach for reconstructing B maps is presented first. It takes B‐induced slice profile distortions over off‐resonance frequencies into account. It is then shown how the ratio between the prescribed flip angles can be adjusted to reach a compromise between the level of noise propagated onto B maps and the width of the range in which the field can be mapped. Lastly, several solutions are proposed for reducing the B‐dependent pollution of regions distal to the image slice which participates significantly in the inaccuracy of B mapping. These methods were experimentally tested by comparison with gold standard B maps obtained on a phantom using a non‐selective and thus much slower technique. As they are independent and lead to significant improvements, these solutions can be combined to achieve high precision and fast B mapping using spin‐echo DAM. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Myocardial T measurements in iron‐overloaded thalassemia: An in vivo study to investigate optimal methods of quantification

Reproducible and accurate myocardial T measurements are required for the quantification of iron in heart tissue in transfused thalassemia. The aim of this study was to determine the best method to measure the myocardial T from multi‐gradient‐echo data acquired both with and without black‐blood preparation. Sixteen thalassemia patients from six centers were scanned twice locally, within 1 week, using an optimized bright‐blood T sequence and then subsequently scanned at the standardization center in London within 4 weeks, using a T sequence both with and without black‐blood preparation. Different curve‐fitting models (monoexponential, truncation, and offset) were applied to the data and the results were compared by means of reproducibility. T measurements obtained using the bright‐ and black‐blood techniques. The black‐blood data were well fitted by the monoexponential model, which suggests that a more accurate measure of T can be obtained by removing the main source of errors in the bright‐blood data. For bright‐blood data, the offset model appeared to underestimate T values substantially and was less reproducible. The truncation model gave rise to more reproducible T measurements, which were also closer to the values obtained from the black‐blood data. Magn Reson Med 60:1082–1089, 2008. © 2008 Wiley‐Liss, Inc.     

A steep ramp test is valid for estimating maximal power and oxygen uptake during a standard ramp test in type 2 diabetes

A short maximal steep ramp test (SRT, 25 W/10 s) has been proposed to guide exercise interventions in type 2 diabetes, but requires validation. This study aims to (a) determine the relationship between W_max and reached during SRT and the standard ramp test (RT); (b) obtain test‐retest reliability; and (c) document electrocardiogram (ECG) abnormalities during SRT. Type 2 diabetes patients (35 men, 26 women) performed a cycle ergometer‐based RT (women 1.2; men 1.8 W/6 s) and SRT on separate days. A random subgroup (n = 42) repeated the SRT. ECG, heart rate, and were monitored. W_max during RT: 193 ± 63 (men) and 106 ± 33 W (women). W_max during SRT: 193 ± 63 (men) and 188 ± 55 W (women). The relationship between RT and SRT was described by men RT (mL/min) = 152 + 7.67 × W_max SRT1 (r: 0.859); women RT (mL/min) = 603 + 4.75 × W_max SRT1 (r: 0.771); intraclass correlation coefficients between first (SRT1) and second SRT W_max (SRT2) were men 0.951 [95% confidence interval (CI) 0.899–0.977] and women 0.908 (95% CI 0.727–0.971). No adverse events were noted during any of the exercise tests. This validation study indicates that the SRT is a low‐risk, accurate, and reliable test to estimate maximal aerobic capacity during the RT to design exercise interventions in type 2 diabetes patients.     

Ultrashort T relaxometry for quantitation of highly concentrated superparamagnetic iron oxide (SPIO) nanoparticle labeled cells

A new method was developed to measure ultrashort T relaxation in tissues containing a focal area of superparamagnetic iron oxide (SPIO) nanoparticle‐labeled cells in which the T decay is too short to be accurately measured using regular gradient echo T mapping. The proposed method utilizes the relatively long T₂ relaxation of SPIO‐labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T decay curve. MRI experiments in phantoms and rats with SPIO‐labeled tumors demonstrated that it can detect ultrashort T down to 1 ms or less. The measured T values were about 10% higher than those from the ultrashort TE (UTE) technique. The shorter the TE, the less the measurements deviated from the UTE T mapping. Combined with the regular T mapping, this technique is expected to provide quantitation of highly concentrated iron‐labeled cells from direct cell transplantation. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.